syntax in assignment statement l value

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Lvalue and Rvalue

Kenneth Leroy Busbee

Some programming languages use the idea of l-values and r-values , deriving from the typical mode of evaluation on the left and right hand side of an assignment statement. An lvalue refers to an object that persists beyond a single expression. An rvalue is a temporary value that does not persist beyond the expression that uses it. [1]

Lvalue and Rvalue refer to the left and right side of the assignment operator. The  Lvalue  (pronounced: L value) concept refers to the requirement that the operand on the left side of the assignment operator is modifiable, usually a variable.  Rvalue  concept pulls or fetches the value of the expression or operand on the right side of the assignment operator. Some examples:

The value 39 is pulled or fetched (Rvalue) and stored into the variable named age (Lvalue); destroying the value previously stored in that variable.

If the expression has a variable or named constant on the right side of the assignment operator, it would pull or fetch the value stored in the variable or constant. The value 18 is pulled or fetched from the variable named voting_age and stored into the variable named age.

If the expression is a test expression or Boolean expression, the concept is still an Rvalue one. The value in the identifier named age is pulled or fetched and used in the relational comparison of less than.

This is illegal because the identifier JACK_BENNYS_AGE does not have Lvalue properties. It is not a modifiable data object, because it is a constant.

Some uses of the Lvalue and Rvalue can be confusing in languages that support increment and decrement operators. Consider:

Postfix increment says to use my existing value then when you are done with the other operators; increment me. Thus, the first use of the oldest variable is an Rvalue context where the existing value of 55 is pulled or fetched and then assigned to the variable age; an Lvalue context. The second use of the oldest variable is an Lvalue context wherein the value of the oldest is incremented from 55 to 56.

• cnx.org: Programming Fundamentals – A Modular Structured Approach using C++
• Wikipedia: Value (computer science) ↵

Programming Fundamentals Copyright © 2018 by Kenneth Leroy Busbee is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License , except where otherwise noted.

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Verilog assign statement

Hardware schematic.

Signals of type wire or a similar wire like data type requires the continuous assignment of a value. For example, consider an electrical wire used to connect pieces on a breadboard. As long as the +5V battery is applied to one end of the wire, the component connected to the other end of the wire will get the required voltage.

In Verilog, this concept is realized by the assign statement where any wire or other similar wire like data-types can be driven continuously with a value. The value can either be a constant or an expression comprising of a group of signals.

Assign Syntax

The assignment syntax starts with the keyword assign followed by the signal name which can be either a single signal or a concatenation of different signal nets. The drive strength and delay are optional and are mostly used for dataflow modeling than synthesizing into real hardware. The expression or signal on the right hand side is evaluated and assigned to the net or expression of nets on the left hand side.

Delay values are useful for specifying delays for gates and are used to model timing behavior in real hardware because the value dictates when the net should be assigned with the evaluated value.

• LHS should always be a scalar or vector net or a concatenation of scalar or vector nets and never a scalar or vector register.
• RHS can contain scalar or vector registers and function calls.
• Whenever any operand on the RHS changes in value, LHS will be updated with the new value.
• assign statements are also called continuous assignments and are always active

In the following example, a net called out is driven continuously by an expression of signals. i1 and i2 with the logical AND & form the expression.

If the wires are instead converted into ports and synthesized, we will get an RTL schematic like the one shown below after synthesis.

Continuous assignment statement can be used to represent combinational gates in Verilog.

The module shown below takes two inputs and uses an assign statement to drive the output z using part-select and multiple bit concatenations. Treat each case as the only code in the module, else many assign statements on the same signal will definitely make the output become X.

Assign reg variables

It is illegal to drive or assign reg type variables with an assign statement. This is because a reg variable is capable of storing data and does not require to be driven continuously. reg signals can only be driven in procedural blocks like initial and always .

Implicit Continuous Assignment

When an assign statement is used to assign the given net with some value, it is called explicit assignment. Verilog also allows an assignment to be done when the net is declared and is called implicit assignment.

