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10.4 The do-until Statement

The do-until statement is similar to the while statement, except that it repeatedly executes a statement until a condition becomes true, and the test of the condition is at the end of the loop, so the body of the loop is always executed at least once. As with the condition in an if statement, the condition in a do-until 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 a do-until statement is a vector or a matrix, it is considered true only if it is non-empty and all of the elements are non-zero.

Octave's do-until statement looks like this:

do
  body
until (condition)

Here body is a statement or list of statements that we call the body of the loop, and condition is an expression that controls how long the loop keeps running.

This example creates a variable fib that contains the first ten elements of the Fibonacci sequence.

fib = ones (1, 10);
i = 2;
do
  i++;
  fib (i) = fib (i-1) + fib (i-2);
until (i == 10)

A newline is not required between the do keyword and the body; but using one makes the program clearer unless the body is very simple.