while - while loop to repeat when condition

is true - MATLAB (original) (raw)

• Documentation
• Examples
• Functions
• Apps
• Videos
• Answers

Main Content

while loop to repeat when condition is true

Syntax

while expression statements end

Description

while _`expression`_, _`statements`_, end evaluates an expression, and repeats the execution of a group of statements in a loop while the expression is true. An expression is true when its result is nonempty and contains only nonzero elements (logical or real numeric). Otherwise, the expression is false.

example

Examples

collapse all

Use a while loop to calculate factorial(10).

n = 10; f = n; while n > 1 n = n-1; f = f*n; end disp(['n! = ' num2str(f)])

Count the number of lines of code in the file magic.m. Skip blank lines and comments using a continue statement. continue skips the remaining instructions in the while loop and begins the next iteration.

fid = fopen('magic.m','r'); count = 0; while ~feof(fid) line = fgetl(fid); if isempty(line) || strncmp(line,'%',1) || ~ischar(line) continue end count = count + 1; end count

Sum a sequence of random numbers until the next random number is greater than an upper limit. Then, exit the loop using a break statement.

limit = 0.8; s = 0;

while 1 tmp = rand; if tmp > limit break end s = s + tmp; end

More About

collapse all

An expression can include relational operators (such as < or ==) and logical operators (such as &&, ||, or ~). Use the logical operators and and or to create compound expressions. MATLAB® evaluates compound expressions from left to right, adhering to operator precedence rules.

Within the conditional expression of a while...end block, logical operators & and | behave as short-circuit operators. This behavior is the same as && and ||, respectively. Since && and || consistently short-circuit in conditional expressions and statements, it is good practice to use && and || instead of & and | within the expression. For example,

x = 42; while exist('myfunction.m','file') && (myfunction(x) >= pi) disp('Expressions are true') break end

The first part of the expression evaluates to false. Therefore, MATLAB does not need to evaluate the second part of the expression, which would result in an undefined function error.

Tips

• If you inadvertently create an infinite loop (that is, a loop that never ends on its own), stop execution of the loop by pressingCtrl+C.
• If the conditional expression evaluates to a matrix, MATLAB evaluates the statements only if all elements in the matrix are true (nonzero). To execute statements if any element is true, wrap the expression in the any function.
• To programmatically exit the loop, use a break statement. To skip the rest of the instructions in the loop and begin the next iteration, use a continue statement.
• When nesting a number of while statements, each while statement requires an end keyword.
• The MATLABwhile loop is similar to a do...while loop in other programming languages, such as C and C++. However,while evaluates the conditional expression at the beginning of the loop rather than the end.
  do % Not valid MATLAB syntax
  statements
  while expression
  To mimic the behavior of a do...while loop, set the initial condition of while to true and place the conditional expression inside the loop. For example, implement thedo...while loop above by using a MATLABwhile loop.
  while true
  statements
  if ~expression
  break
  end
  end

Extended Capabilities

Version History

Introduced before R2006a