if, else if, else: Choosing a Path

In Module 4, Amara wrote the leap year rule as one expression, and it printed 1 or 0. Her calendar app wanted a sentence instead: "2024 has 366 days".

A 1 on the screen is not a sentence. To print one of two sentences, the program has to choose which line runs.

Every program so far ran every line, top to bottom, every time. Today the road forks.

if: a block that runs only when the answer is not zero

An if statement asks a question. It runs a block of code only when the answer is true. In C, "true" means "not zero", the Module 2 rule.

if (condition) {
    statements
}

#include <stdio.h>

int main(void)
{
    int stock = 7;

    if (stock < 10) {
        printf("reorder\n");
    }
    printf("stock checked\n");
    return 0;
}

reorder
stock checked

Change the 7 to 12 and only the second line prints. The printf inside the braces is skipped; the one after them is not.

So an if does not end the program or choose an exit. It decides whether one block runs, and then the program continues.

else: exactly one of two paths

An else gives the false answer a block of its own. Exactly one of the two blocks runs: never both, and never neither.

#include <stdio.h>

int main(void)
{
    int year = 2024;
    int leap = (year % 4 == 0 && year % 100 != 0) || year % 400 == 0;

    if (leap) {
        printf("%d has 366 days\n", year);
    } else {
        printf("%d has 365 days\n", year);
    }
    return 0;
}

2024 has 366 days

The expression is Amara's from Module 4, unchanged. What is new is that its 1 or 0 now decides which printf runs.

So the leap year rule did not change at all. It stopped being an answer to print and became an answer to act on.

The else if ladder: the first true condition wins

Most real questions have more than two answers. C chains them with else if, and the chain is usually called a ladder.

if (condition_1) {
    runs when condition_1 is true
} else if (condition_2) {
    runs when condition_1 was false and condition_2 is true
} else {
    runs when every condition above was false
}

#include <stdio.h>

int main(void)
{
    int marks = 72;

    if (marks >= 80) {
        printf("distinction\n");
    } else if (marks >= 60) {
        printf("merit\n");
    } else if (marks >= 40) {
        printf("pass\n");
    } else {
        printf("fail\n");
    }
    return 0;
}

merit

A trace table is the tool for reading a ladder. One row per input, and a column for every test C actually evaluated.

Read the middle column for 72. The test >= 40 never ran. The ladder stopped at the first true answer.

Now the ordering rule. Bob, who always rushes, writes the same ladder starting from the easiest test.

#include <stdio.h>

int main(void)
{
    int marks = 95;

    if (marks >= 40) {
        printf("pass\n");
    } else if (marks >= 60) {
        printf("merit\n");
    } else if (marks >= 80) {
        printf("distinction\n");
    } else {
        printf("fail\n");
    }
    return 0;
}

pass

Trace 95 through it. The first test, >= 40, is true, so the ladder stops there. The same happens for 62 and for 100.

Every mark from 40 up passes the first test, so the merit and distinction blocks can never run. The compiler says nothing; it cannot know what Bob meant.

So the rule for any ladder is this. When one condition's range contains another's, the smaller range must come first. Here, "80 and above" sits inside "40 and above", so it goes higher up.

Any int is a condition, and 0 is false

The brackets after if do not need a comparison. Any expression with a number works, and only zero means no.

#include <stdio.h>

int main(void)
{
    int stock = 0;
    int n = -7;

    if (stock) {
        printf("in stock\n");
    } else {
        printf("sold out\n");
    }

    if (n % 2) {
        printf("%d is odd\n", n);
    }
    return 0;
}

sold out
-7 is odd

if (stock) means exactly if (stock != 0). The short form reads well for a count that answers "is there any".

The second test is subtler. -7 % 2 is -1, the Module 4 sign rule, and -1 is not zero. So the test is true for negative odd numbers too, where n % 2 == 1 would have been false.

So a condition is not a special kind of value. It is a number, and only zero says no.

