Alpha transparency for overlap in Matplotlib - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the transparency of the scatter plot points.

Matplotlib

import matplotlib.pyplot as plt

plt.scatter([1, 2, 3], [4, 5, 6], alpha=[1])
plt.show()

Drag options to blanks, or click blank then click option'

A1.5

B0.5

C-0.3

D'0.5'

Attempts:

3 left

💡 Hint

Common Mistakes

Using alpha values greater than 1 or less than 0 causes errors.

Passing alpha as a string instead of a number.

The alpha parameter controls transparency and must be between 0 (fully transparent) and 1 (fully opaque). 0.5 sets half transparency.

2fill in blank

medium

Complete the code to plot two overlapping scatter plots with transparency.

Matplotlib

import matplotlib.pyplot as plt

plt.scatter([1, 2, 3], [4, 5, 6], alpha=[1], color='red')
plt.scatter([2, 3, 4], [5, 6, 7], alpha=0.3, color='blue')
plt.show()

Drag options to blanks, or click blank then click option'

A0.8

B2

C-1

D'0.8'

Attempts:

3 left

💡 Hint

Common Mistakes

Using alpha values outside the 0-1 range.

Passing alpha as a string.

Setting alpha to 0.8 makes the red points slightly transparent so overlapping with blue points is visible.

3fill in blank

hard

Fix the error in the code to correctly apply transparency to the scatter plot.

Matplotlib

import matplotlib.pyplot as plt

plt.scatter([1, 2, 3], [4, 5, 6], alpha=[1])
plt.show()

Drag options to blanks, or click blank then click option'

A'0.5'

B2

C0.5

D-0.5

Attempts:

3 left

💡 Hint

Common Mistakes

Passing alpha as a string causes a TypeError.

Using values outside the 0-1 range causes ValueError.

Alpha must be a float between 0 and 1, not a string or invalid number.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps words to their lengths only if length is greater than 3.

Matplotlib

words = ['data', 'ai', 'science', 'ml']
lengths = {word: [1] for word in words if [2]

Drag options to blanks, or click blank then click option'

Alen(word)

Blen(word) > 3

Cword

Dword > 3

Attempts:

3 left

💡 Hint

Common Mistakes

Using the word itself instead of its length as the value.

Checking if the word (string) is greater than 3 instead of length.

The dictionary comprehension maps each word to its length only if the length is greater than 3.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps uppercase words to their lengths only if length is greater than 4.

Matplotlib

words = ['apple', 'data', 'science', 'ai']
result = [1]: [2] for w in words if [3]

Drag options to blanks, or click blank then click option'

Aw.upper()

Blen(w)

Clen(w) > 4

Dw.lower()

Attempts:

3 left

💡 Hint

Common Mistakes

Using w.lower() instead of w.upper() for keys.

Not filtering words by length correctly.

The comprehension creates a dictionary with uppercase words as keys and their lengths as values, filtering words longer than 4 characters.

Practice

1. What does the alpha parameter control in a matplotlib plot?

easy

A. The size of plot elements

B. The color of plot elements

C. The shape of plot elements

D. The transparency level of plot elements

Solution

1. Step 1: Understand the role of alpha in plotting

   The alpha parameter sets how transparent or see-through a plot element is.

2. Step 2: Compare with other visual properties

   Color changes hue, size changes dimension, shape changes form, but only alpha controls transparency.

3. Final Answer:

   The transparency level of plot elements -> Option D

4. Quick Check:

   Alpha = transparency [OK]

Hint: Alpha means transparency from 0 (clear) to 1 (solid) [OK]

Common Mistakes:

• Confusing alpha with color
• Thinking alpha changes size
• Assuming alpha changes shape

2. Which of the following is the correct way to set alpha transparency to 0.5 in a matplotlib scatter plot?

easy

A. plt.scatter(x, y, alpha=0.5)

B. plt.scatter(x, y, transparency=0.5)

C. plt.scatter(x, y, opacity=0.5)

D. plt.scatter(x, y, alpha=5)

Solution

1. Step 1: Recall the correct parameter name for transparency

   Matplotlib uses alpha to set transparency, not transparency or opacity.