Combinational Logic Design

Consider the following digital circuit made from combinational gates and the corresponding Verilog code.

Combinational logic requires the inputs to be continuously driven to maintain the output unlike sequential elements like flip flops where the value is captured and stored at the edge of a clock. So an assign statement fits the purpose the well because the output o is updated whenever any of the inputs on the right hand side change.

After design elaboration and synthesis, we do get to see a combinational circuit that would behave the same way as modeled by the assign statement.

See that the signal o becomes 1 whenever the combinational expression on the RHS becomes true. Similarly o becomes 0 when RHS is false. Output o is X from 0ns to 10ns because inputs are X during the same time.

Click here for a slideshow with simulation example !

问 错误:赋值语句中的语法l-value EN

这是Verilog代码。

我得到了以下错误。你能帮我了解如何修复它们吗？

Stack Overflow用户

发布于 2020-12-20 19:35:17

您的代码中有一些语法错误。

不要使用 assign 关键字为 reg ( F1 )赋值。

不要将模块实例( sub )放在 always 块中。

使用 sub 模块实例的实例名称。

请勿创建与模块( sub )同名的 task 。在这种情况下，不需要 task/endtask 。

这是一个为我干净利落地编译的版本：

https://stackoverflow.com/questions/65377518

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Programming Fundamentals/Lvalue and Rvalue

• 2 Discussion
• 3 Key Terms
• 4 References

Overview [ edit | edit source ]

Some programming languages use the idea of L-values and R-values, deriving from the typical mode of evaluation on the left and right-hand side of an assignment statement. An L-value refers to an object that persists beyond a single expression. An R-value is a temporary value that does not persist beyond the expression that uses it. [1]

Discussion [ edit | edit source ]

L-value and R-value refer to the left and right side of the assignment operator. The L-value (pronounced: L value) concept refers to the requirement that the operand on the left side of the assignment operator is modifiable, usually a variable. R-value concept pulls or fetches the value of the expression or operand on the right side of the assignment operator. Some examples:

The value 39 is pulled or fetched (R-value) and stored into the variable named age (L-value); destroying the value previously stored in that variable.

If the expression has a variable or named constant on the right side of the assignment operator, it would pull or fetch the value stored in the variable or constant. The value 18 is pulled or fetched from the variable named voting_age and stored into the variable named age.

If the expression is a test expression or Boolean expression, the concept is still an R-value. The value in the identifier named age is pulled or fetched and used in the relational comparison of less than.

This is illegal because the identifier JACK_BENNYS_AGE does not have L-value properties. It is not a modifiable data object, because it is a constant.

Some uses of the L-value and R-value can be confusing in languages that support increment and decrement operators. Consider:

Postfix increment says to use my existing value then when you are done with the other operators; increment me. Thus, the first use of the oldest variable is an R-value context where the existing value of 55 is pulled or fetched and then assigned to the variable age; an L-value context. The second use of the oldest variable is an L-value context wherein the value of the oldest is incremented from 55 to 56.

Key Terms [ edit | edit source ]

References [ edit | edit source ].

• cnx.org: Programming Fundamentals – A Modular Structured Approach using C++
• ↑ Wikipedia: Value (computer science)

• Book:Programming Fundamentals

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lvalue:- 

lvalue simply means an object that has an identifiable location in memory (i.e. having an address).

• In any assignment statement “lvalue” must have the capability to store the data.
• lvalue cannot be a function, expression (like a+b) or a constant (like 3 , 4 , etc.).

