### 10.1 The if Statement

The `if` statement is Octave’s decision-making statement. There are three basic forms of an `if` statement. In its simplest form, it looks like this:

    if (condition)
      then-body
    endif

condition is an expression that controls what the rest of the statement will do. The then-body is executed only if condition is true.

The condition in an `if` statement is considered true if its value is nonzero, and false if its value is zero. If the value of the conditional expression in an `if` statement is a vector or a matrix, it is considered true only if it is non-empty and all of the elements are nonzero. The conceptually equivalent code when condition is a matrix is shown below.

    if (matrix) ≡ if (all (matrix(:)))

The second form of an if statement looks like this:

    if (condition)
      then-body
    else
      else-body
    endif

If condition is true, then-body is executed; otherwise, else-body is executed.

Here is an example:

    if (rem (x, 2) == 0)
      printf ("x is even\n");
    else
      printf ("x is odd\n");
    endif

In this example, if the expression `rem (x, 2) == 0` is true (that is, the value of `x` is divisible by 2), then the first `printf` statement is evaluated, otherwise the second `printf` statement is evaluated.

The third and most general form of the `if` statement allows multiple decisions to be combined in a single statement. It looks like this:

    if (condition)
      then-body
    elseif (condition)
      elseif-body
    else
      else-body
    endif

Any number of `elseif` clauses may appear. Each condition is tested in turn, and if one is found to be true, its corresponding body is executed. If none of the conditions are true and the `else` clause is present, its body is executed. Only one `else` clause may appear, and it must be the last part of the statement.

In the following example, if the first condition is true (that is, the value of `x` is divisible by 2), then the first `printf` statement is executed. If it is false, then the second condition is tested, and if it is true (that is, the value of `x` is divisible by 3), then the second `printf` statement is executed. Otherwise, the third `printf` statement is performed.

    if (rem (x, 2) == 0)
      printf ("x is even\n");
    elseif (rem (x, 3) == 0)
      printf ("x is odd and divisible by 3\n");
    else
      printf ("x is odd\n");
    endif

Note that the `elseif` keyword must not be spelled `else if`, as is allowed in Fortran. If it is, the space between the `else` and `if` will tell Octave to treat this as a new `if` statement within another `if` statement’s `else` clause. For example, if you write

    if (c1)
      body-1
    else if (c2)
      body-2
    endif

Octave will expect additional input to complete the first `if` statement. If you are using Octave interactively, it will continue to prompt you for additional input. If Octave is reading this input from a file, it may complain about missing or mismatched `end` statements, or, if you have not used the more specific `end` statements (`endif`, `endfor`, etc.), it may simply produce incorrect results, without producing any warning messages.

It is much easier to see the error if we rewrite the statements above like this,

    if (c1)
      body-1
    else
      if (c2)
        body-2
      endif

using the indentation to show how Octave groups the statements. See Functions and Scripts.