Control flow and loops

In plain English

Control flow decides which lines of a program actually run. An if statement runs a block only when its condition is true, and an else block runs when it is false. A switch statement picks one case out of several by matching a single value, and each case ends with break so the program does not fall through into the next one. Loops repeat a block: a for loop is best when you know how many times to run, and a while loop is best when you keep going until a condition becomes false. break exits a loop immediately, and continue skips the rest of the current pass and starts the next one. An off-by-one error happens when a loop bound is one too small or one too large, so it skips the last item or runs once too many.

Step by step

1. 1Write an if/else that classifies a lesson length.
2. 2Add a switch that matches a specific minute count.
3. 3Count from 0 to 4 with a for loop.
4. 4Write a while loop and skip one value with continue.
5. 5Run the loop bounds by hand to check for an off-by-one.

#include <stdio.h>

int main(void) {
    int minutes = 12;

    if (minutes > 10) {
        printf("long lesson\n");
    } else {
        printf("short lesson\n");
    }

    switch (minutes) {
        case 10:
            printf("ten minutes\n");
            break;
        case 12:
            printf("twelve minutes\n");
            break;
        default:
            printf("another length\n");
    }

    for (int i = 0; i < 5; i++) {
        printf("for %d\n", i);
    }

    int j = 0;
    while (j < 3) {
        if (j == 1) {
            j++;
            continue;
        }
        printf("while %d\n", j);
        j++;
    }

    return 0;
}

Common mistakes

• Forgetting break in a switch case lets execution fall through into the next case by accident.
• Using <= instead of < in a loop that starts at zero runs the body one time too many.
• Placing the increment after continue can skip it, which makes the loop run forever.
• Checking the loop variable after the loop has ended reads a value that may be out of scope.