Conditional Execution

Until now, your program can only execute multiple steps after each other, and then stops. But you can also let your application leave out some code, or execute some other only in specific circumstances.

Conditions

First you need to know what a condition is. Conditions are expressions, that return a boolean value, true or false.

Relational Operators

For example you can compare two values, and if they are equal, return true:

foo == 10

There are also other available operators for conditions:

foo > 10 // the number foo must be greater than 10

foo < 10 // foo must be smaller than 10 to return true

foo >= 10 // foo needs to be greater than or equal to 10. the same as foo > 9

foo <= 10

foo != 10 // returns true only if foo not contains 10

Of course you can also just use a boolean variable, a function that returns a boolean value as a condition, or even true or false directly.

Logical Operators

Sometimes it is useful to combine two conditions, for example if you want that a variable to be between 10 and 20.

This can be accomplished with logical operators:

number <= 20 && number >= 10

And (&&) returns only true, if both expressions on the left and right side are true.

Logical Or (||) instead needs only one true expression to also return true:

// returns true when foo is smaller
// than 20 or when it is exactly 30
foo < 20 || foo == 30

A third operator is !, which simply turns true into false and vice-versa:

bool condition = !(5 > 6) // true

You can combine these operations further to build more complex conditions.

If

Sometimes you have code that should only run, when a specific condidtion is met. Do that with the if statement:

if (condition) {
    // here is your code
}

An example would be:

if (foo == 10) {
    stdout.printf ("foo is 10\n");
}

In this case only when the variable foo exactly contains the number 10, the program prints out “foo is 10”.

In the example foo == 10 is a condition.

Else

You can also execute code whenever an if-statement was not executed:

if (foo == 10) {
    stdout.printf ("foo is 10\n");
} else {
    stdout.printf ("foo is not 10\n");
}

It covers all other remaining cases, when foo is for example 11, 12, 9, etc. but not 10.

Else If

To further extend that concept, you can also specify alternative code sections, with a different condition, that get checked and run when the condition in if returned false:

if (c == 0) {
    stdout.printf ("0\n");
} else if (c == 1) {
    stdout.printf ("1\n");
} else if (c == 2) {
    stdout.printf ("2\n");
}

stdout.printf ("done\n");

If c == 0 here is true, the program prints out 0. If not, the next else if is checked. If it also not true, the next one is taken etc. And when one condition is true, the corresponding code block is executed and after that the programs continues with code after the last else if and prints out “done”.

Of course it can have also an additional else part at the end:

if (c == 0) {
    stdout.printf ("c is 0\n");
} else if (c == 1) {
    stdout.printf ("c is 1\n");
} else {
    stdout.printf ("c is not 0 and also not 1\n");
}

Switch

With switch the code checks if a expression equals exactly a value, and executes then code for that value:

switch (fruit) {
    case "apple":
        stdout.printf ("An apple.\n");
        break;
    case "cherry":
        stdout.printf ("A cherry.\n");
        break;
    case "banana":
        stdout.printf ("A banana.\n");
        break;
    case "blueberry":
    case "raspberry":
        stdout.printf ("A berry.\n");
        break;
    default:
        stdout.printf ("This is no fruit.\n");
        break;
}

Here it is checked first if fruit constains “apple”, if yes it is printed out and then the program exits with break; the switch statement. If not, then it is checked for “cherry” with the same process and so on. As you can see with “blueberry”, you can also leave the break; out. Then the program just continues to execute the following commands without caring about another case statement.

Also useful

• If the code inside of a if or else block or any other code block from a statement is only one line, you can avoid the curly brackets:

  if (foo == 10)
      stdout.printf ("foo is 10\n");