6.4: for Loops

A for loop is used when we know ahead of time how many times the repeated statements should run. The variable that controls the loop is called the loop variable or iterator.

General Format of a for Loop

 

for iterator = startValue:endValue
    statements to repeat
end

 

In the following example, a for loop is used to the message Hello World ten times:

Example \(\PageIndex{1}\)

Using a for loop.

 

for i = 1:10
    disp('Hello World')
end

 

The loop variable i starts at 1. MATLAB runs the statement inside the loop, then automatically increases i to the next value. This continues until i reaches 10. We do not need to write i = i + 1 in a MATLAB for loop; MATLAB does that automatically.

Note

Use a for loop when you can say, “I need to repeat this exactly n times,” or “I need to process every element from index 1 to index n.”

 

Example: Sum of the First N Integers

The following function takes an integer n and adds all integers from 1 through n.

 

function outSum = sumFirstNIntegers(n)
% sumFirstNIntegers returns the sum of integers from 1 to n.

outSum = 0;

for i = 1:n
    outSum = outSum + i;
end

end

If we call the function with n = 8, MATLAB computes:

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8

The result is 36.

 

Accumulator Variables

In the previous example, outSum is called an accumulator variable. An accumulator is a variable that starts with an initial value and is updated during each loop iteration. Each time through the loop, MATLAB takes the old value of outSum and adds the current value of i. This pattern appears frequently in programming.

Note

Key Pattern

Initialize the accumulator before the loop. Update the accumulator inside the loop.