Lesson 4

Loops

Short answer: a loop repeats a block until a condition says stop. Use for when you know how many times; use while when you don't.

for: three parts, one line

for (start; condition; step) reads as: set up, test before each pass, move forward after each pass.

#include <iostream>
using namespace std;

int main() {
    int n;
    cin >> n;

    int sum = 0;
    for (int i = 1; i <= n; i++) {
        sum += i;
    }
    cout << "sum = " << sum << endl;
    return 0;
}

For n = 5 that is 1+2+3+4+5 = 15. Change n in the input box. This exact shape is Sum from 1 to N.

while: when the count is unknown

Chewing digits off a number is the perfect while: you cannot know in advance how many digits there are.

#include <iostream>
using namespace std;

int main() {
    int n;
    cin >> n;

    int digits = 0;
    while (n > 0) {
        n /= 10;      // chop off the last digit
        digits++;
    }
    cout << "digits = " << digits << endl;
    return 0;
}

That n /= 10 is integer division doing the chopping — the same rule from lesson 2.

Nested loops: rows and columns

#include <iostream>
using namespace std;

int main() {
    for (int row = 1; row <= 3; row++) {
        for (int col = 1; col <= 4; col++) {
            cout << "(" << row << "," << col << ") ";
        }
        cout << endl;
    }
    return 0;
}

The inner loop finishes completely for each single pass of the outer loop. This is the machine behind Star Pyramid and Multiplication Table.

The three bugs

• Off by one. i < n vs i <= n. Trace one tiny case by hand (n = 1) before trusting it.
• The stray semicolon. for (...) ; — that ; is an empty body; the loop spins and does nothing else.
• Infinite loop. If nothing inside moves the condition toward false, it never ends. On this site a runaway loop is killed after 3 seconds, but the habit matters on a real machine.