Custom colormaps in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Custom Colormap Creation

What is the output of this code snippet that creates a custom colormap and applies it to a simple gradient?

Matplotlib

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.colors import LinearSegmentedColormap

colors = [(1, 0, 0), (0, 1, 0), (0, 0, 1)]  # Red, Green, Blue
custom_cmap = LinearSegmentedColormap.from_list('mycmap', colors, N=3)
data = np.linspace(0, 1, 3).reshape(1, 3)
plt.imshow(data, cmap=custom_cmap)
plt.axis('off')
plt.show()
print(custom_cmap(0.5))

A [0.0, 0.5, 0.5, 1.0]

B [1.0, 0.0, 0.0, 1.0]

C [0.0, 1.0, 0.0, 1.0]

D [0.5, 0.5, 0.0, 1.0]

💡 Hint

Remember that the colormap interpolates colors between the given points. The value 0.5 is halfway between red and green in the list.

The custom colormap has three colors: red at 0.0, green at 0.5, and blue at 1.0. The value 0.5 corresponds exactly to green, so the output color is green with full opacity.

❓ data_output

intermediate

Number of Colors in Custom Colormap

Given this custom colormap created with N=5, how many distinct colors will it contain?

Matplotlib

from matplotlib.colors import LinearSegmentedColormap
colors = ['yellow', 'purple']
custom_cmap = LinearSegmentedColormap.from_list('custom', colors, N=5)
print(len(custom_cmap.colors) if hasattr(custom_cmap, 'colors') else 'No attribute colors')

A 1

B 2

C 5

D No attribute colors

💡 Hint

Check if the LinearSegmentedColormap object has a 'colors' attribute.

The LinearSegmentedColormap object does not have a 'colors' attribute. The N parameter controls the resolution internally but does not create a list attribute named 'colors'.

❓ visualization

advanced

Visualizing a Custom Diverging Colormap

Which option produces a plot with a smooth diverging colormap from blue to white to red?

Matplotlib

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.colors import LinearSegmentedColormap

colors = ['blue', 'white', 'red']
custom_cmap = LinearSegmentedColormap.from_list('diverging', colors, N=256)
data = np.outer(np.ones(100), np.linspace(-1, 1, 100))
plt.imshow(data, cmap=custom_cmap)
plt.colorbar()
plt.show()

A A plot with a gradient from blue on the left, white in the center, and red on the right

B A plot with a gradient from red on the left, white in the center, and blue on the right

C A plot with only blue color throughout

D A plot with only red color throughout

💡 Hint

The order of colors in the list defines the gradient direction.

The colormap is created from blue to white to red, so the gradient goes from blue on the low end to red on the high end with white in the middle.

🧠 Conceptual

advanced

Understanding Colormap Normalization

If you create a custom colormap with colors ['black', 'yellow'] and apply it to data ranging from 0 to 100, what happens if you normalize the data with vmin=20 and vmax=80 before plotting?

A Values below 20 map to black, values above 80 map to yellow, and values between 20 and 80 map to colors between black and yellow

B Values below 20 map to yellow, values above 80 map to black

C All values map to colors between black and yellow regardless of their range

D Normalization has no effect on the colormap application

💡 Hint

Normalization rescales data to the 0-1 range before applying the colormap.

Normalization with vmin=20 and vmax=80 rescales data so that 20 maps to 0 (black) and 80 maps to 1 (yellow). Values outside this range are clipped to the nearest color.

🔧 Debug

expert

Identify the Error in Custom Colormap Usage

What error will this code raise when trying to create and use a custom colormap?

Matplotlib

import matplotlib.pyplot as plt
from matplotlib.colors import LinearSegmentedColormap

colors = [(1, 0, 0), (0, 1)]  # Incorrect tuple length
custom_cmap = LinearSegmentedColormap.from_list('error_cmap', colors)
data = [[0, 1], [1, 0]]
plt.imshow(data, cmap=custom_cmap)
plt.show()

A TypeError: 'tuple' object cannot be interpreted as an integer

B ValueError: color sequence must be RGB or RGBA tuples

C IndexError: tuple index out of range

D No error, plot displays correctly

💡 Hint

Check the format of the color tuples in the list.

The second color tuple has only two elements instead of three or four, causing a ValueError when matplotlib tries to interpret it as RGB or RGBA.