c multiple variable assignment

1.4 — Variable assignment and initialization

In the previous lesson ( 1.3 -- Introduction to objects and variables ), we covered how to define a variable that we can use to store values. In this lesson, we’ll explore how to actually put values into variables and use those values.

As a reminder, here’s a short snippet that first allocates a single integer variable named x , then allocates two more integer variables named y and z :

Variable assignment

After a variable has been defined, you can give it a value (in a separate statement) using the = operator . This process is called assignment , and the = operator is called the assignment operator .

By default, assignment copies the value on the right-hand side of the = operator to the variable on the left-hand side of the operator. This is called copy assignment .

Here’s an example where we use assignment twice:

This prints:

When we assign value 7 to variable width , the value 5 that was there previously is overwritten. Normal variables can only hold one value at a time.

One of the most common mistakes that new programmers make is to confuse the assignment operator ( = ) with the equality operator ( == ). Assignment ( = ) is used to assign a value to a variable. Equality ( == ) is used to test whether two operands are equal in value.

Initialization

One downside of assignment is that it requires at least two statements: one to define the variable, and another to assign the value.

These two steps can be combined. When an object is defined, you can optionally give it an initial value. The process of specifying an initial value for an object is called initialization , and the syntax used to initialize an object is called an initializer .

In the above initialization of variable width , { 5 } is the initializer, and 5 is the initial value.

Different forms of initialization

Initialization in C++ is surprisingly complex, so we’ll present a simplified view here.

There are 6 basic ways to initialize variables in C++:

You may see the above forms written with different spacing (e.g. int d{7}; ). Whether you use extra spaces for readability or not is a matter of personal preference.

Default initialization

When no initializer is provided (such as for variable a above), this is called default initialization . In most cases, default initialization performs no initialization, and leaves a variable with an indeterminate value.

We’ll discuss this case further in lesson ( 1.6 -- Uninitialized variables and undefined behavior ).

Copy initialization

When an initial value is provided after an equals sign, this is called copy initialization . This form of initialization was inherited from C.

Much like copy assignment, this copies the value on the right-hand side of the equals into the variable being created on the left-hand side. In the above snippet, variable width will be initialized with value 5 .

Copy initialization had fallen out of favor in modern C++ due to being less efficient than other forms of initialization for some complex types. However, C++17 remedied the bulk of these issues, and copy initialization is now finding new advocates. You will also find it used in older code (especially code ported from C), or by developers who simply think it looks more natural and is easier to read.

For advanced readers

Copy initialization is also used whenever values are implicitly copied or converted, such as when passing arguments to a function by value, returning from a function by value, or catching exceptions by value.

Direct initialization

When an initial value is provided inside parenthesis, this is called direct initialization .

Direct initialization was initially introduced to allow for more efficient initialization of complex objects (those with class types, which we’ll cover in a future chapter). Just like copy initialization, direct initialization had fallen out of favor in modern C++, largely due to being superseded by list initialization. However, we now know that list initialization has a few quirks of its own, and so direct initialization is once again finding use in certain cases.

Direct initialization is also used when values are explicitly cast to another type.

One of the reasons direct initialization had fallen out of favor is because it makes it hard to differentiate variables from functions. For example:

List initialization

The modern way to initialize objects in C++ is to use a form of initialization that makes use of curly braces. This is called list initialization (or uniform initialization or brace initialization ).

List initialization comes in three forms:

As an aside…

Prior to the introduction of list initialization, some types of initialization required using copy initialization, and other types of initialization required using direct initialization. List initialization was introduced to provide a more consistent initialization syntax (which is why it is sometimes called “uniform initialization”) that works in most cases.

Additionally, list initialization provides a way to initialize objects with a list of values (which is why it is called “list initialization”). We show an example of this in lesson 16.2 -- Introduction to std::vector and list constructors .

List initialization has an added benefit: “narrowing conversions” in list initialization are ill-formed. This means that if you try to brace initialize a variable using a value that the variable can not safely hold, the compiler is required to produce a diagnostic (usually an error). For example:

In the above snippet, we’re trying to assign a number (4.5) that has a fractional part (the .5 part) to an integer variable (which can only hold numbers without fractional parts).

