for Loop in C++

In C++, for loop is an entry-controlled loop that is used to execute a block of code repeatedly for the given number of times. It is generally preferred over while and do-while loops in case the number of iterations is known beforehand.

Let's take a look at an example:

#include <bits/stdc++.h>
using namespace std;

int main() {

  	// for loop to print "Hi" 5 times
    for (int i = 5; i < 10; i++) {
      	cout << "Hi" << endl;
    }
  
    return 0;
}

Hi
Hi
Hi
Hi
Hi

In the above program, for loop prints the text "Hi" 5 times. It starts with the loop variable i set to 0 and increments i by 1 after each iteration. The loop continues as long as i is less than 5, so it runs for i values 0, 1, 2, 3, and 4, printing "Hi" in each iteration.

for Loop Syntax

The syntax of for loop in C++ is shown below:

for ( initialization; test condition; updation) { 
    // body of for loop
}

where,

• Initialization: Initialize the loop variable to some initial value.
• Test Condition: This specifies the test condition. If the condition evaluates to true, then body of the loop is executed, and loop variable is updated according to update expression. If evaluated false, loop is terminated.
• Update Expression: After executing the loop body, this expression increments/decrements the loop variable by some value.

Note: The loop variable can also be declared in the initialization section but the scope of the loop variables that are declared in the initialization section is limited to the for loop block.

Working of a for Loop in C++

The working of for loop is as shown below:

1. Initialization: Control enters the loop, and initialization is done.
2. Condition Check: The condition is tested.
   1. If true, the flow enters the loop body.
   2. If false, the loop terminates, and control exits the loop.
3. Execution of Body: The statements inside the body of the loop are executed.
4. Update: The loop variable is updated.
5. Repeat: The flow returns to Step 2 (Condition Check) for the next iteration.
6. Exit: Once the condition becomes false, the loop terminates, and control exits the loop.

Flowchart of for Loop

Examples of for Loop

The below examples demonstrate how to use the for loop in a C++ program along with the different possible variations of the loop.

Print Numbers in Reverse Order

#include <iostream>
using namespace std;

int main() {
    
  	// Initial value of number
    int n = 5;
  
  	// Initialization of loop variable
    int i;
    for (i = n; i >= 1; i--)
        cout << i << " ";
    return 0;
}

5 4 3 2 1 

In the above program, the loop variable i is iterated from n to 1 and in each test condition is checked (is i>=1). If true then it prints the value of i followed by a space and decrement i. When the condition is false loop terminates.

You may have noticed that we have not used braces {} in the body of the loop. We can skip braces {} till there is only one statement in the loop.

Print a Square Pattern using Nested Loops

#include <iostream>
using namespace std;

int main() {
  	
    // Outer loop to print each row
    for (int i = 0; i < 4; i++) {

        // Inner loop to print each 
      	// character in each row
        for (int j = 0; j < 4; j++) {
            cout << "*" << " ";
        }
        cout << endl;
    }
  
  	return 0;
}

* * * * 
* * * * 
* * * * 
* * * * 

The above program uses nested for loops to print a 4x4 matrix of asterisks (*). Here, the outer loop (i) iterates over rows and the inner loop (j) iterates over columns. In each iteration, inner loop prints an asterisk, and a space. Also, a new line is added after each row is printed to shift the output to the next line.

Use Multiple Loop Variables in for Loop

#include <iostream>
using namespace std;

int main() {
  
    // Defining two variable
    int m, n;

    // Loop having multiple variable and updations
    for (m = 1, n = 1; m <= 3; m += 1, n += 2) {
        cout << "iteration " << m << endl;
        cout << "m is: " << m << endl;
        cout << "j is: " << n << endl;
    }

    return 0;
}

iteration 1
m is: 1
j is: 1
iteration 2
m is: 2
j is: 3
iteration 3
m is: 3
j is: 5

The above program uses for loop with multiple variables (here m and n). It increments and updates both variables in each iteration and prints their respective values.

Infinite for Loop

When no parameters are given to the for loop, it repeats endlessly due to the lack of input parameters, making it a kind of infinite loop.

#include <iostream>
using namespace std;

int main() {
  	
    // Skip Initialization, test 
    // and update conditions
  	// for infinite for loop
    for (;;) {
        cout << "gfg" << endl;
    }
  
    return 0;
}

Output

gfg
gfg
.
.
.
infinite times

Other Types of for Loop in C++

The above explained for loop is the actual legacy for loop that has been the part of the language since the beginning. But the different versions of for loop were added later in the languages. They are:

1. Range-Based for Loop in C++

C++ range-based for loops execute for loops over a range of values, such as all the elements in a container, in a more readable way than the traditional for loops. It is much simpler as compared to traditional for loop. But the disadvantage of this is that it has limited applications.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    int nums[] = {1, 2, 3, 4, 5};

    // Range-based for loop to print 
    // elements of the an array
    for (int num : nums)
        cout << num << " ";

    return 0;
}

1 2 3 4 5 

In the above code, we use a range-based for loop to print each element of the array, which automatically handles the iteration without requiring explicit variables to update or check conditions, unlike a traditional for loop where you need to manually manage the index and loop condition.

2. for_each Loop in C++

C++ for_each loop is not a loop but an algorithm that mimics the range based for loop. It accepts a function that executes over each of the container elements. This loop is defined in the header file <algorithm> and hence has to be included for the successful operation of this loop.

Example:

#include <bits/stdc++.h>
using namespace std;

void print(int num) {
    cout << num << " ";
}

int main() {
    int nums[] = {1, 2, 3, 4, 5};

    // Using for_each to print 
    // each element of the array
    for_each(begin(nums), end(nums), print);

    return 0;
}

1 2 3 4 5 

In the above code, we initializes an array nums with values {1, 2, 3, 4, 5}. It then uses for_each to apply the print function to each element, printing each number in the array.

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