Reshaping arrays in MATLAB

Introduction

Reshaping arrays lets you change the shape or size of your data without changing its values. It helps organize data in a way that fits your needs.

You have a list of numbers and want to turn it into a matrix with rows and columns.

You want to prepare data for a function that needs a specific shape.

You want to flatten a matrix into a single row or column.

You want to split a long list into smaller blocks for easier processing.

Syntax

MATLAB

reshapedArray = reshape(originalArray, newRowCount, newColumnCount);

The total number of elements must stay the same before and after reshaping.

You can reshape to more than 2 dimensions by adding more size arguments.

Examples

This turns a 1-by-6 row vector into a 2-by-3 matrix.

MATLAB

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

This reshapes a 2-by-2 matrix into a 4-by-1 column vector.

MATLAB

A = [1 2; 3 4];
B = reshape(A, 4, 1);

This reshapes a 1-by-8 vector into a 3D array with size 2-by-2-by-2.

MATLAB

A = 1:8;
B = reshape(A, 2, 2, 2);

Reshape must keep the total number of elements the same.

MATLAB

A = 1:5;
% reshape(A, 2, 3) would cause an error because 5 elements can't fill 6 spots.

Sample Program

This program starts with a 1-by-12 row vector. It reshapes it into a 3-by-4 matrix, then reshapes it again into a 3D array with size 2-by-2-by-3. It prints the arrays before and after reshaping.

MATLAB

originalArray = 1:12;
disp('Original array:');
disp(originalArray);

reshapedArray = reshape(originalArray, 3, 4);
disp('Reshaped array (3 rows, 4 columns):');
disp(reshapedArray);

reshapedArray2 = reshape(originalArray, 2, 2, 3);
disp('Reshaped array into 3D (2x2x3):');
disp(reshapedArray2);

Important Notes

Reshape operation runs in O(1) time because it only changes how data is viewed, not the data itself.

Space complexity is O(1) extra space since no new data is created, just a new view.

A common mistake is trying to reshape to a size that doesn't match the total number of elements.

Use reshape when you want to reorganize data without changing values. Use other functions like transpose if you want to flip rows and columns.

Summary

Reshape changes the shape of an array without changing its data.

The total number of elements must stay the same before and after reshaping.

It works for vectors, matrices, and multi-dimensional arrays.

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