Copy and direct initialization would simply drop the fractional part, resulting in the initialization of value 4 into variable width . Your compiler may optionally warn you about this, since losing data is rarely desired. However, with list initialization, your compiler is required to generate a diagnostic in such cases.

Conversions that can be done without potential data loss are allowed.

To summarize, list initialization is generally preferred over the other initialization forms because it works in most cases (and is therefore most consistent), it disallows narrowing conversions, and it supports initialization with lists of values (something we’ll cover in a future lesson). While you are learning, we recommend sticking with list initialization (or value initialization).

Best practice

Prefer direct list initialization (or value initialization) for initializing your variables.

Author’s note

Bjarne Stroustrup (creator of C++) and Herb Sutter (C++ expert) also recommend using list initialization to initialize your variables.

In modern C++, there are some cases where list initialization does not work as expected. We cover one such case in lesson 16.2 -- Introduction to std::vector and list constructors .

Because of such quirks, some experienced developers now advocate for using a mix of copy, direct, and list initialization, depending on the circumstance. Once you are familiar enough with the language to understand the nuances of each initialization type and the reasoning behind such recommendations, you can evaluate on your own whether you find these arguments persuasive.

Value initialization and zero initialization

When a variable is initialized using empty braces, value initialization takes place. In most cases, value initialization will initialize the variable to zero (or empty, if that’s more appropriate for a given type). In such cases where zeroing occurs, this is called zero initialization .

Q: When should I initialize with { 0 } vs {}?

Use an explicit initialization value if you’re actually using that value.

Use value initialization if the value is temporary and will be replaced.

Initialize your variables

Initialize your variables upon creation. You may eventually find cases where you want to ignore this advice for a specific reason (e.g. a performance critical section of code that uses a lot of variables), and that’s okay, as long as the choice is made deliberately.

Related content

For more discussion on this topic, Bjarne Stroustrup (creator of C++) and Herb Sutter (C++ expert) make this recommendation themselves here .

We explore what happens if you try to use a variable that doesn’t have a well-defined value in lesson 1.6 -- Uninitialized variables and undefined behavior .

Initialize your variables upon creation.

Initializing multiple variables

In the last section, we noted that it is possible to define multiple variables of the same type in a single statement by separating the names with a comma:

We also noted that best practice is to avoid this syntax altogether. However, since you may encounter other code that uses this style, it’s still useful to talk a little bit more about it, if for no other reason than to reinforce some of the reasons you should be avoiding it.

You can initialize multiple variables defined on the same line:

Unfortunately, there’s a common pitfall here that can occur when the programmer mistakenly tries to initialize both variables by using one initialization statement:

In the top statement, variable “a” will be left uninitialized, and the compiler may or may not complain. If it doesn’t, this is a great way to have your program intermittently crash or produce sporadic results. We’ll talk more about what happens if you use uninitialized variables shortly.

The best way to remember that this is wrong is to consider the case of direct initialization or brace initialization:

Because the parenthesis or braces are typically placed right next to the variable name, this makes it seem a little more clear that the value 5 is only being used to initialize variable b and d , not a or c .

Unused initialized variables warnings

Modern compilers will typically generate warnings if a variable is initialized but not used (since this is rarely desirable). And if “treat warnings as errors” is enabled, these warnings will be promoted to errors and cause the compilation to fail.

Consider the following innocent looking program:

When compiling this with the g++ compiler, the following error is generated:

and the program fails to compile.

There are a few easy ways to fix this.

• If the variable really is unused, then the easiest option is to remove the defintion of x (or comment it out). After all, if it’s not used, then removing it won’t affect anything.
• Another option is to simply use the variable somewhere:

But this requires some effort to write code that uses it, and has the downside of potentially changing your program’s behavior.

The [[maybe_unused]] attribute C++17

In some cases, neither of the above options are desirable. Consider the case where we have a bunch of math/physics values that we use in many different programs:

If we use these a lot, we probably have these saved somewhere and copy/paste/import them all together.

However, in any program where we don’t use all of these values, the compiler will complain about each variable that isn’t actually used. While we could go through and remove/comment out the unused ones for each program, this takes time and energy. And later if we need one that we’ve previously removed, we’ll have to go back and re-add it.

To address such cases, C++17 introduced the [[maybe_unused]] attribute, which allows us to tell the compiler that we’re okay with a variable being unused. The compiler will not generate unused variable warnings for such variables.