L-value : “l-value” refers to memory location which identifies an object. l-value may appear as either left hand or right hand side of an assignment operator(=). l-value often represents as identifier. Expressions referring to modifiable locations are called “ modifiable l-values “. A modifiable l-value cannot have an array type, an incomplete type, or a type with the const attribute. For structures and unions to be modifiable lvalues , they must not have any members with the const attribute. The name of the identifier denotes a storage location, while the value of the variable is the value stored at that location. An identifier is a modifiable lvalue if it refers to a memory location and if its type is arithmetic, structure, union or pointer. For example, if ptr is a pointer to a storage region, then *ptr is a modifiable l-value that designates the storage region to which ptr points. In C, the concept was renamed as “locator value” , and referred to expressions that locate (designate) objects. The l-value is one of the following:

• The name of the variable of any type i.e. , an identifier of integral, floating, pointer, structure, or union type.
• A subscript ([ ]) expression that does not evaluate to an array.
• A unary-indirection (*) expression that does not refer to an array
• An l-value expression in parentheses.
• A const object (a nonmodifiable l-value).
• The result of indirection through a pointer, provided that it isn’t a function pointer.
• The result of member access through pointer(-> or .)

p = &a; // ok, assignment of address // at l-value

&a = p; // error: &a is an r-value

( x < y ? y : x) = 0; // It’s valid because the ternary // expression preserves the "lvalue-ness" // of both its possible return values

r-value simply means, an object that has no identifiable location in memory (i.e. having an address).

• Anything that is capable of returning a constant expression or value.
• Expression like a+b will return some constant.

R-value : r-value” refers to data value that is stored at some address in memory. A r-value is an expression, that can’t have a value assigned to it, which means r-value can appear on right but not on left hand side of an assignment operator(=). 

Note : The unary & (address-of) operator requires an l-value as its operand. That is, &n is a valid expression only if n is an l-value. Thus, an expression such as &12 is an error. Again, 12 does not refer to an object, so it’s not addressable. For instance, 

Remembering the mnemonic, that l-values can appear on the left of an assignment operator while r-values can appear on the right.

Reference: https://msdn.microsoft.com/en-us/library/bkbs2cds.aspx

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12.2 — Value categories (lvalues and rvalues)

Before we talk about our first compound type (lvalue references), we’re going to take a little detour and talk about what an lvalue is.

In lesson 1.10 -- Introduction to expressions , we defined an expression as “a combination of literals, variables, operators, and function calls that can be executed to produce a singular value”.

For example:

In the above program, the expression 2 + 3 is evaluated to produce the value 5, which is then printed to the console.

In lesson 6.4 -- Increment/decrement operators, and side effects , we also noted that expressions can produce side effects that outlive the expression:

In the above program, the expression ++x increments the value of x , and that value remains changed even after the expression has finished evaluating.

Besides producing values and side effects, expressions can do one more thing: they can evaluate to objects or functions. We’ll explore this point further in just a moment.

The properties of an expression

To help determine how expressions should evaluate and where they can be used, all expressions in C++ have two properties: a type and a value category.

The type of an expression

The type of an expression is equivalent to the type of the value, object, or function that results from the evaluated expression. For example:

For v1 , the compiler will determine (at compile time) that a division with two int operands will produce an int result, so int is the type of this expression. Via type inference, int will then be used as the type of v1 .

For v2 , the compiler will determine (at compile time) that a division with a double operand and an int operand will produce a double result. Remember that arithmetic operators must have operands of matching types, so in this case, the int operand gets converted to a double , and a floating point division is performed. So double is the type of this expression.

The compiler can use the type of an expression to determine whether an expression is valid in a given context. For example:

In the above program, the print(int) function is expecting an int parameter. However, the type of the expression we’re passing in (the string literal "foo" ) does not match, and no conversion can be found. So a compile error results.

Note that the type of an expression must be determinable at compile time (otherwise type checking and type deduction wouldn’t work) -- however, the value of an expression may be determined at either compile time (if the expression is constexpr) or runtime (if the expression is not constexpr).

The value category of an expression

Now consider the following program:

One of these assignment statements is valid (assigning value 5 to variable x ) and one is not (what would it mean to assign the value of x to the literal value 5 ?). So how does the compiler know which expressions can legally appear on either side of an assignment statement?

The answer lies in the second property of expressions: the value category . The value category of an expression (or subexpression) indicates whether an expression resolves to a value, a function, or an object of some kind.