2. Step 2: Check the value range for alpha

   Alpha values must be between 0 and 1, so 0.5 is valid, but 5 is not.

3. Final Answer:

   plt.scatter(x, y, alpha=0.5) -> Option A

4. Quick Check:

   Alpha param and value 0.5 [OK]

Hint: Use alpha=0.5 for half transparency in matplotlib [OK]

Common Mistakes:

• Using wrong parameter names like transparency or opacity
• Setting alpha value outside 0-1 range
• Confusing alpha with color or size

3. What will be the effect of this code snippet on the plot?

import matplotlib.pyplot as plt
x = [1, 2, 3, 4]
y1 = [1, 4, 9, 16]
y2 = [2, 3, 5, 7]
plt.plot(x, y1, alpha=0.3)
plt.plot(x, y2, alpha=0.7)
plt.show()

medium

A. The second line will be invisible

B. Both lines will have the same transparency

C. The first line will be more transparent than the second

D. The first line will be solid and the second transparent

Solution

1. Step 1: Understand alpha values in the code

   The first plot line uses alpha=0.3, which means it is 30% opaque (70% transparent). The second uses alpha=0.7, which is 70% opaque.

2. Step 2: Compare transparency levels

   Lower alpha means more transparent, so the first line is more see-through than the second.

3. Final Answer:

   The first line will be more transparent than the second -> Option C

4. Quick Check:

   Lower alpha = more transparent [OK]

Hint: Lower alpha means more see-through, higher means more solid [OK]

Common Mistakes:

• Thinking higher alpha means more transparent
• Assuming alpha affects color, not transparency
• Confusing which line has which alpha

4. Identify the error in this code that tries to set alpha transparency for overlapping scatter points:

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [4, 5, 6]
plt.scatter(x, y, alpha='0.5')
plt.show()

medium

A. Alpha parameter is not valid for scatter plots

B. Alpha value should be a float, not a string

C. x and y lists must be numpy arrays

D. Missing plt.figure() before plotting

Solution

1. Step 1: Check the alpha parameter type

   Alpha expects a numeric float between 0 and 1, but here it is passed as a string '0.5'.

2. Step 2: Understand matplotlib type requirements

   Passing alpha as a string will cause a TypeError or unexpected behavior.

3. Final Answer:

   Alpha value should be a float, not a string -> Option B

4. Quick Check:

   Alpha must be float, not string [OK]

Hint: Alpha must be a number, not text, for transparency [OK]

Common Mistakes:

• Passing alpha as a string instead of float
• Thinking alpha is invalid for scatter plots
• Assuming lists must be numpy arrays

5. You have two overlapping histograms plotted with:

plt.hist(data1, bins=20, alpha=0.4)
plt.hist(data2, bins=20, alpha=0.6)

How would you adjust alpha values to better see where the two datasets overlap clearly?

hard

A. Set both alphas to 0.5 for balanced transparency

B. Set alpha of data1 to 1 and data2 to 0 for full visibility

C. Set both alphas to 1 to make histograms solid

D. Set alpha of data1 to 0 and data2 to 0.5 to hide data1

Solution

1. Step 1: Understand the goal of alpha in overlapping plots

   Alpha helps see overlapping areas by making plots semi-transparent.

2. Step 2: Balance transparency for both datasets

   Setting both alphas to 0.5 makes both histograms equally transparent, so overlaps are visible clearly.

3. Step 3: Evaluate other options

   Setting alpha to 1 or 0 hides or fully shows one dataset, losing overlap clarity.

4. Final Answer:

   Set both alphas to 0.5 for balanced transparency -> Option A

5. Quick Check:

   Equal alpha ~ balanced overlap visibility [OK]

Hint: Use equal moderate alpha for clear overlap visibility [OK]

Common Mistakes:

• Making one histogram fully opaque and the other invisible
• Using alpha=1 which hides overlap
• Setting alpha=0 hides data completely