The following program should generate no warnings/errors:

Additionally, the compiler will likely optimize these variables out of the program, so they have no performance impact.

In future lessons, we’ll often define variables we don’t use again, in order to demonstrate certain concepts. Making use of [[maybe_unused]] allows us to do so without compilation warnings/errors.

Question #1

What is the difference between initialization and assignment?

Show Solution

Initialization gives a variable an initial value at the point when it is created. Assignment gives a variable a value at some point after the variable is created.

Question #2

What form of initialization should you prefer when you want to initialize a variable with a specific value?

Direct list initialization (aka. direct brace initialization).

Question #3

What are default initialization and value initialization? What is the behavior of each? Which should you prefer?

Default initialization is when a variable initialization has no initializer (e.g. int x; ). In most cases, the variable is left with an indeterminate value.

Value initialization is when a variable initialization has an empty brace (e.g. int x{}; ). In most cases this will perform zero-initialization.

You should prefer value initialization to default initialization.

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C Language: Integer Variables

This C tutorial explains how to declare and use integer variables with syntax and examples.

Description

There are the following integer types available in the C Language:

• unsigned short int
• unsigned int
• unsigned long int

For the purposes of this tutorial, we will focus on the basic int type.

The syntax for declaring integer variables is:

Or the syntax for declaring multiple integer variables is:

Parameters or Arguments

To declare an integer variable with a type other than int, simply replace the int keyword in the syntax above with your selected type.

For example:

Example - Declaring a variable

Let's look at an example of how to declare an integer variable in the C language.

In this example, the variable named age would be defined as an int.

Below is an example C program where we declare this variable:

This C program would print "TechOnTheNet.com is over 10 years old."

Example - Declaring a variable and assigning a value

You can define a variable as an integer and assign a value to it in a single declaration.

In this example, the variable named age would be defined as an integer and assigned the value of 10.

Below is an example C program where we declare this variable and assign the value:

Example - Declaring multiple variables in a statement

If your variables are the same type, you can define multiple variables in one declaration statement.

In this example, two variables called age and reach would be defined as integers.

Below is an example C program where we declare these two variables:

This C program would print "TechOnTheNet.com is over 10 years old and reaches over 100 countries."

Example - Declaring multiple variables in a statement and assigning values

If your variables are the same type, you can define multiple variables in one declaration statement. You can also assign the variables a value in the declaration statement.

In this example, two variables called age and reach would be defined as integers and be assigned the values 10 and 100, respectively.

Below is an example C program where we declare these two variables and assign their values:

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C++ 17 | New ways to Assign values to Variables

• Assigning function to a variable in C++
• Different Ways to Initialize a Variable in C++
• Inline Variables in C++ 17
• How to Store Vectors as Values in a Map?
• Scope of Variables in C++
• Can Global Variables be dangerous ?
• Swap Two Numbers Without Third Variable in C++
• How to Access Global Variable if there is a Local Variable with Same Name in C/ C++?
• Assigning Values to Variables
• Assigning values to variables in R programming - assign() Function
• How to Set Variable to Cell Value in Excel VBA?
• What are the default values of static variables in C?
• How to use Variables in Python3?
• Assigning multiple variables in one line in Python
• Variables under the hood in Python
• Different ways to declare variable as constant in C
• C# | Types of Variables
• How to print a variable name in C?
• C | Variable Declaration and Scope | Question 2

C++ 17 introduced many new ways to declare a variable. Earlier assignment and declaration was done using “=”

But now 2 more ways are introduced in C++17 . They are:

Explanation In C++ 17, the variable can be declared by providing values through parenthesis too. The difference between constructor initialization and the old normal way of initialization is that it will always return last value in the parenthesis no matter what it’s magnitude or sign is.

For example,

The value stored in a would be 1.

Unlike Constructor initialization, this assigning method can only take one value in the braces. Providing multiple values would return a compile error.

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6.1. Multiple assignment ¶

I haven’t said much about it, but it is legal in C++ to make more than one assignment to the same variable. The effect of the second assignment is to replace the old value of the variable with a new value.

The active code below reassigns fred from 5 to 7 and prints both values out.

The output of this program is 57 , because the first time we print fred his value is 5, and the second time his value is 7.

The active code below reassigns fred from 5 to 7 without printing out the initial value.

