Reshaping arrays in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the reshape function do in MATLAB?

The reshape function changes the size and shape of an array without changing its data. It rearranges elements into a new dimension.

beginner

How do you reshape a 1x6 array into a 2x3 array in MATLAB?

Use reshape(array, 2, 3). This changes the array to have 2 rows and 3 columns.

beginner

What must be true about the number of elements when reshaping an array?

The total number of elements must stay the same before and after reshaping. For example, a 6-element array can be reshaped to 2x3 or 3x2, but not 4x2.

intermediate

What happens if you try to reshape an array to a size that doesn't match the number of elements?

MATLAB gives an error because the reshape function requires the total number of elements to remain constant.

intermediate

How does MATLAB fill the reshaped array?

MATLAB fills the reshaped array column-wise, taking elements from the original array in column-major order.

What does reshape(A, 3, 2) do if A is a 1x6 array?

A Changes A into a 2-row, 3-column array

B Changes A into a 3-row, 2-column array

C Flattens A into a 1x6 array

D Throws an error

The reshape function changes the shape to 3 rows and 2 columns, keeping all elements.

What must be true for reshape to work without error?

A The new shape must have the same total number of elements as the original

B The new shape must have fewer elements

C The new shape must have more elements

D The new shape must be square

The total number of elements must remain the same to reshape successfully.

If you have a 2x3 matrix, which reshape is valid?

A reshape(A, 1, 5)

B reshape(A, 4, 2)

C reshape(A, 6, 2)

D reshape(A, 3, 2)

2x3 has 6 elements, so reshaping to 3x2 (also 6 elements) is valid.

How does MATLAB fill elements when reshaping?

A Row-wise

B It reverses the order

C Column-wise

D Randomly

MATLAB fills reshaped arrays column-wise, following its column-major order.

What happens if you try reshape(A, 4, 2) on a 2x3 matrix?

A It throws an error

B It works fine

C It truncates extra elements

D It pads with zeros

Because 2x3 has 6 elements and 4x2 requires 8, MATLAB throws an error.

Explain how the reshape function works in MATLAB and what rules must be followed.

Think about how you can rearrange a box of items without adding or losing any.

Describe a real-life example that helps you understand reshaping arrays.

Imagine moving books from one shelf layout to another.

Practice

1. What does the MATLAB function reshape do to an array?

easy

A. Sorts the array elements in ascending order

B. Changes the data values but keeps the shape the same

C. Deletes elements from the array

D. Changes the shape of the array without changing its data

Solution

1. Step 1: Understand the purpose of reshape

   The reshape function changes how the array is organized in terms of rows and columns but does not modify the actual data values.

2. Step 2: Confirm data remains unchanged

   The total number of elements stays the same, so no data is lost or changed, only rearranged.

3. Final Answer:

   Changes the shape of the array without changing its data -> Option D

4. Quick Check:

   reshape changes shape, not data [OK]

Hint: Remember reshape only rearranges elements, data stays same [OK]

Common Mistakes:

• Thinking reshape changes data values
• Assuming reshape deletes elements
• Confusing reshape with sorting

2. Which of the following is the correct syntax to reshape a 1x6 vector A into a 2x3 matrix in MATLAB?

easy

A. reshape(A, 2, 3)

B. reshape(2, 3, A)

C. reshape(A, [3 2])

D. reshape(A, 3, 2)

Solution

1. Step 1: Recall reshape syntax

   The correct syntax is reshape(array, rows, columns). So the array comes first, then the new dimensions.

2. Step 2: Check dimensions for 2x3

   To reshape into 2 rows and 3 columns, use reshape(A, 2, 3). Options with swapped dimensions or wrong order are incorrect.

3. Final Answer:

   reshape(A, 2, 3) -> Option A

4. Quick Check:

   reshape(array, rows, cols) [OK]

Hint: Syntax is reshape(array, rows, columns) [OK]

Common Mistakes:

• Putting dimensions before the array
• Swapping rows and columns
• Using brackets incorrectly

3. What is the output of the following MATLAB code?

A = 1:6;
B = reshape(A, 3, 2);
disp(B);

medium

A. [1 4; 2 5; 3 6]

B. [1 2; 3 4; 5 6]

C. [1 3 5; 2 4 6]

D. [1 2 3; 4 5 6]

Solution

1. Step 1: Understand reshape fills column-wise

   MATLAB fills the reshaped matrix column by column from the original vector. The vector A = [1 2 3 4 5 6].

2. Step 2: Fill 3 rows and 2 columns

   Fill first column: 1, 2, 3; second column: 4, 5, 6. So matrix B is:
   [1 4;
   2 5;
   3 6]

3. Final Answer:

   [1 4; 2 5; 3 6] -> Option A

4. Quick Check:

   reshape fills column-wise [OK]

Hint: Remember MATLAB fills columns first in reshape [OK]

Common Mistakes:

• Assuming row-wise filling
• Mixing up rows and columns
• Confusing output format

4. Identify the error in this MATLAB code:

A = 1:8;
B = reshape(A, 3, 3);

medium

A. Variable A is not defined correctly

B. reshape dimensions do not match number of elements

C. Syntax error in reshape function

D. No error, code runs fine

Solution

1. Step 1: Count elements in A

   Vector A has 8 elements (1 to 8).

2. Step 2: Check reshape target size

   reshape tries to make a 3x3 matrix which needs 9 elements, but A has only 8.

3. Final Answer:

   reshape dimensions do not match number of elements -> Option B

4. Quick Check:

   Elements must match reshape size [OK]

Hint: Check total elements before reshaping [OK]

Common Mistakes:

• Ignoring element count mismatch
• Assuming reshape pads missing elements
• Thinking reshape auto adjusts size

5. Given a 3D array A of size 2x3x4, which MATLAB command correctly reshapes it into a 6x4 matrix without changing the order of elements?

hard

A. reshape(A, [2 3 4])

B. reshape(A, 4, 6)

C. reshape(A, [6 4])

D. reshape(A, [4 6])

Solution

1. Step 1: Calculate total elements

   Original size is 2x3x4 = 24 elements total.

2. Step 2: Confirm reshape size matches total elements

   6x4 = 24 elements, so reshape to [6 4] is valid.

3. Step 3: Use correct syntax for reshape with vector size

   reshape accepts size as a vector, so reshape(A, [6 4]) is correct and clearer for multi-dimensional arrays.

4. Final Answer:

   reshape(A, [6 4]) -> Option C

5. Quick Check:

   Use vector size for multi-dim reshape [OK]

Hint: Use reshape(A, [rows cols]) for multi-dim arrays [OK]

Common Mistakes:

• Swapping rows and columns
• Using wrong syntax without brackets
• Ignoring total element count