freeCodeCamp/guide/english/cplusplus/for-loop/index.md

---
title: For Loop
---

A `for` loop is a repetitive statement that is used to check for some condition and then, based upon the condition a block of code, is executed repeatedly until the specified condition is satisfied.

The `for` loop is distinguished from other looping statements through an explicit loop counter or loop variable which allows the body of the loop to know the exact sequencing of each iteration.

Hence a `for` loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.

For loop is an entry controlled loop unlike do-while loop.

## Syntax

```
for (init; condition; increment ) 
{
   update_statement(s);
}
```

It is allowed to place the increment inside the for loop like in a while loop. Meaning a syntax like this can also work.

```
for ( init; condition;) 
{
   update_statement(s);
   increment;
}
```

It is also allowed to ignore the init variables. For example:
```
int a = 1;
for (; a <= 10 ;)
{
    cout << a << '\n';
    a++;
}
```

However, you can't pass zero arguments, since empty condition is defaulted as false, and so the `for` loop won't run at all.

### init
This step allows you to declare and initialize any loop control variables. This step is performed first and only once.  
Note that the variables declared in init can only be used inside the brackets of the For Loop.

### condition

Next the condition is evaluated. If it holds true, the body of the loop is executed. If it holds false, the body of the loop does not execute and flow of control jumps outside the loop.

### increment
The increment statement is used to alter the loop variable by using simple increment operations and executes after the completion of the body of the loop.

### update
The update statement is used to alter the loop variable by using simple operations like addition, subtraction, multiplication or division and executes after the execution of the body of the loop.

You will often see an increment operation as the update statement (e.g. i++, count++). This is often seen as one of the distinguishing features and possible name sources for the C++ language.

## IMPLEMENTATION:

```C++
#include <iostream>
using std::cout; // Here we use the scope resolution operator to define the scope of the standard functions as std
using std::endl;

int main () {
   // for loop execution
   for( int a = 10; a < 20; a++ ) { // The loop will run till the value of a is less than 20
      cout << "value of a: " << a << endl;
   }

   return 0;
}
```

Output:
```output
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19
```

## Single lined loop
The body of the for loop need not be enclosed in braces if the loop iterates over only one statement.

### Example
```c++
   #include<iostream.h>
   using std::cout;
   
   int main () {
   // Single line for loop
   for( int a = 10; a < 20; a++ ) 
      cout << "value of a: " << a << endl;
   

   return 0;
}
```

This would generate the same output as the previous program.

Output:
```output
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19
```

### Explanation
Here, the initialization condition is first set to `a=10`. The loop first checks for this condition. It then checks for the condition expression i.e. `a < 20` which holds true as `10 < 20` (for the first case). Now the body of the loop is executed and we get the output "Value of a: 10". Then the update expression is executed which adds the number 1 to 'a' and the value of 'a' gets updated to 11 and the same steps are followed (as above) until the value of v reaches less than 20 i.e 19.

## Range-based for-loop
C++ also has what we call "range-based" `for` loops which iterate through all the elements of a container (e.g., array).

### Syntax

```c++
for ( element: container )
   statement(s);
}
```

```c++
int[5] array = { 1, 2, 3, 4, 5 }
for ( int i: array )
   cout << i << endl;
}
```

Output:
```output
1
2
3
4
5
```

## Applications of the for loops

### Use as infinite loops

This C-style for-loop is commonly the source of an infinite loop since the fundamental steps of iteration are completely in the control of the programmer. In fact, when infinite loops are intended, this type of for-loop can be used (with empty expressions), such as:
```
for (;;)
   //loop body
```

## Additional Resources
- [Range for-loop](https://guide.freecodecamp.org/cplusplus/range-for-loop)
- [More on For Loops](http://www.cplusplus.com/doc/tutorial/control/)