Multiple images in subplot grid in Matplotlib - Step-by-Step Execution

Concept Flow - Multiple images in subplot grid

Start

↓

Create Figure and Subplots Grid

↓

Load or Generate Images

↓

Assign Each Image to a Subplot

↓

Display Images with plt.imshow()

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Adjust Layout and Show Plot

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End

The flow shows creating a grid of subplots, loading images, placing each image in a subplot, and displaying them together.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

fig, axs = plt.subplots(2, 2)
for i in range(2):
    for j in range(2):
        axs[i, j].imshow(np.random.rand(10,10))
plt.show()

This code creates a 2x2 grid of subplots and displays a random image in each subplot.

Execution Table

Step	Action	Variable/Function	Result/State
1	Create figure and 2x2 subplots grid	plt.subplots(2, 2)	fig object and axs 2x2 array created
2	Start outer loop i=0	i=0	Ready to fill first row
3	Start inner loop j=0	j=0	Ready to fill subplot axs[0,0]
4	Generate random image	np.random.rand(10,10)	10x10 array with random floats
5	Display image in axs[0,0]	axs[0,0].imshow()	Image shown in first subplot
6	Inner loop j=1	j=1	Ready to fill subplot axs[0,1]
7	Generate random image	np.random.rand(10,10)	10x10 array with random floats
8	Display image in axs[0,1]	axs[0,1].imshow()	Image shown in second subplot
9	Outer loop i=1	i=1	Ready to fill second row
10	Inner loop j=0	j=0	Ready to fill subplot axs[1,0]
11	Generate random image	np.random.rand(10,10)	10x10 array with random floats
12	Display image in axs[1,0]	axs[1,0].imshow()	Image shown in third subplot
13	Inner loop j=1	j=1	Ready to fill subplot axs[1,1]
14	Generate random image	np.random.rand(10,10)	10x10 array with random floats
15	Display image in axs[1,1]	axs[1,1].imshow()	Image shown in fourth subplot
16	Show all subplots	plt.show()	Window opens with 4 images in grid
17	End	-	All images displayed in subplot grid

💡 All subplots filled with images and displayed, execution ends.

Variable Tracker

Variable	Start	After Step 4	After Step 7	After Step 11	After Step 14	Final
i	N/A	0	0	1	1	1
j	N/A	0	1	0	1	1
axs	N/A	2x2 Axes array	2x2 Axes array	2x2 Axes array	2x2 Axes array	2x2 Axes array
image_array	N/A	random 10x10 array	random 10x10 array	random 10x10 array	random 10x10 array	last random 10x10 array

Key Moments - 3 Insights

Why do we use axs[i, j].imshow() inside nested loops?

Because axs is a 2D array of subplot axes, using axs[i, j] targets the correct subplot to display each image, as shown in steps 5, 8, 12, and 15.

What happens if we call plt.imshow() without specifying the subplot?

It will display the image on the current active axes or create a new figure, not placing images in the grid properly. The execution_table shows images assigned explicitly to axs[i, j].

Why do we call plt.show() only once after the loops?

plt.show() renders the entire figure with all subplots at once. Calling it inside loops would open multiple windows. Step 16 shows the single call after all images are set.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'i' when the image is displayed in axs[1,0]?

A 2

B 0

C 1

D None

💡 Hint

Check step 12 in the execution_table where axs[1,0] is used.

At which step does the inner loop variable 'j' change from 0 to 1 for the first time?

A Step 6

B Step 3

C Step 5

D Step 7

💡 Hint

Look at the execution_table rows where j changes inside the inner loop.

If we change plt.subplots(2, 2) to plt.subplots(3, 1), how would the variable 'axs' change?

A It becomes a 3x1 array of axes

B It becomes a 1D array with 3 axes

C It becomes a single Axes object

D It becomes a 2x3 array of axes

💡 Hint

Check how plt.subplots returns axes arrays depending on rows and columns.

Concept Snapshot

Use plt.subplots(rows, cols) to create a grid of subplots.
Each subplot is accessed by axs[row, col] if rows and cols > 1.
Use axs[i, j].imshow(image) to display images in each subplot.
Call plt.show() once after all images are assigned.
This displays multiple images neatly in a grid layout.

Full Transcript

This visual execution trace shows how to display multiple images in a grid using matplotlib. First, a figure and a 2x2 grid of subplots are created. Then, nested loops iterate over rows and columns. At each subplot position axs[i, j], a random 10x10 image array is generated and displayed using imshow. After filling all subplots, plt.show() displays the window with all images arranged in the grid. Variables i and j track the current subplot indices. The variable axs holds the array of subplot axes. Key moments clarify why images are assigned to specific subplots and why plt.show() is called once at the end. The quiz tests understanding of loop variables and subplot structure. This method helps visualize multiple images side by side clearly.