Nested loops in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nested loop in MATLAB?

A nested loop is a loop inside another loop. The inner loop runs completely every time the outer loop runs once.

beginner

How does the flow of execution work in nested loops?

The outer loop runs one iteration, then the inner loop runs all its iterations. This repeats until the outer loop finishes all iterations.

beginner

Write a simple nested loop structure in MATLAB that prints numbers 1 to 3 in the outer loop and 1 to 2 in the inner loop.

for i = 1:3 for j = 1:2 disp([i, j]) end end

beginner

Why are nested loops useful?

Nested loops help when you need to repeat actions in multiple dimensions, like rows and columns in a matrix or combinations of items.

intermediate

What should you be careful about when using nested loops?

Nested loops can slow down your program if they run many times. Try to keep loops efficient or avoid deep nesting if possible.

In MATLAB, what happens first in nested loops?

A The loops run randomly

B The outer loop runs all its iterations before the inner loop starts

C Both loops run simultaneously

D The inner loop runs all its iterations for each outer loop iteration

The inner loop completes all its iterations every time the outer loop runs once.

Which keyword starts a loop in MATLAB?

A for

B loop

C whileloop

D repeat

MATLAB uses 'for' to start a counted loop.

What is the output of this nested loop? for i=1:2 for j=1:2 disp(i+j) end end

A 1 2 2 3

B 2 3 3 4

C 3 4 5 6

D 1 1 2 2

The sums are: (1+1)=2, (1+2)=3, (2+1)=3, (2+2)=4.

What is a common use case for nested loops?

A Processing elements in a 2D matrix

B Reading a single value

C Printing a single message

D Defining a function

Nested loops are often used to process rows and columns in matrices.

What can happen if nested loops have too many iterations?

A Program runs faster

B Program ignores inner loops

C Program may slow down significantly

D Program crashes immediately

Many nested iterations increase computation time and slow the program.

Explain how nested loops work in MATLAB and give a simple example.

Think about how one loop runs inside another and how many times each runs.

Describe a real-life situation where nested loops would be useful.

Imagine checking every seat in a theater row by row.

Practice

1. What does a nested loop in MATLAB do?
for i = 1:2
for j = 1:3
disp([i, j])
end
end

easy

A. Runs one loop inside another to repeat tasks multiple times

B. Runs two loops one after another without overlap

C. Creates a single loop with combined ranges

D. Stops execution after the first loop finishes

Solution

1. Step 1: Understand the loop structure

   The outer loop runs with i from 1 to 2, and inside it, the inner loop runs with j from 1 to 3.

2. Step 2: Identify nested loop behavior

   For each i, the inner loop runs fully for all j values, repeating tasks multiple times.

3. Final Answer:

   Runs one loop inside another to repeat tasks multiple times -> Option A

4. Quick Check:

   Nested loops repeat inner loop for each outer loop iteration [OK]

Hint: Nested loops repeat inner loop fully for each outer loop step [OK]

Common Mistakes:

• Thinking loops run sequentially without nesting
• Confusing nested loops with single combined loops
• Assuming loops stop early without completing inner loop

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

easy

A. for i = 1:3
disp(i)
for j = 1:2
disp(j)
end

B. for i = 1:3
for j = 1:2
disp(i+j)
end

C. for i = 1:3
for j = 1:2
disp(i+j)
end

D. for i = 1:3
for j = 1:2
disp(i+j)
end
end

Solution

1. Step 1: Check proper loop nesting and end statements

   for i = 1:3 for j = 1:2 disp(i+j) end end correctly nests the inner loop inside the outer loop and closes both with end.

2. Step 2: Identify syntax errors in other options

   The other options miss proper indentation or have missing end statements causing syntax errors.

3. Final Answer:

   for i = 1:3 for j = 1:2 disp(i+j) end end -> Option D

4. Quick Check:

   Proper nested loops need matching end for each for [OK]

Hint: Each for loop must have its own end statement [OK]

Common Mistakes:

• Missing end for inner or outer loop
• Incorrect indentation causing confusion
• Starting loops without proper nesting

3. What is the output of this MATLAB code?

for i = 1:2
  for j = 1:2
    fprintf('%d ', i*j)
  end
  fprintf('\n')
end

medium

A. 1 2 2 4

B. 1 1
2 2

C. 1 2
2 4

D. 2 4
1 2

Solution

1. Step 1: Calculate inner loop outputs for each outer loop value

   When i=1, j=1:2, outputs are 1*1=1 and 1*2=2. When i=2, j=1:2, outputs are 2*1=2 and 2*2=4.

2. Step 2: Format output with new lines

   Each inner loop prints values with space, then outer loop prints newline. So output lines are '1 2 ' and '2 4 '.

3. Final Answer:

   1 2 2 4 -> Option C

4. Quick Check:

   Multiply i and j in nested loops for output [OK]

Hint: Multiply outer and inner loop vars for each output [OK]

Common Mistakes:

• Ignoring newline after inner loop
• Mixing up i and j values
• Printing all values in one line

4. Find the error in this nested loop code:

for i = 1:3
  for j = 1:2
    disp(i+j)
end

medium

A. Missing end for outer loop

B. Inner loop range is incorrect

C. disp function is used incorrectly

D. Loop variables i and j are not initialized

Solution

1. Step 1: Check loop endings

   The inner loop has an end, but the outer loop does not have its closing end.

2. Step 2: Identify syntax error cause

   Missing the outer loop's end causes MATLAB to throw an error.

3. Final Answer:

   Missing end for outer loop -> Option A

4. Quick Check:

   Each for loop needs its own end [OK]

Hint: Count ends: one per for loop [OK]

Common Mistakes:

• Forgetting to close outer loop
• Assuming one end closes both loops
• Misreading error messages

5. You want to create a 3x3 multiplication table using nested loops in MATLAB. Which code correctly fills matrix M with products of row and column indices?

hard

A. M = zeros(3);
for i = 1:3
for j = 1:3
M(j,i) = i+j;
end
end

B. M = zeros(3);
for i = 1:3
for j = 1:3
M(i,j) = i*j;
end
end

C. M = zeros(3);
for i = 1:3
for j = 1:3
M(i,j) = i+j;
end
end

D. M = zeros(3);
for i = 1:3
for j = 1:3
M(i,j) = i-j;
end
end

Solution

1. Step 1: Understand the multiplication table goal

   The table should store products of row and column indices, so M(i,j) = i*j.

2. Step 2: Check each option's assignment

   M = zeros(3); for i = 1:3 for j = 1:3 M(i,j) = i*j; end end correctly assigns M(i,j) = i*j. The other options assign sums or differences, not products.

3. Final Answer:

   M = zeros(3); for i = 1:3 for j = 1:3 M(i,j) = i*j; end end -> Option B

4. Quick Check:

   Multiplication table uses product of indices [OK]

Hint: Assign M(i,j) = i*j for multiplication table [OK]

Common Mistakes:

• Using addition or subtraction instead of multiplication
• Swapping indices in assignment
• Not initializing matrix before loops