while

while loop to repeat when condition is true

while expression
statements
end

Description

example

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.

Examples

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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)])
n! = 3628800

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
count = 34
fclose(fid);

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

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Expression

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 pressing Ctrl+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 MATLAB while 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 the do...while loop above by using a MATLAB while loop.

while true
statements
if ~expression
break
end
end