Conditionals and loops

Matlab has a standard if-elseif-else conditional; for example:

>> t = rand(1);
>> if t > 0.75
      s = 0;
   elseif t < 0.25
      s = 1;
   else
      s = 1-2*(t-0.25);
   end
>> s
s =
     0
>> t
t =
    0.7622

The logical operators in Matlab are <, >, <=, >=, == (logical equals), and ~= (not equal). These are binary operators which return the values 0 and 1 (for scalar arguments):

>> 5>3
ans =
     1
>> 5<3
ans =
     0
>> 5==3
ans =
     0

Thus the general form of the if statement is

if expr1
   statements
elseif expr2
   statements
 .
 .
 .
else
   statements
end

The first block of statements following a nonzero expr executes.

Matlab provides two types of loops, a for-loop (comparable to a Fortran do-loop or a C for-loop) and a while-loop. A for-loop repeats the statements in the loop as the loop index takes on the values in a given row vector:

>> for i=[1,2,3,4]
      disp(i^2)
   end
     1
     4
     9
    16

(Note the use of the built-in function disp, which simply displays its argument.) The loop, like an if-block, must be terminated by end. This loop would more commonly be written as

>> for i=1:4
      disp(i^2)
   end
     1
     4
     9
    16

(recall that 1:4 is the same as [1,2,3,4]).

The while-loop repeats as long as the given expr is true (nonzero):

>> x=1;
>> while 1+x > 1
      x = x/2;
   end
>> x
x =
   1.1102e-16