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For loop

The first type of loop that we are going to learn is the "for loop".

This is probably the most common and well-known type of loop, for reasons that will become clear in a moment.

First, let's see its syntax:

for (initialization; condition; update) {
    // code to be repeatedly executed
}

Explanation​

  • The initialization statement sets an initial value for the counter variable. This happens only at the beginning of the loop.
  • Before every iteration, including the first one, the condition of the for loop is checked and the iterations continue until the condition is evaluated to false. In that case, the loop terminates
  • At every iteration, after the successful execution of statements inside the body of the loop, the counter is updated with a specified increment (or decrement).

For example, this is a loop that iterates 10 times with a counter starting from 0 and an increment of 1 for each iteration (the value of i is printed to console):

for (int i=0; i<10; i++) { // ++i is fine too
    cout << i << endl; // executed 10 times
}
Output
0
1
2
3
4
5
6
7
8
9
Note

The letter i (and the subsequent j, k and so on if needed) is conventionally used as the identifier (name) of the loop counter. It typically stands for "index" or "iterator" (or maybe "integer", since it's the obvious choice for a loop counter), but you can use any letter you want.

For loops are used to repeat a section of code a known number of times, so they are especially useful when the number of iterations is known ahead of time. They let us clearly define and change the values of loop variables and integrate a counter directly, unlike the other kinds of loops (while and do-while).

The incrementation can also be bigger than one or negative. If you decrease the counter, you must be cautious about the condition for exiting the loop. See this example:

for (int i=20; i>0; i-=2) { // or i=i-2
    cout << i << endl; // executed 10 times
}
Output
20
18
16
14
12
10
8
6
4
2

Off-by-one error​

A frequently occurring logic error for programmers is the "off-by-one error", in which the number of iterations of a loop is stated improperly, being either 1 less or 1 more than we would expect. In other words, there's an error in the initial value of the loop variable or in the end condition of the loop.

// loop n times
for (int i=1; i<n; i++) { ... } // ❌ n-1 times
for (int i=0; i<=n; i++) { ... } // ❌ n+1 times
for (int i=0; i<n; i++) { ... } // ✔️ n times

Nested loops​

C and C++ allow to use one loop inside another loop. This is known as "nesting" and is more often done with for loops, but while and do-while loops can also be used (you can mix different types of loops too).

You nest a loop inside another in this way:

for (int i=x; i<n; i++) {
    for (int j=y; j<m; j++) {
        // code executed n*m times
    }
    // code executed n times
}
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