However, if we do not print fred the first time, the output is only 7 because the value of fred is just 7 when it is printed.

This kind of multiple assignment is the reason I described variables as a container for values. When you assign a value to a variable, you change the contents of the container, as shown in the figure:

When there are multiple assignments to a variable, it is especially important to distinguish between an assignment statement and a statement of equality. Because C++ uses the = symbol for assignment, it is tempting to interpret a statement like a = b as a statement of equality. It is not!

An assignment statement uses a single = symbol. For example, x = 3 assigns the value of 3 to the variable x . On the other hand, an equality statement uses two = symbols. For example, x == 3 is a boolean that evaluates to true if x is equal to 3 and evaluates to false otherwise.

First of all, equality is commutative, and assignment is not. For example, in mathematics if \(a = 7\) then \(7 = a\) . But in C++ the statement a = 7; is legal, and 7 = a; is not.

Furthermore, in mathematics, a statement of equality is true for all time. If \(a = b\) now, then \(a\) will always equal \(b\) . In C++, an assignment statement can make two variables equal, but they don’t have to stay that way!

The third line changes the value of a but it does not change the value of b , and so they are no longer equal. In many programming languages an alternate symbol is used for assignment, such as <- or := , in order to avoid confusion.

Although multiple assignment is frequently useful, you should use it with caution. If the values of variables are changing constantly in different parts of the program, it can make the code difficult to read and debug.

• Checking if a is equal to b
• Assigning a to the value of b
• Setting the value of a to 4

Q-4: What will print?

• There are no spaces between the numbers.
• Remember, in C++ spaces must be printed.
• Carefully look at the values being assigned.

Q-5: What is the correct output?

• Remember that printing a boolean results in either 0 or 1.
• Is x equal to y?
• x is equal to y, so the output is 1.

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Assignment (=)

The assignment ( = ) operator is used to assign a value to a variable or property. The assignment expression itself has a value, which is the assigned value. This allows multiple assignments to be chained in order to assign a single value to multiple variables.

A valid assignment target, including an identifier or a property accessor . It can also be a destructuring assignment pattern .

An expression specifying the value to be assigned to x .

Return value

The value of y .

Thrown in strict mode if assigning to an identifier that is not declared in the scope.

Thrown in strict mode if assigning to a property that is not modifiable .

Description

The assignment operator is completely different from the equals ( = ) sign used as syntactic separators in other locations, which include:

• Initializers of var , let , and const declarations
• Default values of destructuring
• Default parameters
• Initializers of class fields

All these places accept an assignment expression on the right-hand side of the = , so if you have multiple equals signs chained together:

This is equivalent to:

Which means y must be a pre-existing variable, and x is a newly declared const variable. y is assigned the value 5 , and x is initialized with the value of the y = 5 expression, which is also 5 . If y is not a pre-existing variable, a global variable y is implicitly created in non-strict mode , or a ReferenceError is thrown in strict mode. To declare two variables within the same declaration, use:

Simple assignment and chaining

Value of assignment expressions.

The assignment expression itself evaluates to the value of the right-hand side, so you can log the value and assign to a variable at the same time.

Unqualified identifier assignment

The global object sits at the top of the scope chain. When attempting to resolve a name to a value, the scope chain is searched. This means that properties on the global object are conveniently visible from every scope, without having to qualify the names with globalThis. or window. or global. .

Because the global object has a String property ( Object.hasOwn(globalThis, "String") ), you can use the following code:

So the global object will ultimately be searched for unqualified identifiers. You don't have to type globalThis.String ; you can just type the unqualified String . To make this feature more conceptually consistent, assignment to unqualified identifiers will assume you want to create a property with that name on the global object (with globalThis. omitted), if there is no variable of the same name declared in the scope chain.

In strict mode , assignment to an unqualified identifier in strict mode will result in a ReferenceError , to avoid the accidental creation of properties on the global object.

Note that the implication of the above is that, contrary to popular misinformation, JavaScript does not have implicit or undeclared variables. It just conflates the global object with the global scope and allows omitting the global object qualifier during property creation.

Assignment with destructuring

The left-hand side of can also be an assignment pattern. This allows assigning to multiple variables at once.

For more information, see Destructuring assignment .

Specifications

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• Assignment operators in the JS guide
• Destructuring assignment

Multiple assignment in Python: Assign multiple values or the same value to multiple variables

In Python, the = operator is used to assign values to variables.

