With loop control statements, you can repeatedly execute a block of code. There are two types of loops:
for
statements
loop a specific number of times, and keep track of each iteration
with an incrementing index variable.
For example, preallocate a 10-element vector, and calculate five values:
x = ones(1,10); for n = 2:6 x(n) = 2 * x(n - 1); end
while
statements
loop as long as a condition remains true.
For example, find the first integer n
for
which factorial(n)
is a 100-digit number:
n = 1; nFactorial = 1; while nFactorial < 1e100 n = n + 1; nFactorial = nFactorial * n; end
Each loop requires the end
keyword.
It is a good idea to indent the loops for readability, especially when they are nested (that is, when one loop contains another loop):
A = zeros(5,100); for m = 1:5 for n = 1:100 A(m, n) = 1/(m + n - 1); end end
You can programmatically exit a loop using a break
statement, or skip to the next
iteration of a loop using a continue
statement.
For example, count the number of lines in the help for the magic
function
(that is, all comment lines until a blank line):
fid = fopen('magic.m','r'); count = 0; while ~feof(fid) line = fgetl(fid); if isempty(line) break elseif ~strncmp(line,'%',1) continue end count = count + 1; end fprintf('%d lines in MAGIC help\n',count); fclose(fid);
Tip If you inadvertently create an infinite loop (a loop that never ends on its own), stop execution of the loop by pressing Ctrl+C. |