For Loops

Welcome to one of the most important concepts in programming: the for loop. With it, we can tell the computer to do something many times, automatically

Imagine we want to print all the numbers from 1 to 10

#include <stdio.h>

int main(void) {
    printf("%d\n", 1);
    printf("%d\n", 2);
    printf("%d\n", 3);
    // ... and so on, ten times
    return 0;
}

Tedious. For 1 to 1000 it's impossible. for saves us

#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 10; i++) {
        printf("%d\n", i);
    }
    return 0;
}

Run it. You'll see the numbers from 1 to 10, one per line

The for loop in C has three parts between the parentheses, separated by ;

#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 10; i++) {
        printf("%d\n", i);
    }
    return 0;
}

int i = 1: the starting point. We declare a variable i and set it to 1
i <= 10: the condition. As long as this is true, the loop keeps running
i++: what to do after each iteration. We increase i by 1

So i takes the values 1, 2, 3, ..., 10. When i becomes 11, the condition 11 <= 10 is false and the loop ends

A small detail: in older C (before C99), you couldn't declare int i inside for. You had to declare it beforehand. In modern C (C99 and later, which we use), declaring it inside is fine and idiomatic

We can count by 2s, count down, do anything we want

#include <stdio.h>

int main(void) {
    // counting by 2s
    for (int i = 0; i <= 10; i = i + 2) {
        printf("%d\n", i);
    }

    // counting down
    for (int i = 10; i >= 1; i--) {
        printf("%d\n", i);
    }
    return 0;
}

i-- means i = i - 1

Be careful: if we forget to update i, we get an infinite loop

#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 10; ) {
        printf("%d\n", i);
    }
    return 0;
}

i stays 1 forever, the condition stays true forever, and the program prints 1 until something stops it. The platform stops it after 5 seconds, but on real systems an infinite loop can freeze your computer. Always make sure your condition can become false

Mission: The Magnetic Tape Reels

The computing center's storage room has 101 magnetic tape reels, numbered from 0 to 100, sitting on a long shelf. Most reels just show their number on the label, but reels 10 and 50 have a special label: they hold the shift operators' coffee stash, not data.

Write a for loop from 0 to 100. For each number, if it is 10 or 50, print COFFEE instead of the number. Otherwise print the number itself.

Example

Your program should print