If/Else If/Else

Sometimes we need more than two branches. Picture a mainframe booting up in the morning at the computing center. Depending on how many seconds remain until it's fully up, the system goes through different stages

If 100 seconds remain → check memory

If 60 seconds remain → check peripherals

If 20 seconds remain → load the operating system kernel

If 10 seconds remain → start system processes

Otherwise → do nothing special

In C we chain branches with else if

#include <stdio.h>

int main(void) {
    int seconds = 100;

    if (seconds == 100) {
        printf("Checking memory\n");
    } else if (seconds == 60) {
        printf("Checking peripherals\n");
    } else if (seconds == 20) {
        printf("Loading operating system kernel\n");
    } else if (seconds == 10) {
        printf("Starting system processes\n");
    } else {
        printf("%d seconds have no effect\n", seconds);
    }

    return 0;
}

Let's trace what happens if we change seconds to 60, 20, 10, 42

The chain runs top to bottom. At the first branch that is true, C runs that block and then exits the whole chain. The remaining branches are never checked. So for seconds = 60, only one line is printed, not all of them

Why not just write a bunch of separate ifs? Like this

#include <stdio.h>

int main(void) {
    int seconds = 60;
    if (seconds == 100) { printf("100\n"); }
    if (seconds == 60) { printf("60\n"); }
    if (seconds == 20) { printf("20\n"); }
    else { printf("other\n"); }
    return 0;
}

The problem: each if is independent. The else at the end belongs only to the last if. So for seconds = 60, the third condition fails (60 != 20), and the else would print "other": wrong, we already handled 60 above

Rule of thumb: when testing the same variable against multiple values, chain with if / else if / else

C also has a switch statement that fits this pattern well, but we'll leave it for a more advanced lesson. For now, if / else if / else is enough

We can nest ifs inside each other

#include <stdio.h>

int main(void) {
    int seconds = 5;
    int error_detected = 0;

    if (seconds < 10) {
        if (error_detected) {
            printf("Error detected. Aborting launch\n");
        } else {
            printf("No error detected. Starting the system...\n");
        }
    }
    return 0;
}

Nested ifs are fine, but if you go 5 levels deep, the code becomes unreadable. Try to keep things flat when you can

Mission: Mainframe Boot Sequence

You are the shift operator. The boot timer shows how many seconds remain until full boot, while a separate sensor tells you whether any hardware error was detected.

Write a program that, inside main

• declares an int named seconds
• declares an int named error_detected
• uses an if / else if / else chain:
  • if seconds is 100 → print Checking memory
  • otherwise if seconds is 60 → print Checking peripherals
  • otherwise if seconds is 30 → print Loading operating system kernel
  • otherwise if seconds is 10 → print Starting system processes
  • otherwise if seconds is less than 10 → check error_detected: if it is 1, print Error detected. Aborting launch, otherwise print No error detected. Starting the system...
  • otherwise → print %d seconds have no effect (with the number of seconds in place of %d)

Example

For a seconds of 5 and an error_detected of 1, your program would print something like

Error detected. Aborting launch