Prior to C++11, there were only two possible value categories: lvalue and rvalue .

In C++11, three additional value categories ( glvalue , prvalue , and xvalue ) were added to support a new feature called move semantics .

Author’s note

In this lesson, we’ll stick to the pre-C++11 view of value categories, as this makes for a gentler introduction to value categories (and is all that we need for the moment). We’ll cover move semantics (and the additional three value categories) in a future chapter.

Lvalue and rvalue expressions

An lvalue (pronounced “ell-value”, short for “left value” or “locator value”, and sometimes written as “l-value”) is an expression that evaluates to an identifiable object or function (or bit-field).

The term “identity” is used by the C++ standard, but is not well-defined. An entity (such as an object or function) that has an identity can be differentiated from other similar entities (typically by comparing the addresses of the entity).

Entities with identities can be accessed via an identifier, reference, or pointer, and typically have a lifetime longer than a single expression or statement.

In the above program, the expression x is an lvalue expression as it evaluates to variable x (which has an identifier).

Since the introduction of constants into the language, lvalues come in two subtypes: a modifiable lvalue is an lvalue whose value can be modified. A non-modifiable lvalue is an lvalue whose value can’t be modified (because the lvalue is const or constexpr).

An rvalue (pronounced “arr-value”, short for “right value”, and sometimes written as r-value ) is an expression that is not an lvalue. Rvalue expressions evaluate to a value. Commonly seen rvalues include literals (except C-style string literals, which are lvalues) and the return value of functions and operators that return by value. Rvalues aren’t identifiable (meaning they have to be used immediately), and only exist within the scope of the expression in which they are used.

You may be wondering why return5() , x + 1 , and static_cast<int>(d) are rvalues: the answer is because these expressions produce temporary values that are not identifiable objects.

Key insight

Lvalue expressions evaluate to an identifiable object. Rvalue expressions evaluate to a value.

Now we can answer the question about why x = 5 is valid but 5 = x is not: an assignment operation requires the left operand of the assignment to be a modifiable lvalue expression, and the right operand to be an rvalue expression. The latter assignment ( 5 = x ) fails because the left operand expression 5 isn’t an lvalue.

Related content

A full list of lvalue and rvalue expressions can be found here . In C++11, rvalues are broken into two subtypes: prvalues and xvalues, so the rvalues we’re talking about here are the sum of both of those categories.

If you’re not sure whether an expression is an lvalue or rvalue, try taking its address using operator& , which requires its operand to be an lvalue. If &(expression); compiles, expression must be an lvalue:

A C-style string literal is an lvalue because C-style strings (which are C-style arrays) decay to a pointer. The decay process only works if the array is an lvalue (and thus has an address that can be stored in the pointer). C++ inherited this for backwards compatibility.

We cover array decay in lesson 17.8 -- C-style array decay .

Lvalue to rvalue conversion

Let’s take a look at this example again:

If x is an lvalue expression that evaluates to variable x , then how does y end up with value 5 ?

The answer is that lvalue expressions will implicitly convert to rvalue expressions in contexts where an rvalue is expected but an lvalue is provided. The initializer for an int variable is expected to be an rvalue expression. Thus lvalue expression x undergoes an lvalue-to-rvalue conversion, which evaluates to value 5 , which is then used to initialize y .

We said above that the assignment operator expects the right operand to be an rvalue expression, so why does code like this work?

In this case, y is an lvalue expression, but it undergoes an lvalue-to-rvalue conversion, which evaluates to the value of y (2), which is then assigned to x .

Now consider this example:

In this statement, the variable x is being used in two different contexts. On the left side of the assignment operator, x is an lvalue expression that evaluates to variable x . On the right side of the assignment operator, x + 1 is an rvalue expression that evaluates to the value 3 .

Now that we’ve covered lvalues, we can get to our first compound type: the lvalue reference .

