[ MATLAB Tutorial - Lession 6] Loops

There may be a situation when you need to execute a block of code several number of times. In general, statements are executed sequentially. The first statement in a function is executed first, followed by the second, and so on.

Programming languages provide various control structures that allow for more complicated execution paths.

A loop statement allows us to execute a statement or group of statements multiple times and following is the general form of a loop statement in most of the programming languages −

MATLAB provides following types of loops to handle looping requirements. Click the following links to check their detail −

Sr.No.	Loop Type & Description
1	while loop Repeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body.
2	for loop Executes a sequence of statements multiple times and abbreviates the code that manages the loop variable.
3	nested loops You can use one or more loops inside any another loop.

The while loop repeatedly executes statements while condition is true.

While loop Syntax

The syntax of a while loop in MATLAB is −

while <expression>
   <statements>
end

The while loop repeatedly executes program statement(s) as long as the expression remains true.

An expression is true when the result is nonempty and contains all nonzero elements (logical or real numeric). Otherwise, the expression is false.

Example

Create a script file and type the following code −

 Live Demo

a = 10;
% while loop execution 
while( a < 20 )
   fprintf('value of a: %d\n', a);
   a = a + 1;
end

When you run the file, it displays the following result −

value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19

A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.

2.For loop Syntax

The syntax of a for loop in MATLAB is −

for index = values
   <program statements>
            ...
end

values has one of the following forms −

Sr.No.	Format & Description
1	initval:endval increments the index variable from initval to endval by 1, and repeats execution of program statements until index is greater than endval.
2	initval:step:endval increments index by the value step on each iteration, or decrements when step is negative.
3	valArray creates a column vector index from subsequent columns of array valArray on each iteration. For example, on the first iteration, index = valArray(:,1). The loop executes for a maximum of n times, where n is the number of columns of valArray, given by numel(valArray, 1, :). The input valArray can be of any MATLAB data type, including a string, cell array, or struct.

Example 1

Create a script file and type the following code −

 Live Demo

for a = 10:20 
   fprintf('value of a: %d\n', a);
end

When you run the file, it displays the following result −

value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19
value of a: 20

Example 2

Create a script file and type the following code −

 Live Demo

for a = 1.0: -0.1: 0.0
   disp(a)
end

When you run the file, it displays the following result −

1
0.90000
0.80000
0.70000
0.60000
0.50000
0.40000
0.30000
0.20000
0.10000
0

Example 3

Create a script file and type the following code −

 Live Demo

for a = [24,18,17,23,28]
   disp(a)
end

When you run the file, it displays the following result −

24

18

17

23

28

MATLAB allows to use one loop inside another loop. Following section shows few examples to illustrate the concept.

3. Nested loop Syntax

The syntax for a nested for loop statement in MATLAB is as follows −

for m = 1:j
   for n = 1:k
      <statements>;
   end
end

The syntax for a nested while loop statement in MATLAB is as follows −

while <expression1>
   while <expression2>
      <statements>
   end
end

Example

Let us use a nested for loop to display all the prime numbers from 1 to 100. Create a script file and type the following code −

 Live Demo

for i = 2:100
   for j = 2:100
      if(~mod(i,j)) 
         break; % if factor found, not prime
      end 
   end
   if(j > (i/j))
      fprintf('%d is prime\n', i);
   end
end

When you run the file, it displays the following result −

2 is prime
3 is prime
5 is prime
7 is prime
11 is prime
13 is prime
17 is prime
19 is prime
23 is prime
29 is prime
31 is prime
37 is prime
41 is prime
43 is prime
47 is prime
53 is prime
59 is prime
61 is prime
67 is prime
71 is prime
73 is prime
79 is prime
83 is prime
89 is prime
97 is prime

Loop Control Statements

Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.

MATLAB supports the following control statements. Click the following links to check their detail.

Sr.No.	Control Statement & Description
1	break statement Terminates the loop statement and transfers execution to the statement immediately following the loop.
2	continue statement Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.

The break statement terminates execution of for or while loop. Statements in the loop that appear after the break statement are not executed.

In nested loops, break exits only from the loop in which it occurs. Control passes to the statement following the end of that loop.

1. Break Flow Diagram

Example

Create a script file and type the following code −

 Live Demo

a = 10;
% while loop execution 
while (a < 20 )
   fprintf('value of a: %d\n', a);
   a = a + 1;
      if( a > 15)
         % terminate the loop using break statement 
         break;
      end 
end

When you run the file, it displays the following result −

value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15

The continue statement is used for passing control to next iteration of for or while loop.

The continue statement in MATLAB works somewhat like the break statement. Instead of forcing termination, however, 'continue' forces the next iteration of the loop to take place, skipping any code in between.

2.Continue Flow Diagram

Example

Create a script file and type the following code −

 Live Demo

a = 9;
%while loop execution 
while a < 20
   a = a + 1; 
   if a == 15
      % skip the iteration 
      continue;
   end 
fprintf('value of a: %d\n', a);
end

When you run the file, it displays the following result −

value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 16
value of a: 17
value of a: 18
value of a: 19
value of a: 20