Basic scatter plot with plt.scatter in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple scatter plot code

What will be the output of this code snippet that uses plt.scatter to plot points?

Matplotlib

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

A An error because plt.scatter requires color argument.

B A line plot connecting points (1,4), (2,5), and (3,6).

C A scatter plot with three points at coordinates (1,4), (2,5), and (3,6) displayed.

D A bar chart with bars at x=1,2,3 and heights 4,5,6.

💡 Hint

Remember, plt.scatter plots points, not lines or bars.

plt.scatter plots individual points at the given x and y coordinates. It does not connect points or create bars.

❓ data_output

intermediate

Number of points plotted by plt.scatter

How many points will be plotted by this code?

Matplotlib

import matplotlib.pyplot as plt
x = [10, 20, 30, 40]
y = [1, 2, 3, 4]
plt.scatter(x, y)
plt.show()

A 4

B 3

C 5

D 0

💡 Hint

Count the number of x or y values given.

There are 4 x values and 4 y values, so 4 points are plotted.

🔧 Debug

advanced

Identify the error in scatter plot code

What error will this code produce?

Matplotlib

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

A ValueError: x and y must be the same size

B TypeError: plt.scatter() missing required positional argument

C No error, plot shows 2 points

D IndexError: list index out of range

💡 Hint

Check if x and y lists have the same length.

plt.scatter requires x and y to have the same number of elements to pair points correctly.

❓ visualization

advanced

Effect of marker size in plt.scatter

What will be the visual difference when running this code compared to default marker size?

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [4, 5, 6]
plt.scatter(x, y, s=200)
plt.show()

A Points will appear smaller than default size.

B Code will raise an error due to invalid size argument.

C Points will be connected by lines.

D Points will appear larger than default size.

💡 Hint

The 's' parameter controls marker size in plt.scatter.

Setting s=200 increases the marker size, making points bigger on the plot.

🚀 Application

expert

Plotting two groups with different colors using plt.scatter

Which code correctly plots two groups of points in different colors on the same scatter plot?

A

import matplotlib.pyplot as plt
x1, y1 = [1,2], [3,4]
x2, y2 = [5,6], [7,8]
plt.scatter(x1, y1)
plt.scatter(x2, y2)
plt.show()

B

import matplotlib.pyplot as plt
x1, y1 = [1,2], [3,4]
x2, y2 = [5,6], [7,8]
plt.scatter(x1, y1, color='red')
plt.scatter(x2, y2, color='blue')
plt.show()

C

import matplotlib.pyplot as plt
x = [1,2,5,6]
y = [3,4,7,8]
plt.scatter(x, y, color=['red', 'red', 'blue', 'blue'])
plt.show()

D

import matplotlib.pyplot as plt
x = [1,2,5,6]
y = [3,4,7,8]
plt.scatter(x, y, c='red blue')
plt.show()

💡 Hint

Use separate plt.scatter calls with color argument for each group.

Option B plots two groups separately with specified colors. Option B fails because color list must be a sequence of valid color specs, but matplotlib expects a single color or array of floats for colormap. Option B plots both groups with default color. Option B has invalid color string causing error.

Practice

1. What does the plt.scatter(x, y) function do in matplotlib?

easy

A. It creates a line plot connecting x and y points.

B. It creates a scatter plot showing points for x and y data.

C. It creates a bar chart for x and y values.

D. It creates a histogram of x values only.

Solution

1. Step 1: Understand the purpose of plt.scatter

   The function plt.scatter is designed to plot individual points based on two lists or arrays, x and y, showing their relationship.

2. Step 2: Compare with other plot types

   Unlike line plots or bar charts, scatter plots show points without connecting lines or bars.