A rule of thumb to identify lvalue and rvalue expressions:

• Lvalue expressions are those that evaluate to variables or other identifiable objects that persist beyond the end of the expression.
• Rvalue expressions are those that evaluate to literals or values returned by functions/operators that are discarded at the end of the expression.

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4.5: L Value and R Value

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• Page ID 11257

• Kenneth Leroy Busbee
• Houston Community College via OpenStax CNX

They refer to on the left and right side of the assignment operator. The  Lvalue  (pronounced: L value) concept refers to the requirement that the operand on the left side of the assignment operator is modifiable, usually a variable.  Rvalue  concept pulls or fetches the value of the expression or operand on the right side of the assignment operator. Some examples:

The value 39 is pulled or fetched (Rvalue) and stored into the variable named age (Lvalue); destroying the value previously stored in that variable.

If the expression has a variable or named constant on the right side of the assignment operator, it would pull or fetch the value stored in the variable or constant. The value 18 is pulled or fetched from the variable named voting_age and stored into the variable named age.

If the expression is a  test expression  or  Boolean expression , the concept is still an Rvalue one. The value in the identifier named age is pulled or fetched and used in the relational comparison of less than.

This is illegal because the identifier JACK_BENNYS_AGE does not have Lvalue properties. It is not a modifiable data object, because it is a constant.

Some uses of the Lvalue and Rvalue can be confusing.

Postfix increment says to use my existing value then when you are done with the other operators; increment me. Thus, the first use of the oldest variable is an Rvalue context where the existing value of 55 is pulled or fetched and then assigned to the variable age; an Lvalue context. The second use of the oldest variable is an Lvalue context where in the value of oldest is incremented from 55 to 56.

Definitions

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L-Value and R-Value Expressions

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Expressions that refer to memory locations are called "l-value" expressions. An l-value represents a storage region's "locator" value, or a "left" value, implying that it can appear on the left of the equal sign ( = ). L-values are often identifiers.

Expressions referring to modifiable locations are called "modifiable l-values." A modifiable l-value can't have an array type, an incomplete type, or a type with the const attribute. For structures and unions to be modifiable l-values, they must not have any members with the const attribute. The name of the identifier denotes a storage location, while the value of the variable is the value stored at that location.

An identifier is a modifiable l-value if it refers to a memory location and if its type is arithmetic, structure, union, or pointer. For example, if ptr is a pointer to a storage region, then *ptr is a modifiable l-value that designates the storage region to which ptr points.

Any of the following C expressions can be l-value expressions:

An identifier of integral, floating, pointer, structure, or union type

A subscript ( [ ] ) expression that doesn't evaluate to an array

A member-selection expression ( -> or . )

A unary-indirection ( * ) expression that doesn't refer to an array

An l-value expression in parentheses

A const object (a nonmodifiable l-value)

The term "r-value" is sometimes used to describe the value of an expression and to distinguish it from an l-value. All l-values are r-values but not all r-values are l-values.

Microsoft Specific

Microsoft C includes an extension to the ANSI C standard that allows casts of l-values to be used as l-values, as long as the size of the object isn't lengthened through the cast. (For more information, see Type-Cast Conversions .) The following example illustrates this feature:

The default for Microsoft C is that the Microsoft extensions are enabled. Use the /Za compiler option to disable these extensions.

END Microsoft Specific

Operands and Expressions

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Additional resources

Lvalues and rvalues

Expressions can be categorized into one of the following value categories:

An expression can appear on the left side of an assignment expression if the expression is not const qualified.

An rvalue reference that is to expire.

Rvalues include both xvalues and prvalues. Lvalues and xvalues can be referred as glvalues.

• An array type
• An incomplete type
• A const -qualified type
• A structure or union type with one of its members qualified as a const type

The term rvalue refers to a data value that is stored at some address in memory. An rvalue is an expression that cannot have a value assigned to it. Both a literal constant and a variable can serve as an rvalue. When an lvalue appears in a context that requires an rvalue, the lvalue is implicitly converted to an rvalue. The reverse, however, is not true: an rvalue cannot be converted to an lvalue. Rvalues always have complete types or the void type.

