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Pass by Value

  • Arguments Pass by Value The values from the calling function are copied to the parameters in the called function at the time the function is called. The called function can change the values of the parameter variables all it wants. This change will not be reflected in the variables passed as arguments in the calling function.


The general syntax of passing arguments by value is:


The following example makes a function square() that takes a number n as a parameter to the function and prints the square of the function within the function.

fn square(mut n:i32){
  n = n * n;
  println!("The value of n inside function : {}", n);
fn main() {
  let n = 4;
  println!("The value of n before function call : {}", n);
  println!("Invoke Function");
  println!("\nThe value of n after function call : {}", n);


The value of n before function call : 4
Invoke Function
The value of n inside function : 16

The value of n after function call : 4


The above program is of two parts, the user defined function square() and the driver function main() where the function is being called.

  • User defined function The function ` square() is defined from line 1 to line 4`.

    On line 2 n is multiplied with itself and the value is saved in n. The square of the argument thus calculated is displayed to the screen on line 3.

  • Driver function

The driver function main() is defined from line 5 to line 11.

On line 6, a variable n is defined.
On line 9, the function `square()` is invoked which takes n as an argument to the function.
After the function call, the value of the n is printed

Note: The value of n is not changed

# Quiz

Test your understanding of passing arguments by value as a function parameter.

  1. What is the output of the following code?
fn swap( x:i32, y:i32 ) {
  let temp = y;
  let y = x;
  let x = temp;
  println!("x : {}, y : {}", x , y);
fn main() {
  let x = 10;
  let y = 9;
  swap( x, y );
  println!("x : {}, y : {}", x , y);


x : 9, y : 10
x : 10, y : 9


x : 9, y : 10
x : 9, y : 10