For loops in MATLAB

Introduction

For loops help you repeat a set of actions many times without writing the same code again and again.

When you want to add numbers from 1 to 10.

When you want to print each item in a list one by one.

When you want to perform the same calculation on many values.

When you want to create a pattern by repeating steps.

When you want to process rows in a table or matrix.

Syntax

MATLAB

for index = startValue:endValue
    % commands to repeat
end

The index changes from startValue to endValue, one step at a time.

The commands inside the loop run once for each value of index.

Examples

This prints numbers 1 to 5, one number per line.

MATLAB

for i = 1:5
    disp(i)
end

This counts down from 10 to 2 by steps of 2 and prints each number.

MATLAB

for x = 10:-2:2
    disp(x)
end

This prints each letter in the array one by one.

MATLAB

for letter = ['a', 'b', 'c']
    disp(letter)
end

Sample Program

This program adds numbers from 1 to 5 using a for loop and then prints the total.

MATLAB

sum = 0;
for n = 1:5
    sum = sum + n;
end
fprintf('Sum of numbers 1 to 5 is %d\n', sum);

Important Notes

Remember to end the loop with end in MATLAB.

You can use any variable name for the loop index, not just i.

The step size in the loop can be changed by adding a third number in the range, like 1:2:9.

Summary

For loops repeat commands for a set number of times.

The loop index changes each time to control repetition.

Use end to close the loop block.

Practice

1. What does a for loop do in MATLAB?

easy

A. Stops the program immediately

B. Runs commands only once

C. Creates a new variable

D. Repeats a set of commands a fixed number of times

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop is designed to repeat commands multiple times based on a range or list.

2. Step 2: Compare options with the definition

   Only Repeats a set of commands a fixed number of times correctly describes repeating commands a fixed number of times.

3. Final Answer:

   Repeats a set of commands a fixed number of times -> Option D

4. Quick Check:

   For loop = repeat commands [OK]

Hint: For loops repeat commands multiple times [OK]

Common Mistakes:

• Thinking for loops run only once
• Confusing for loops with if statements
• Believing for loops stop the program

2. Which of the following is the correct syntax for a for loop in MATLAB?

easy

A. for i = 1:5
disp(i)
end

B. for (i = 1; i <= 5; i++)
disp(i)
end

C. loop i from 1 to 5
disp(i)
end

D. for i in range(1,5):
disp(i)

Solution

1. Step 1: Identify MATLAB for loop syntax

   MATLAB uses 'for variable = start:end' and ends with 'end'.

2. Step 2: Check each option

   for i = 1:5 disp(i) end matches MATLAB syntax. Options B, C, and D use other languages' syntax.

3. Final Answer:

   for i = 1:5\n disp(i)\nend -> Option A

4. Quick Check:

   MATLAB for loop = for i = 1:5 ... end [OK]

Hint: MATLAB for loops use 'for var = start:end' and end with 'end' [OK]

Common Mistakes:

• Using parentheses like C or JavaScript
• Using Python or other language syntax
• Forgetting the 'end' keyword

3. What is the output of this MATLAB code?

sum = 0;
for i = 1:3
  sum = sum + i;
end
disp(sum)

medium

A. 6

B. 3

C. 0

D. Error

Solution

1. Step 1: Trace the loop iterations

   The loop runs i = 1, 2, 3. sum starts at 0.

2. Step 2: Calculate sum after each iteration

   sum = 0+1=1, then 1+2=3, then 3+3=6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   1+2+3 = 6 [OK]

Hint: Add numbers 1 to 3 inside loop [OK]

Common Mistakes:

• Forgetting to initialize sum
• Adding outside the loop
• Confusing loop range

4. Identify the error in this MATLAB for loop:

for i = 1:4
  disp(i)
  i = i + 1;
end

medium

A. Missing 'end' keyword

B. Modifying loop variable inside the loop

C. Incorrect loop range syntax

D. disp function is invalid

Solution

1. Step 1: Review loop variable usage

   The loop variable 'i' is controlled by MATLAB and should not be changed inside the loop.

2. Step 2: Identify the problem

   Changing 'i' inside the loop causes unexpected behavior or errors.

3. Final Answer:

   Modifying loop variable inside the loop -> Option B

4. Quick Check:

   Do not change loop variable inside for loop [OK]

Hint: Never change the loop variable inside the loop [OK]

Common Mistakes:

• Changing loop variable inside loop body
• Forgetting 'end' keyword (not the case here)
• Misunderstanding disp function

5. How can you use a for loop in MATLAB to create a vector v containing squares of numbers from 1 to 5?

hard

A. v = [];
for i = 1:5
v = v + i^2;
end

B. for i = 1:5
v(i) = i*2;
end

C. v = zeros(1,5);
for i = 1:5
v(i) = i^2;
end

D. v = 1:5;
for i = 1:5
v(i) = i + 2;
end

Solution

1. Step 1: Initialize vector to store results

   Pre-allocate vector v with zeros to hold 5 elements.

2. Step 2: Use for loop to assign squares

   For each i from 1 to 5, assign v(i) = i squared.

3. Final Answer:

   v = zeros(1,5);\nfor i = 1:5\n v(i) = i^2;\nend -> Option C

4. Quick Check:

   Pre-allocate vector and assign squares in loop [OK]

Hint: Pre-allocate vector, assign squares inside loop [OK]

Common Mistakes:

• Not pre-allocating vector before loop
• Adding instead of assigning inside loop
• Using wrong operation for squares