For example, all assignment operators evaluate their right operand and assign that value to their left operand. The left operand must be a modifiable lvalue or a reference to a modifiable object.

The following expressions are xvalues:

• The result of calling a function whose return type is of an rvalue reference type
• A cast to an rvalue reference
• A nonstatic data member of a non-reference type accessed through an xvalue expression
• A pointer to member access expression in which the first operand is an xvalue expression and the second operand is of a pointer to member type

See the following example:

In this example, The initializer for rvalue reference b is an xvalue because it is a result of a cast to an rvalue reference. A call to the function f() produces an xvalue because the return type of this function is of the int&& type. The initializer for rvalue reference rc is an xvalue because it is an expression that accesses a nonstatic non-reference data member c through an xvalue expression.

• Lvalue-to-rvalue conversions

CS105: Introduction to Python

Variables and assignment statements.

Computers must be able to remember and store data. This can be accomplished by creating a variable to house a given value. The assignment operator = is used to associate a variable name with a given value. For example, type the command:

in the command line window. This command assigns the value 3.45 to the variable named a . Next, type the command:

in the command window and hit the enter key. You should see the value contained in the variable a echoed to the screen. This variable will remember the value 3.45 until it is assigned a different value. To see this, type these two commands:

You should see the new value contained in the variable a echoed to the screen. The new value has "overwritten" the old value. We must be careful since once an old value has been overwritten, it is no longer remembered. The new value is now what is being remembered.

Although we will not discuss arithmetic operations in detail until the next unit, you can at least be equipped with the syntax for basic operations: + (addition), - (subtraction), * (multiplication), / (division)

For example, entering these command sequentially into the command line window:

would result in 12.32 being echoed to the screen (just as you would expect from a calculator). The syntax for multiplication works similarly. For example:

would result in 35 being echoed to the screen because the variable b has been assigned the value a * 5 where, at the time of execution, the variable a contains a value of 7.

After you read, you should be able to execute simple assignment commands using integer and float values in the command window of the Repl.it IDE. Try typing some more of the examples from this web page to convince yourself that a variable has been assigned a specific value.

In programming, we associate names with values so that we can remember and use them later. Recall Example 1. The repeated computation in that algorithm relied on remembering the intermediate sum and the integer to be added to that sum to get the new sum. In expressing the algorithm, we used th e names current and sum .

In programming, a name that refers to a value in this fashion is called a variable . When we think of values as data stored somewhere i n the computer, we can have a mental image such as the one below for the value 10 stored in the computer and the variable x , which is the name we give to 10. What is most important is to see that there is a binding between x and 10.

The term variable comes from the fact that values that are bound to variables can change throughout computation. Bindings as shown above are created, and changed by assignment statements . An assignment statement associates the name to the left of the symbol = with the value denoted by the expression on the right of =. The binding in the picture is created using an assignment statemen t of the form x = 10 . We usually read such an assignment statement as "10 is assigned to x" or "x is set to 10".

If we want to change the value that x refers to, we can use another assignment statement to do that. Suppose we execute x = 25 in the state where x is bound to 10.Then our image becomes as follows:

Choosing variable names

Suppose that we u sed the variables x and y in place of the variables side and area in the examples above. Now, if we were to compute some other value for the square that depends on the length of the side , such as the perimeter or length of the diagonal, we would have to remember which of x and y , referred to the length of the side because x and y are not as descriptive as side and area . In choosing variable names, we have to keep in mind that programs are read and maintained by human beings, not only executed by machines.

Note about syntax

In Python, variable identifiers can contain uppercase and lowercase letters, digits (provided they don't start with a digit) and the special character _ (underscore). Although it is legal to use uppercase letters in variable identifiers, we typically do not use them by convention. Variable identifiers are also case-sensitive. For example, side and Side are two different variable identifiers.

There is a collection of words, called reserved words (also known as keywords), in Python that have built-in meanings and therefore cannot be used as variable names. For the list of Python's keywords See 2.3.1 of the Python Language Reference.