You can assign values to multiple variables in one line.

Assign multiple values to multiple variables

Assign the same value to multiple variables.

You can assign multiple values to multiple variables by separating them with commas , .

You can assign values to more than three variables, and it is also possible to assign values of different data types to those variables.

When only one variable is on the left side, values on the right side are assigned as a tuple to that variable.

If the number of variables on the left does not match the number of values on the right, a ValueError occurs. You can assign the remaining values as a list by prefixing the variable name with * .

For more information on using * and assigning elements of a tuple and list to multiple variables, see the following article.

• Unpack a tuple and list in Python

You can also swap the values of multiple variables in the same way. See the following article for details:

• Swap values ​​in a list or values of variables in Python

You can assign the same value to multiple variables by using = consecutively.

For example, this is useful when initializing multiple variables with the same value.

After assigning the same value, you can assign a different value to one of these variables. As described later, be cautious when assigning mutable objects such as list and dict .

You can apply the same method when assigning the same value to three or more variables.

Be careful when assigning mutable objects such as list and dict .

If you use = consecutively, the same object is assigned to all variables. Therefore, if you change the value of an element or add a new element in one variable, the changes will be reflected in the others as well.

If you want to handle mutable objects separately, you need to assign them individually.

after c = []; d = [] , c and d are guaranteed to refer to two different, unique, newly created empty lists. (Note that c = d = [] assigns the same object to both c and d .) 3. Data model — Python 3.11.3 documentation

You can also use copy() or deepcopy() from the copy module to make shallow and deep copies. See the following article.

• Shallow and deep copy in Python: copy(), deepcopy()

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C/C++ and MPI artial pivoting technique

In Assignment 2, you need to tackle the programming tasks as described below using C/C++  and MPI. You also need to write a report on your work.  Gaussian elimination with partial pivoting is a technique used to solve a system of linear  equations. It adds multiples of each row to later rows to transform the matrix into triangular  matrices L and U. Partial pivoting technique is applied to make the algorithm stable; that is, in  each elimination iteration, rows are swapped so that the leading element A(i,i) is the largest in  the leading column to prevent division by zero. A serial pseudocode is given below.  // Gaussian elimination with partial pivoting   for (i=0; i<n-1; i++)   //find and record k where |A(k,i)|=A(j,i)|   if (|A(k,i)|== 0)   //exit with a warning that A is singular, or nearly so   else if (k != i)   //swap row i and row k of A   //store multiplier in place of A(j,i)   for (j=i+1; j<n; j++)   A[j][i] = A[j][i] / A[i][i]; //now each |quotient|≤ 1   //subtract multiple of row A(i,:) to zero out A(j,i)   for (j=i+1; j<n; j++)   for (k=i+1; k<n; k++)   A[j][k] −= A[j][i] * A[i][k];  In distributed memory machines data need to be partitioned and distributed to processes. In this  assignment you need to use Column block cyclic partitioning for data distribution.  In your implementation:  • Processes are organized as a one dimensional, or 1D array;  • Column block cyclic partitioning must be adopted for the data distribution;  • You need to write two MPI programs: one without loop unrolling, and the other  with loop unrolling added to improve the performance:  o In the program without loop unrolling the block size b in column block cyclic  partitioning must be a variable to handle different block sizes asked by the user.  Then your program needs to ask for matrix size N and block size b as two userdefined  input parameters;  o In the program with loop unrolling, the block size b in column block cyclic  partitioning is fixed and set to 8, and the loop unrolling factor is set to 4 to  simplify the implementation. Then your program needs to ask for matrix size N  as one user-defined input parameter.  • You must submit both programs! They will be marked separately.  • You must use derived datatypes for process communication for the purposes of  efficiency and readability;  • Your main programs must conduct a self-check for correctness; i.e., compare the  results of a basic Gaussian elimination with partial pivoting as done in gepp_0.c and  your parallel computing procedure with the same input data set. Your programs must be able to compile and run on Microsoft Azure Virtual Machines.  Report: You must write a report. The report should be concise, clear (3-6 A4 pages) and  contain the following sections:  1. Problem definition and requirements  2. Algorithm design and implementation  3. Testing  4. Discussion  5. Known issues in you program  6. Manual (e.g. how to compile and run the program, input and output)  Your assignments will be marked on correctness of results, the efficiency of your algorithm,  program logic and readability, and quality of your report.  You MUST attempt this assignment individually.  AI use policy: You are permitted to use Artificial Intelligence (AI) tools to help you develop  your programs and write your report. You are required to complete and submit the  “Acknowledgement of AI Use Form”, even if you do not use AI tools at all. You will not lose  marks for using AI tools as long as you correctly acknowledge your use of them.  Submission Requirements  1. Your submission must be made by 11:59pm on Friday, 24 May, 2024 (Sydney time).  2. Create a tar or zip file that contains your makefile and source files (e.g., .c and .h files)  and “Acknowledgement of AI Use Form”. DO NOT INCLUDE ANY OBJECT OR  BINARY FILES.  • Submit only one .tar or .zip file to the “Assignment 2 code submission inbox”.  3. Save your report in pdf format.  • Submit your report to the “Assignment 2 report submission inbox”.   WX：codinghelp

