C Foundations Lesson 4 of 7

if, for and while in C

25 min, 4 exercises

Builds on lessons 1 to 3. Start at lesson 1

Learn

Conditions are numbers

Programs get interesting when they can choose and repeat.

Choosing uses if, else if and else. The condition goes in parentheses:

if (score >= 90) {
    printf("A\n");
} else if (score >= 50) {
    printf("pass\n");
} else {
    printf("try again\n");
}

Early C had no boolean type, and a condition is still just a number. 0 means false, anything else means true. Comparisons like x > 3 produce the int 1 or 0. (<stdbool.h> gives you bool, true and false as friendlier names.)

That is why this classic bug compiles happily:

if (level = 5)   // stores 5 in level; 5 is nonzero, so always true

You wanted ==. Some compilers warn about this one; this one does not, so learn to spot it by eye.

Learn

Loops and remainders

Repeating uses loops. A for loop packs start, condition and step into one line:

for (int i = 1; i <= 5; i++) {
    printf("%d\n", i);
}

A while loop keeps going while its condition is true, which suits "I do not know how many times yet" situations. break leaves a loop early, continue skips to the next round.

In memory, i is one int: 4 bytes. Each round writes the new value into the same 4 bytes, and the old value is gone. A loop never keeps a list of past values unless you store them yourself.

The % operator gives the remainder of a division: 10 % 3 is 1. If n % 2 == 0, n is even. You will use this constantly.

Some programs here read numbers that are typed in. scanf("%d", &n); reads one whole number into the int called n; call it again for the next number. Write it exactly like that for now, with the &: the pointers lesson explains what the & does.

Try

Run it: a running total

The example is already in the editor. Run it, change one thing, and run it again.

example.c to run
#include <stdio.h>

int main(void) {
    int total = 0;
    int n = 1;
    while (total < 20) {
        total = total + n;
        printf("added %d, total is %d\n", n, total);
        n++;
    }
    if (total % 2 == 0) {
        printf("%d is even\n", total);
    } else {
        printf("%d is odd\n", total);
    }
    return 0;
}

Practice

FizzBuzz to fifteen

For each number from 1 to 15, print Fizz if it divides by 3, Buzz if it divides by 5, FizzBuzz if it divides by both, and otherwise the number itself. One per line.

exercise_1.c to run
#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 15; i++) {
        printf("%d\n", i);
    }
    return 0;
}

Practice

Count down from input

The program reads a number n with scanf. Print a countdown from n to 1, one number per line, then print Liftoff!. With the input below, the output is:

  • 5
  • 4
  • 3
  • 2
  • 1
  • Liftoff!

Your program reads the Input box under the editor.

exercise_2.c to run
#include <stdio.h>

int main(void) {
    int n;
    scanf("%d", &n);
    // count down here
    return 0;
}

Break/fixFix the bug

Everyone gets gold

The program reads three race scores and should only give gold for a perfect 100. Everyone gets gold, and the scores print wrong too. Fix it. With the input below it should print:

  • 70: keep going
  • 100: gold
  • 85: keep going

Your program reads the Input box under the editor.

exercise_3.c to run
#include <stdio.h>

int main(void) {
    int s;
    for (int i = 0; i < 3; i++) {
        scanf("%d", &s);
        if (s = 100) {
            printf("%d: gold\n", s);
        } else {
            printf("%d: keep going\n", s);
        }
    }
    return 0;
}

Your turn

Longest practice streak

A practice app logs the minutes you played guitar each day. The input is a count n, then n daily totals. A day counts towards a streak if it has at least 30 minutes. Print the longest run of counting days in a row as longest streak N days.

Milestones

  1. Streak in the middle
  2. Every session under target
  3. Streak at the end

On your own. Hints open after your first check.

Your program reads the Input box under the editor. Check also tries 2 other inputs.

exercise_4.c to run
#include <stdio.h>
C, 32-bit: int and pointers are 4 bytes