Syntax and Sema ntic Errors

Now that we know how to write arithmetic expressions and assignment statements in Python, we can pause and think about what Python does if we write something that the Python interpreter cannot interpret. Python informs us about such problems by giving an error message. Broadly speaking there are two categories for Python errors:

• Syntax errors: These occur when we write Python expressions or statements that are not well-formed according to Python's syntax. For example, if we attempt to write an assignment statement such as 13 = age , Python gives a syntax error. This is because Python syntax says that for an assignment statement to be well-formed it must contain a variable on the left hand side (LHS) of the assignment operator "=" and a well-formed expression on the right hand side (RHS), and 13 is not a variable.
• Semantic errors: These occur when the Python interpreter cannot evaluate expressions or execute statements because they cannot be associated with a "meaning" that the interpreter can use. For example, the expression age + 1 is well-formed but it has a meaning only when age is already bound to a value. If we attempt to evaluate this expression before age is bound to some value by a prior assignment then Python gives a semantic error.

Even though we have used numerical expressions in all of our examples so far, assignments are not confined to numerical types. They could involve expressions built from any defined type. Recall the table that summarizes the basic types in Python.

The following video shows execution of assignment statements involving strings. It also introduces some commonly used operators on strings. For more information see the online documentation. In the video below, you see the Python shell displaying "=> None" after the assignment statements. This is unique to the Python shell presented in the video. In most Python programming environments, nothing is displayed after an assignment statement. The difference in behavior stems from version differences between the programming environment used in the video and in the activities, and can be safely ignored.

Distinguishing Expressions and Assignments

So far in the module, we have been careful to keep the distinction between the terms expression and statement because there is a conceptual difference between them, which is sometimes overlooked. Expressions denote values; they are evaluated to yield a value. On the other hand, statements are commands (instructions) that change the state of the computer. You can think of state here as some representation of computer memory and the binding of variables and values in the memory. In a state where the variable side is bound to the integer 3, and the variable area is yet unbound, the value of the expression side + 2 is 5. The assignment statement side = side + 2 , changes the state so that value 5 is bound to side in the new state. Note that when you type an expression in the Python shell, Python evaluates the expression and you get a value in return. On the other hand, if you type an assignment statement nothing is returned. Assignment statements do not return a value. Try, for example, typing x = 100 + 50 . Python adds 100 to 50, gets the value 150, and binds x to 150. However, we only see the prompt >>> after Python does the assignment. We don't see the change in the state until we inspect the value of x , by invoking x .

What we have learned so far can be summarized as using the Python interpreter to manipulate values of some primitive data types such as integers, real numbers, and character strings by evaluating expressions that involve built-in operators on these types. Assignments statements let us name the values that appear in expressions. While what we have learned so far allows us to do some computations conveniently, they are limited in their generality and reusability. Next, we introduce functions as a means to make computations more general and reusable.

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Lvalue and Rvalue

Kenneth Leroy Busbee

Some programming languages use the idea of l-values and r-values , deriving from the typical mode of evaluation on the left and right hand side of an assignment statement. An lvalue refers to an object that persists beyond a single expression. An rvalue is a temporary value that does not persist beyond the expression that uses it. [1]

Lvalue and Rvalue refer to the left and right side of the assignment operator. The  Lvalue  (pronounced: L value) concept refers to the requirement that the operand on the left side of the assignment operator is modifiable, usually a variable.  Rvalue  concept pulls or fetches the value of the expression or operand on the right side of the assignment operator. Some examples:

The value 39 is pulled or fetched (Rvalue) and stored into the variable named age (Lvalue); destroying the value previously stored in that variable.

If the expression has a variable or named constant on the right side of the assignment operator, it would pull or fetch the value stored in the variable or constant. The value 18 is pulled or fetched from the variable named voting_age and stored into the variable named age.

If the expression is a test expression or Boolean expression, the concept is still an Rvalue one. The value in the identifier named age is pulled or fetched and used in the relational comparison of less than.

