for Loop in C++

The for loop

A for loop is the go-to loop when you know how many times to repeat, or when counting through a range. It gathers everything about the loop's counter into one tidy line.

The three parts

for (int i = 0; i < 5; i++) {
     //  ↑ init    ↑ cond ↑ update
    cout << i << " ";
}

A working example

#include <iostream>
using namespace std;
int main() {
    int sum = 0;
    for (int i = 1; i <= 5; i++) {
        sum += i;
    }
    cout << "Sum 1 to 5 = " << sum << "\n";
    return 0;
}

Looping over an array

A classic for loop uses the index to walk an array:

int a[] = {10, 20, 30};
for (int i = 0; i < 3; i++) {
    cout << a[i] << " ";
}

Range-based for loop

Since C++11, if you just want each element and not its index, the range-based for loop is cleaner:

int a[] = {10, 20, 30};
for (int x : a) {
    cout << x << " ";
}

Use for (int x : a) to read; use for (int &x : a) with a reference if you want to modify the elements.

for vs while

See the while and do-while lesson for the other loops.

Common mistakes

• Using <= where you meant <, running one extra pass (off-by-one).
• Changing the counter inside the body as well as in the update, confusing the count.
• A stray semicolon: for (...); gives an empty body.
• Reading past the end of an array by looping too far.

Summary

• A for loop is best when the number of repeats is known.
• Its three parts are init, condition and update, all on one line.
• A classic for loop uses an index; great for arrays and ranges.
• The range-based for (for (int x : a)) visits each element cleanly.
• Watch off-by-one errors with < vs <=.