The if Statement Next: Up: Statements


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 non-zero 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 all of the elements are non-zero.

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.

warn_assign_as_truth_value Built-in Variable
If the value of warn_assign_as_truth_value is nonzero a warning is issued for statements like 
          if (s = t)
            ...

since such statements are not common and it is likely that the intent was to write 

          if (s == t)
            ...

instead.

There are times when it is useful to write code that contains assignments within the condition of a while or if statement. For example statements like 

          while (c = getc())
            ...

are common in C programming.

It is possible to avoid all warnings about such statements by setting warn_assign_as_truth_value to 0 but that may also let real errors like 

          if (x = 1)  # intended to test (x == 1)!
            ...

slip by.

In such cases it is possible suppress errors for specific statements by writing them with an extra set of parentheses. For example writing the previous example as 

          while ((c = getc()))
            ...

will prevent the warning from being printed for this statement while allowing Octave to warn about other assignments used in conditional contexts.

The default value of warn_assign_as_truth_value is 1.