This is illegal because the identifier JACK_BENNYS_AGE does not have Lvalue properties. It is not a modifiable data object, because it is a constant.

Some uses of the Lvalue and Rvalue can be confusing in languages that support increment and decrement operators. Consider:

Postfix increment says to use my existing value then when you are done with the other operators; increment me. Thus, the first use of the oldest variable is an Rvalue context where the existing value of 55 is pulled or fetched and then assigned to the variable age; an Lvalue context. The second use of the oldest variable is an Lvalue context wherein the value of the oldest is incremented from 55 to 56.

• archive.org: Programming Fundamentals – A Modular Structured Approach using C++
• Wikipedia: Value (computer science) ↵

Programming Fundamentals Copyright © 2018 by Kenneth Leroy Busbee is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License , except where otherwise noted.

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Assignment Statements

Assignment statements in a program come in two forms – with and without mutations. Assignments without mutation are those that give a value to a variable without using the old value of that variable. Assignments with mutation are variable assignments that use the old value of a variable to calculate a value for the variable.

For example, an increment statement like x = x + 1 MUTATES the value of x by updating its value to be one bigger than it was before. In order to make sense of such a statement, we need to know the previous value of x .

In contrast, a statement like y = x + 1 assigns to y one more than the value in x . We do not need to know the previous value of y , as we are not using it in the assignment statement. (We do need to know the value of x ).

Assignments without mutation

We have already seen the steps necessary to process assignment statements that do not involve variable mutation. Recall that we can declare as a premise any assignment statement or claim from a previous logic block involving variables that have not since changed.

For example, suppose we want to verify the following program so the assert statement at the end will hold:

Since none of the statements involve variable mutation, we can do the verification in a single logic block:

Note that we did need to do ∧i so that the last claim was y == z ∧ y == 6 , even though we had previously established the claims y == z and y == 6 . In order for an assert to hold (at least until we switch Logika modes in chapter 10), we need to have established EXACTLY the claim in the assert in a previous logic block.

Assignments with mutation

Assignments with mutation are trickier – we need to know the old value of a variable in order to reason about its new value. For example, if we have the following program:

Then we might try to add the following logic blocks:

…but then we get stuck in the second logic block. There, x is supposed to refer to the CURRENT value of x (after being incremented), but both our attempted claims are untrue. The current value of x is not one more than itself (this makes no sense!), and we can tell from reading the code that x is now 5, not 4.

To help reason about changing variables, Logika has a special name_old value that refers to the OLD value of a variable called name , just before the latest update. In the example above, we can use x_old in the second logic block to refer to x ’s value just before it was incremented. We can now change our premises and finish the verification as follows:

By the end of the logic block following a variable mutation, we need to express everything we know about the variable’s current value WITHOUT using the _old terminology, as its scope will end when the logic block ends. Moreover, we only ever have one _old value available in a logic block – the variable that was most recently changed. This means we will need logic blocks after each variable mutation to process the changes to any related facts.

Variable swap example

Suppose we have the following Logika program:

We can see that this program gets two user input values, x and y , and then swaps their values. So if x was originally 4 and y was originally 6, then at the end of the program x would be 6 and y would be 4.

We would like to be able to assert what we did – that x now has the original value from y , and that y now has the original value from x . To do this, we might invent dummy constants called xOrig and yOrig that represent the original values of those variables. Then we can add our assert:

We can complete the verification by adding logic blocks after assignment statements, being careful to update all we know (without using the _old value) by the end of each block:

Notice that in each logic block, we express as much as we can about all variables/values in the program. In the first logic block, even though xOrig and yOrig were not used in the previous assignment statement, we still expressed how the current values our other variables compared to xOrig and yOrig . It helps to think about what you are trying to claim in the final assert – since our assert involved xOrig and yOrig , we needed to relate the current values of our variables to those values as we progressed through the program.

Last modified by: Julie Thornton Nov 15, 2023