Proposal: Annotate types in multiple assignment

In the latest version of Python (3.12.3), type annotation for single variable assignment is available:

However, in some scenarios like when we want to annotate the tuple of variables in return, the syntax of type annotation is invalid:

In this case, I propose two new syntaxes to support this feature:

• Annotate directly after each variable:
• Annotate the tuple of return:

In other programming languages, as I know, Julia and Rust support this feature in there approaches:

I’m pretty sure this has already been suggested. Did you go through the mailing list and searched for topics here? Without doing that, there’s nothing to discuss here. (Besides linking to them).

Secondly, try to not edit posts, but post a followup. Some people read these topics in mailing list mode and don’t see your edits.

• https://mail.python.org
• https://mail.python.org/archives

For reference, PEP 526 has a note about this in the “Rejected/Postponed Proposals” section:

Allow type annotations for tuple unpacking: This causes ambiguity: it’s not clear what this statement means: x, y: T Are x and y both of type T , or do we expect T to be a tuple type of two items that are distributed over x and y , or perhaps x has type Any and y has type T ? (The latter is what this would mean if this occurred in a function signature.) Rather than leave the (human) reader guessing, we forbid this, at least for now.

Personally I think the meaning of this is rather clear, especially when combined with an assignment, and I would like to see this.

Thank you for your valuable response, both regarding the discussion convention for Python development and the history of this feature.

I have found a related topic here: https://mail.python.org/archives/list/[email protected]/thread/5NZNHBDWK6EP67HSK4VNDTZNIVUOXMRS/

Here’s the part I find unconvincing:

Under what circumstances will fun() be hard to annotate, but a, b will be easy?

It’s better to annotate function arguments and return values, not variables. The preferred scenario is that fun() has a well-defined return type, and the type of a, b can be inferred (there is no reason to annotate it). This idea is presupposing there are cases where that’s difficult, but I’d like to see some examples where that applies.

Does this not work?

You don’t need from __future__ as of… 3.9, I think?

3.10 if you want A | B too: PEP 604 , although I’m not sure which version the OP is using and 3.9 hasn’t reached end of life yet.

We can’t always infer it, so annotating a variable is sometimes necessary or useful. But if the function’s return type is annotated then a, b = fun() allows type-checkers to infer the types of a and b . This stuff isn’t built in to Python and is evolving as typing changes, so what was inferred in the past might be better in the future.

So my question above was: are there any scenarios where annotating the function is difficult, but annotating the results would be easy? That seems like the motivating use case.

Would it be a solution to put it on the line above? And not allow assigning on the same line? Then it better mirrors function definitions.

It’s a long thread, so it might have been suggested already.

Actually, in cases where the called function differs from the user-defined function, we should declare the types when assignment unpacking.

Here is a simplified MWE:

NOTE: In PyTorch, the __call__ function is internally wrapped from forward .

Can’t you write this? That’s shorter than writing the type annotations.

This is the kind of example I was asking for, thanks. Is the problem that typing tools don’t trace the return type through the call because the wrapping isn’t in python?

I still suggest to read the thread you linked, like I’m doing right now.

The __call__ function is not the same as forward . There might be many other preprocessing and postprocessing steps involved inside it.

Yeah, quite a bit of pre-processing in fact… unless you don’t have hooks by the looks of it:

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