If/Else

In real life we make decisions: if it's cold, take a sweater, otherwise a t-shirt is enough. If I'm sleepy, I sleep, otherwise I code

In C we say

#include <stdio.h>

int main(void) {
    int temperature = 10;

    if (temperature < 15) {
        printf("Put on a sweater\n");
    } else {
        printf("A t-shirt is enough\n");
    }

    return 0;
}

A few syntax rules

• The condition goes between parentheses ( )
• The body goes between braces { }
• There is no : at the end

If temperature is less than 15, we enter the if block (it's cold, we bundle up). Otherwise we enter else. Run the code, change the temperature, see what happens

The comparison operators

• < less than
• <= less than or equal
• > greater than
• >= greater than or equal
• == equal (careful, two equal signs)
• != not equal

Be very careful about the difference between = and ==. = assigns, == compares

#include <stdio.h>

int main(void) {
    int x = 4;
    if (x = 4) {
        printf("Boo\n");
    }
    return 0;
}

This is a classic C bug: C accepts this code without any compile error. x = 4 stores 4 in x and returns the value 4, which C treats as "true" (any non-zero value means true). So the code always enters the if, no matter what x was before. Many famous bugs in real-world projects come from exactly this typo. Use == when comparing

We don't always need else. Sometimes we just want to do something if a condition is true, and otherwise do nothing

#include <stdio.h>

int main(void) {
    int user_online = 1;
    if (user_online) {
        printf("Welcome back\n");
    }
    return 0;
}

Wait, user_online = 1? Where is true? Well, C doesn't have a true boolean type by default. It uses whole numbers: 0 means false, anything else (1, 2, -5, ...) means true. We'll see a real bool type in a future lesson, with #include <stdbool.h>

A complete example

#include <stdio.h>

int main(void) {
    int terminal_connected = 1;

    if (terminal_connected) {
        printf("Session open\n");
    } else {
        printf("Terminal disconnected\n");
    }

    return 0;
}

Change terminal_connected to 0 and run again. The result flips. Programming becomes interesting the moment you start playing with values

Mission: Machine Room Thermostat

The computing center's mainframes don't tolerate cold: below zero degrees, the oil in the tape units jams the mechanisms. The thermostat has to warn the shift operator.

Write a program that, inside main

• declares an int named temperature
• uses an if / else: if temperature is less than 0 → print cold alert, otherwise → print temperature normal

Example

For a temperature of -5, your program would print something like

cold alert