For loops in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple for loop with indexing

What is the output of the following MATLAB code?

MATLAB

sumVal = 0;
for i = 1:4
    sumVal = sumVal + i;
end
sumVal

A 10

B 24

C 0

D Error: Undefined variable 'sumVal'

💡 Hint

Think about adding numbers from 1 to 4 step by step.

The loop adds 1 + 2 + 3 + 4 which equals 10.

❓ Predict Output

intermediate

For loop with decrementing index

What will be the value of variable 'result' after running this code?

MATLAB

result = 1;
for k = 5:-1:2
    result = result * k;
end

A 120

B 20

C Error: Invalid for loop range

D 1

💡 Hint

Multiply numbers from 5 down to 2.

The loop multiplies 5*4*3*2 = 120.

🔧 Debug

advanced

Identify the error in this for loop

What error does this MATLAB code produce?

MATLAB

total = 0;
for i = 1 5
    total = total + i;
end

A Syntax error: Missing 'end' statement

B Runtime error: Variable 'total' undefined

C No error, output is 15

D Syntax error: Missing colon in for loop range

💡 Hint

Check the for loop range syntax carefully.

The for loop range must use a colon (1:5), not a space.

❓ Predict Output

advanced

Nested for loops output

What is the value of 'count' after running this nested loop?

MATLAB

count = 0;
for i = 1:3
    for j = 1:2
        count = count + 1;
    end
end

A 5

B 6

C 3

D 2

💡 Hint

Multiply the number of iterations of both loops.

Outer loop runs 3 times, inner loop 2 times, total 3*2=6 increments.

🧠 Conceptual

expert

Effect of modifying loop variable inside for loop

Consider this MATLAB code. What is the final value of 'x' after the loop finishes?

MATLAB

x = 0;
for i = 1:4
    x = x + i;
    i = 10;
end

A 11

B 9

C 10

D 20

💡 Hint

Changing the loop variable inside the loop does not affect the iteration.

The loop variable 'i' is controlled by the for loop and resetting it inside does not change the sequence. So x sums 1+2+3+4=10.

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