3. Final Answer:

   It creates a scatter plot showing points for x and y data. -> Option B

4. Quick Check:

   Scatter plot = points for x and y [OK]

Hint: Scatter plots show points, not lines or bars [OK]

Common Mistakes:

• Confusing scatter plot with line plot
• Thinking it creates bars or histograms
• Assuming it only plots x values

2. Which of the following is the correct syntax to create a scatter plot with x and y lists (x horizontal, y vertical) using matplotlib?

easy

A. plt.scatter(x, y)

B. plt.scatter(y, x)

C. plt.plot.scatter(x, y)

D. scatter.plot(x, y)

Solution

1. Step 1: Recall the correct function call

   The correct function to create a scatter plot in matplotlib is plt.scatter(x, y), where x and y are data lists.

2. Step 2: Identify incorrect syntax

   Options with reversed arguments or wrong method chaining like plt.plot.scatter or scatter.plot are invalid.

3. Final Answer:

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

4. Quick Check:

   Correct syntax = plt.scatter(x, y) [OK]

Hint: Use plt.scatter(x, y) exactly for scatter plots [OK]

Common Mistakes:

• Swapping x and y arguments
• Using plt.plot.scatter instead of plt.scatter
• Calling scatter on wrong object

3. What will be the output of this code?

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

medium

A. An error because plt.show() is missing.

B. A line plot connecting points (1,4), (2,5), and (3,6).

C. A scatter plot with points at (1,4), (2,5), and (3,6).

D. A bar chart with bars at x positions 1, 2, 3.

Solution

1. Step 1: Understand plt.scatter with given data

   The code plots points at coordinates from lists x and y: (1,4), (2,5), and (3,6).

2. Step 2: Confirm plt.show() displays the plot

   The plt.show() command displays the scatter plot window with these points.

3. Final Answer:

   A scatter plot with points at (1,4), (2,5), and (3,6). -> Option C

4. Quick Check:

   Scatter plot points = (1,4), (2,5), (3,6) [OK]

Hint: Scatter plots show points at (x[i], y[i]) pairs [OK]

Common Mistakes:

• Thinking plt.scatter creates line plots
• Assuming plt.show() is optional and causes error
• Confusing scatter with bar chart

4. Identify the error in this code snippet:

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

medium

A. plt.scatter requires numpy arrays, not lists.

B. The code is correct and runs without error.

C. plt.show() is missing to display the plot.

D. x and y lists have different lengths causing an error.

Solution

1. Step 1: Check lengths of x and y lists

   The x list has 3 elements, but y has only 2 elements. This mismatch causes plt.scatter to raise a ValueError.

2. Step 2: Understand plt.scatter input requirements

   Both x and y must have the same number of points to plot pairs correctly.

3. Final Answer:

   x and y lists have different lengths causing an error. -> Option D

4. Quick Check:

   Equal length x and y needed [OK]

Hint: x and y must be same length for scatter plot [OK]

Common Mistakes:

• Ignoring length mismatch of x and y
• Thinking plt.scatter needs numpy arrays only
• Assuming plt.show() missing causes error

5. You want to plot points with x = [1, 2, 3, 4] and y = [10, 20, 30, 40], but color points red if y > 25 and blue otherwise. Which code correctly does this?

hard

A. colors = ['red' if val > 25 else 'blue' for val in y]
plt.scatter(x, y, c=colors)
plt.show()

B. plt.scatter(x, y, color=['red' if y > 25 else 'blue'])
plt.show()

C. plt.scatter(x, y, c='red' if y > 25 else 'blue')
plt.show()

D. colors = ['red' if x > 25 else 'blue' for x in x]
plt.scatter(x, y, c=colors)
plt.show()

Solution

1. Step 1: Create color list based on y values

   We use a list comprehension to assign 'red' if y value is greater than 25, else 'blue'. This creates a color list matching each point.

2. Step 2: Pass color list to plt.scatter with c parameter

   Using c=colors applies the color list to points correctly. Then plt.show() displays the plot.

3. Final Answer:

   colors = ['red' if val > 25 else 'blue' for val in y] plt.scatter(x, y, c=colors) plt.show() -> Option A

4. Quick Check:

   Use list comprehension for colors, pass as c=colors [OK]

Hint: Use list comprehension for colors, pass with c=colors [OK]

Common Mistakes:

• Using 'color' instead of 'c' parameter
• Passing single color string instead of list
• Checking condition on x instead of y