C Conditionals

An introduction to C Conditionals: if/else and switch

Any programming language provides the programmers the ability to perform choices.

We want to do X in some cases, and Y in other cases.

We want to check data, and do choices based on the state of that data.

C provides us 2 ways to do so.

The first is the if statement, with its else helper, and the second is the switch statement.

if

In an if statement, you can check for a condition to be true, and then execute the block provided in the curly brackets:

int a = 1;

if (a == 1) {
  /* do something */
}

You can append an else block to execute a different block if the original condition turns out to be false;

int a = 1;

if (a == 2) {
  /* do something */
} else {
  /* do something else */
}

Beware one common source of bugs - always use the comparison operator == in comparisons, and not the assignment operator =, otherwise the if conditional check will always be true, unless the argument is 0, for example if you do:

int a = 0;

if (a = 0) {
  /* never invoked */
}

Why does this happen? Because the conditional check will look for a boolean result (the result of a comparison), and the 0 number always equates to a false value. Everything else is true, including negative numbers.

You can have multiple else blocks by stacking together multiple if statements:

int a = 1;

if (a == 2) {
  /* do something */
} else if (a == 1) {
  /* do something else */
} else {
  /* do something else again */
}

switch

If you need to do too many if / else / if blocks to perform a check, perhaps because you need to check the exact value of a variable, then switch can be very useful to you.

You can provide a variable as condition, and a series of case entry points for each value you expect:

int a = 1;

switch (a) {
  case 0:
    /* do something */
    break;
  case 1:
    /* do something else */
    break;
  case 2:
    /* do something else */
    break;
}

We need a break keyword at the end of each case, to avoid the next case to be executed when the one before ends. This “cascade” effect can be useful in some creative ways.

You can add a “catch-all” case at the end, labeled default:

int a = 1;

switch (a) {
  case 0:
    /* do something */
    break;
  case 1:
    /* do something else */
    break;
  case 2:
    /* do something else */
    break;
  default:
    /* handle all the other cases */
    break;
}