Multiple images in subplot grid in Matplotlib - Time & Space Complexity

Time Complexity: Multiple images in subplot grid

O(n)

Understanding Time Complexity

When showing many images in a grid using matplotlib, we want to know how the time to draw grows as we add more images.

We ask: How does adding more images affect the total work matplotlib does?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

import matplotlib.pyplot as plt
import numpy as np

fig, axes = plt.subplots(3, 3)
for i, ax in enumerate(axes.flat):
    img = np.random.rand(10, 10)
    ax.imshow(img)
plt.show()

This code creates a 3 by 3 grid of images, each showing a small random 10x10 picture.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Loop over each subplot to draw an image.
• How many times: Once for each image in the grid (rows x columns).

How Execution Grows With Input

As we add more images, the work grows roughly in direct proportion to the number of images.

Input Size (n images)	Approx. Operations
9 (3x3)	9 image draws
100 (10x10)	100 image draws
1024 (32x32)	1024 image draws

Pattern observation: Doubling the number of images roughly doubles the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to draw grows linearly with the number of images shown.

Common Mistake

[X] Wrong: "Adding more images won't affect time much because each image is small."

[OK] Correct: Even small images require drawing work, so more images add up and increase total time.

Interview Connect

Understanding how drawing many images scales helps you explain performance in data visualization tasks clearly and confidently.

Self-Check

"What if we changed the image size from 10x10 to 100x100 pixels? How would the time complexity change?"

Practice

1. What is the main purpose of using plt.subplots when displaying multiple images in a grid?

easy

A. To change the color of images

B. To load images from files automatically

C. To create a grid of axes where each image can be shown separately

D. To save images to disk

Solution

1. Step 1: Understand the role of plt.subplots

   plt.subplots creates a figure and a grid of axes (subplots) to place multiple plots or images.

2. Step 2: Connect to displaying images

   Each axis in the grid can show one image, so it helps organize multiple images neatly.

3. Final Answer:

   To create a grid of axes where each image can be shown separately -> Option C

4. Quick Check:

   plt.subplots = grid for images [OK]

Hint: Remember: plt.subplots makes the grid for multiple images [OK]

Common Mistakes:

• Thinking plt.subplots loads or saves images
• Confusing plt.subplots with image display functions
• Assuming plt.subplots changes image colors

2. Which of the following is the correct way to loop through all axes in a subplot grid to plot images?

easy

A. for ax in axes.flat:

B. for ax in axes.grid():

C. for ax in axes.loop():

D. for ax in axes.images():

Solution

1. Step 1: Identify the axes object type

   When plt.subplots creates multiple axes, they are stored in an array or matrix.

2. Step 2: Use axes.flat to flatten the array for looping

   axes.flat lets you loop over all axes in a simple 1D way.

3. Final Answer:

   for ax in axes.flat: -> Option A

4. Quick Check:

   axes.flat loops all axes [OK]

Hint: Use axes.flat to loop all subplot axes easily [OK]

Common Mistakes:

• Using non-existent methods like axes.grid()
• Trying to loop axes directly without flattening
• Confusing axes with image objects

3. What will be the output of this code snippet?

import matplotlib.pyplot as plt
import numpy as np
images = [np.random.rand(5,5) for _ in range(4)]
fig, axes = plt.subplots(2, 2)
for ax, img in zip(axes.flat, images):
    ax.imshow(img, cmap='gray')
    ax.axis('off')
plt.show()

medium

A. A single image shown in one plot

B. A 2x2 grid showing 4 random grayscale images without axes

C. An error because axes.flat is not iterable

D. A 2x2 grid with empty plots and no images

Solution

1. Step 1: Understand the code flow

   Four random 5x5 images are created and stored in a list. A 2x2 subplot grid is created.

2. Step 2: Loop through axes and images

   Each axis in the 2x2 grid shows one image with grayscale colormap and axes turned off.

3. Final Answer:

   A 2x2 grid showing 4 random grayscale images without axes -> Option B

4. Quick Check:

   Loop + imshow + axis off = grid of images [OK]

Hint: Loop axes.flat with images to plot all in grid [OK]

Common Mistakes:

• Expecting only one image to show
• Thinking axes.flat is not iterable
• Forgetting to turn axes off

4. Identify the error in this code that tries to display 3 images in a 2x2 grid:

fig, axes = plt.subplots(2, 2)
images = [img1, img2, img3]
for i in range(3):
    axes[i].imshow(images[i])
    axes[i].axis('off')

medium

A. axis('off') is not a valid method

B. images list is empty

C. imshow cannot display images in subplots

D. axes is a 2D array, so axes[i] causes an error

Solution

1. Step 1: Check the type of axes

   plt.subplots(2, 2) returns a 2D array of axes, so axes[i] is invalid indexing.

2. Step 2: Correct way to access axes

   Use axes.flat[i] or flatten axes before indexing to access each subplot.

3. Final Answer:

   axes is a 2D array, so axes[i] causes an error -> Option D

4. Quick Check:

   axes 2D array needs flat for 1D access [OK]

Hint: Use axes.flat to index subplots in 1D [OK]

Common Mistakes:

• Indexing 2D axes array as 1D
• Assuming images list is empty
• Misusing axis('off') method

5. You want to display 6 images in a 2x3 grid with titles on each subplot. Which code snippet correctly does this?

hard

A. fig, axes = plt.subplots(2, 3)
for ax, img, i in zip(axes.flat, images, range(6)):
ax.imshow(img)
ax.set_title(f'Image {i+1}')
ax.axis('off')

B. fig, axes = plt.subplots(3, 2)
for i in range(6):
axes[i].imshow(images[i])
axes[i].title(f'Image {i}')
axes[i].axis('off')

C. fig, axes = plt.subplots(2, 3)
for i in range(6):
axes[i].imshow(images[i])
axes[i].set_title('Image')
axes[i].axis('off')

D. fig, axes = plt.subplots(2, 3)
for ax, img in zip(axes, images):
ax.imshow(img)
ax.set_title('Image')
ax.axis('off')

Solution

1. Step 1: Create correct subplot grid

   plt.subplots(2, 3) creates 2 rows and 3 columns, perfect for 6 images.

2. Step 2: Loop through axes.flat and images with index

   Using axes.flat flattens the 2D axes array for easy looping. Adding index with range(6) helps set titles.

3. Step 3: Set image, title, and turn off axes

   Each axis shows one image, sets a title with number, and hides axis ticks.

4. Final Answer:

   fig, axes = plt.subplots(2, 3) for ax, img, i in zip(axes.flat, images, range(6)): ax.imshow(img) ax.set_title(f'Image {i+1}') ax.axis('off') -> Option A

5. Quick Check:

   axes.flat + set_title + axis off = correct grid [OK]

Hint: Use axes.flat and zip(images, range) for titles [OK]

Common Mistakes:

• Indexing 2D axes as 1D without flat
• Using wrong subplot shape for 6 images
• Calling non-existent title() method