Multiple images in subplot grid in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of subplot grid with 2 images

What will be the output of this code that plots two images side by side using matplotlib subplots?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

img1 = np.ones((5,5))
img2 = np.zeros((5,5))

fig, axs = plt.subplots(1, 2)
axs[0].imshow(img1, cmap='gray')
axs[1].imshow(img2, cmap='gray')
plt.show()

A A figure with two images: left is a white square, right is a black square.

B An error because axs is not iterable.

C A single image showing a gradient from black to white.

D A figure with two images stacked vertically.

💡 Hint

Check how subplots are arranged and how images are assigned to axes.

The code creates a 1x2 grid of subplots. The first subplot shows a white 5x5 image (all ones), the second shows a black 5x5 image (all zeros).

❓ data_output

intermediate

Number of axes in a 3x3 subplot grid

How many axes objects are created when you run this code?

Matplotlib

import matplotlib.pyplot as plt
fig, axs = plt.subplots(3, 3)
print(len(axs.flatten()))

A 1

B 3

C 9

D 6

💡 Hint

Count rows times columns in the subplot grid.

A 3x3 grid creates 9 axes objects. Flattening axs gives a list of all 9 axes.

❓ visualization

advanced

Identify the subplot layout from the image

Given a figure with 4 images arranged in a 2x2 grid, which code snippet correctly creates this layout?

A

fig, axs = plt.subplots(1, 4)
for i, ax in enumerate(axs):
    ax.imshow(np.full((3,3), i))

B

fig, axs = plt.subplots(2, 2)
for i, ax in enumerate(axs.flatten()):
    ax.imshow(np.full((3,3), i))

C

fig, axs = plt.subplots(4, 1)
for i, ax in enumerate(axs):
    ax.imshow(np.full((3,3), i))

D

fig, axs = plt.subplots(3, 3)
for i, ax in enumerate(axs.flatten()[:4]):
    ax.imshow(np.full((3,3), i))

💡 Hint

Check the shape of the subplot grid and how images are assigned.

Option B creates a 2x2 grid and fills each subplot with a different image. Others create wrong grid shapes or use extra subplots.

🔧 Debug

advanced

Why does this subplot code raise an error?

This code raises an error. What is the cause?

Matplotlib

import matplotlib.pyplot as plt
fig, axs = plt.subplots(1, 1)
axs.imshow([[1, 2], [3, 4]])
plt.show()

A 'AxesSubplot' object has no attribute 'imshow'

B TypeError because axs is not an array but a single Axes object

C IndexError because axs is not subscriptable

D No error, the code runs fine

💡 Hint

Check the type of axs when subplots(1,1) is used.

When subplots(1,1) is called, axs is a single Axes object, not an array. It has imshow method, so no error occurs.

🚀 Application

expert

Arrange 6 images in a 2x3 grid with shared axes

You want to plot 6 images in a 2 rows by 3 columns grid using matplotlib. You want all subplots to share the same x and y axes to compare images easily. Which code snippet achieves this?

A

fig, axs = plt.subplots(2, 3, sharex=True, sharey=True)
for ax in axs.flatten():
    ax.imshow(np.random.rand(5,5))
plt.show()

B

fig, axs = plt.subplots(2, 3)
for ax in axs.flatten():
    ax.imshow(np.random.rand(5,5))
plt.show()

C

fig, axs = plt.subplots(6, 1, sharex=True, sharey=True)
for ax in axs:
    ax.imshow(np.random.rand(5,5))
plt.show()

D

fig, axs = plt.subplots(3, 2, sharex=True, sharey=True)
for ax in axs.flatten():
    ax.imshow(np.random.rand(5,5))
plt.show()

💡 Hint

Check the shape of the grid and the sharex/sharey parameters.

Option A creates a 2x3 grid with shared x and y axes, matching the requirement. Option A swaps rows and columns, A lacks shared axes, C creates a 6x1 grid.

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