Lesson 5.2. Nested if and indentation

Analyze the following code. Knowing that a>b, what does this line display?

if ( a>b ) if ( b>a ) printf (“OK”); else printf (“KO”);

This example raises two questions:

To what if does the else refer?
How should the code be presented to make it more readable?

Nested if

When if are nested, an else always refers to the last if encountered that has not yet been assigned an else.

In our initial example, we see below that the code displays KO, so the else refers to the last if, i.e. the if ( b>a ) test.

Tester l'exemple en ligne

By the way, note that the compiler alerts us to this ambiguous writing:

main.c:7:40: warning: add explicit braces to avoid dangling else [-Wdangling-else]
  if ( a>b ) if ( b>a ) printf ("OK"); else printf ("KO");

Indenting

If you have been following this course since the beginning, you probably noticed that the source codes are always presented in the same way. After the opening curly braces, the code is shifted to the right:

if (var > 1) {
    printf ("Message");
    if (var > 2) {
        printf ("Another message");
        if (var > 3) {
            printf ("One more message");
            if (var > 4) {
                ...

This type of code presentation is called indenting. The rule is simple:

With each opening brace, the code is shifted one level to the right.
With each closing brace, the code is shifted one level to the left.

If the curly brace is omitted because there is only one statement in the block, this instruction is still indented. Let's go back to the example in the introduction, this code layout is much more understandable:

if ( a>b ) {

    if ( b>a ) 
        printf ("OK"); 
    else 
        printf ("KO");
}

If indentation does not change the execution of the code, it is essential to structure the code; primarily for you, but also for others. Some IDEs offer automatic indentation. On Qt Creator, select your code (ctrl-a to select all), then ctrl-i to automatically indent the selection.

Tester l'exemple en ligne

Tabulation or spaces?

To indent a code, you can use the tab key or put spaces, usually 2 or 4. Whichever you choose, it will not change the execution of the of the program. The files are smaller with tabs, but with spaces the display of the code will be more consistent from one editor to another.

It's funny to note that this debate has animated the developer community for years. This debate is so heated that the authors of the Silicon Valley series devoted a scene to it:

Personally, I'm an unconditional fan of tabs.

Tabs are better! 😉

Exercise

Using nested if, write a program that displays the honors of a grade entered by the user:

greater than or equal to 16: Highest honors
greater than or equal to 16: High honors
greater than or equal to 12 : With honors
greater than or equal to 10 : Passing
less than 10 : Rejected

Enter your grade: 13.8
With honors

Faire l'exercice

Solution

Quiz

What does the following code display?

if (1) if (0) printf ("Crac"); else printf ("Boum");

Nothing

Crac

Boum

CracBoum

Check Bravo! The else is associated to the if (0).

What does the following code display?

if (1) {
    if (0) 
        printf ("Crac"); 
    else 
        printf ("Boum");
printf ("Hue");
}

Crac

Boum

CracBoum

BoumHue

Check

What does the following code display?

int score = 75;
if (score>=95) printf ("Grade A\n");
if (score>=85) printf ("Grade B\n");
if (score>=73) printf ("Grade C\n");
if (score>=64) printf ("Grade D\n");

Grade A

Grade B

Grade C

Grade D

Check Bravo! There is no else, so multiple tests can run.

Why indenting your code is important

To clarify ideas during development.

To make it easier to understand the code.

To avoid bugs.

Without indentation, it doesn't compile.

Check

Lesson 5.1 Summary Lesson 5.3

Lesson 5.2. Nested if and indentation

Nested if

Indenting

Tabulation or spaces?

Exercise

Quiz

See also