If else

Syntax

The general syntax for an if-else block in C++ is

if (condition A)
{
  // Code to run if condition A is true.
}
else if (condition B)
{
  // Code to run if condition B is true.
}
else
{
  // Code to run if both conditions A and B are false.
}

Note that both the else if and the else blocks are optional, that is, you can of course have a single if block on its own.

Example 1

Assume we want to implement a stepwise function that returns the input value if it’s positive, otherwise it returns 0. This could look as follows:

double f(double x)
{
  if (x > 0)
  {
    return x;
  }
  else
  {
    return 0;
  }
}

Inline if tests

We can also write inline if-else blocks in C++. The general syntax looks like this:

variable = (condition) ? expression_if_true : expression_if_false

Example 2

Now let’s write the function from example 1 using an inline if test instead.

double f(double x)
{
  return (x > 0) ? x : 0;
}

Arguably, this code looks horrible – it is usually better to use the longer and more readable block format for if tests. But for very simple if tests, the shorthand version can be handy.