One equals sign: the bug that compiles

= stores a value. == asks a question. Inside a condition, the first one is almost always a typing slip.

#include <stdio.h>

int main(void)
{
    int marks = 35;

    if (marks = 100) {
        printf("full marks\n");
    }
    printf("marks is now %d\n", marks);
    return 0;
}

full marks
marks is now 100

Two things went wrong at once. The assignment's value is 100, which is not zero, so the test is always true. And the student's real mark has been overwritten.

The Playground compiles this in silence, because it has no -Wall. A local gcc -Wall on GCC 12 says warning: suggest parentheses around assignment used as truth value [-Wparentheses].

So when an if is true for every input you try, look for a lone = first.

Braces on every body, and the bug that taught everyone why

C allows an if without braces when its body is a single statement. This track never uses that permission, and here is the reason.

In February 2014, Apple shipped a fix for its TLS code on iOS and macOS, logged as CVE-2014-1266. A function that verified a server's signature contained two identical goto fail; lines under an if with no braces. Only the first line belonged to the if. The second ran every time, jumped past the real check, and returned success. People called it "goto fail".

You can build the same trap with nothing but printf.

#include <stdio.h>

int main(void)
{
    int marks = 35;

    if (marks >= 40)
        printf("pass\n");
        printf("certificate printed\n");
    return 0;
}

certificate printed

The indentation says both lines belong to the if. The compiler ignores indentation, so only the first one does.

The Playground is silent again. A local gcc -Wall says warning: this 'if' clause does not guard... [-Wmisleading-indentation]. With braces, the question never arises.

So braces are not decoration. They are the only thing that tells C where a body ends.

#include <stdio.h>

int main(void)
{
    int celsius = 0;
    scanf("%d", &celsius);

    if (celsius < 0) {
        printf("freezing\n");
    }
    return 0;
}

freezing

#include <stdio.h>

int main(void)
{
    int n = 0;
    scanf("%d", &n);

    if (n % 2 == 0) {
        printf("%d is even\n", n);
    } else {
        printf("%d is odd\n", n);
    }
    return 0;
}

-7 is odd

#include <stdio.h>

int main(void)
{
    int age = 0;
    scanf("%d", &age);

    if (age < 0) {
        printf("invalid age\n");
    } else if (age < 3) {
        printf("ticket 0\n");
    } else if (age < 12) {
        printf("ticket 150\n");
    } else if (age >= 60) {
        printf("ticket 200\n");
    } else {
        printf("ticket 300\n");
    }
    return 0;
}

ticket 200

if (marks = 100) {
    printf("full marks\n");
}

if (marks >= 40);
{
    printf("pass\n");
}

if (marks =< 40) {
    printf("fail\n");
}

if (marks >= 40) {
    printf("pass\n");
} else if (marks >= 80) {
    printf("distinction\n");
}

#include <stdio.h>

int main(void)
{
    int x = -1;
    int y = 5;

    if (x > 0)
        if (y > 0)
            printf("a\n");
    else
        printf("b\n");

    printf("done\n");
    return 0;
}

#include <stdio.h>

int main(void)
{
    int marks = 0;
    scanf("%d", &marks);

    /* One guard for the impossible marks, then one rung per band. */

    return 0;
}

#include <stdio.h>

int main(void)
{
    int a = 0;
    int b = 0;
    int c = 0;
    scanf("%d %d %d", &a, &b, &c);

    /* Two separate ifs are enough. Equal values must not break it. */

    return 0;
}

#include <stdio.h>

int main(void)
{
    int y = 0;
    scanf("%d", &y);

    /* The Module 4 expression decides; an if else prints the word. */

    return 0;
}

#include <stdio.h>

int main(void)
{
    int a = 0;
    int b = 0;
    int c = 0;
    scanf("%d %d %d", &a, &b, &c);

    /* Is it a triangle at all? Then the narrowest kind first. */

    return 0;
}

Next, an if goes inside another if, and an else loses track of which one it belongs to.