Vertical bar chart with plt.bar in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple vertical bar chart code

What will be the height of the bars in the vertical bar chart produced by this code?

Matplotlib

import matplotlib.pyplot as plt

x = ['A', 'B', 'C']
y = [5, 3, 7]
plt.bar(x, y)
plt.show()

A [5, 7, 3]

B [3, 5, 7]

C [7, 5, 3]

D [5, 3, 7]

💡 Hint

Look at the y values passed to plt.bar as heights.

The heights of the bars correspond exactly to the y list values in order: 5, 3, then 7.

❓ data_output

intermediate

Number of bars in the chart

How many bars will be displayed by this code snippet?

Matplotlib

import matplotlib.pyplot as plt

labels = ['X', 'Y', 'Z', 'W']
values = [10, 20, 15, 5]
plt.bar(labels, values)
plt.show()

A 4

B 2

C 5

D 3

💡 Hint

Count the number of labels or values given.

There are 4 labels and 4 values, so 4 bars will be drawn.

❓ visualization

advanced

Identify the bar colors in the chart

Given this code, what color will the bars be?

Matplotlib

import matplotlib.pyplot as plt

categories = ['Cat1', 'Cat2']
heights = [8, 12]
plt.bar(categories, heights, color='green')
plt.show()

A Green

B Red

C Blue

D Yellow

💡 Hint

Check the color argument in plt.bar.

The color argument is set to 'green', so bars will be green.

🔧 Debug

advanced

Error type from mismatched data lengths

What error will this code raise when run?

Matplotlib

import matplotlib.pyplot as plt

labels = ['A', 'B', 'C']
values = [1, 2]
plt.bar(labels, values)
plt.show()

A TypeError: unsupported operand type(s)

B ValueError: x and height must be the same length

C IndexError: list index out of range

D No error, chart displays with missing bars

💡 Hint

Check if the number of labels matches the number of values.

Matplotlib requires the x and height lists to be the same length; otherwise, it raises a ValueError.

🚀 Application

expert

Calculate total bar height from grouped data

You have this data and code. What is the total height of all bars combined?

Matplotlib

import matplotlib.pyplot as plt

fruits = ['Apple', 'Banana', 'Cherry']
counts = [4, 7, 3]
plt.bar(fruits, counts)
plt.show()

A 10

B 7

C 14

D 4

💡 Hint

Add all the values in the counts list.

Sum of counts is 4 + 7 + 3 = 14, which is the total height of all bars combined.

Practice

1. What does the plt.bar() function do in matplotlib?

easy

A. It creates a scatter plot with dots.

B. It creates a line chart connecting data points.

C. It creates a pie chart showing proportions.

D. It creates a vertical bar chart using categories and their values.

Solution

1. Step 1: Understand the purpose of plt.bar()

   The plt.bar() function is designed to draw vertical bars representing data values for different categories.

2. Step 2: Compare with other chart types

   Other functions like plt.plot() create line charts, plt.scatter() creates scatter plots, and plt.pie() creates pie charts, so they are different from plt.bar().

3. Final Answer:

   It creates a vertical bar chart using categories and their values. -> Option D

4. Quick Check:

   plt.bar() = vertical bar chart [OK]

Hint: Remember: bar = vertical bars for categories [OK]

Common Mistakes:

• Confusing bar chart with line or pie charts
• Thinking plt.bar() draws horizontal bars
• Mixing up plt.bar() with scatter plot functions

2. Which of the following is the correct syntax to create a vertical bar chart with categories ['A', 'B', 'C'] and values [5, 7, 3]?

easy

A. plt.bar(['A', 'B', 'C'], [5, 7, 3])

B. plt.bar([5, 7, 3], ['A', 'B', 'C'])

C. plt.bar('A', 5, 'B', 7, 'C', 3)

D. plt.bar([5, 7, 3])

Solution

1. Step 1: Identify correct parameter order for plt.bar()

   The first argument should be the categories (x-axis labels), and the second argument should be the heights (values).

2. Step 2: Check each option

   plt.bar(['A', 'B', 'C'], [5, 7, 3]) uses categories first and values second, which is correct. plt.bar([5, 7, 3], ['A', 'B', 'C']) reverses them, which is wrong. plt.bar('A', 5, 'B', 7, 'C', 3) uses multiple arguments incorrectly. plt.bar([5, 7, 3]) misses categories.

3. Final Answer:

   plt.bar(['A', 'B', 'C'], [5, 7, 3]) -> Option A

4. Quick Check:

   Categories first, values second in plt.bar() [OK]

Hint: Categories go first, values second in plt.bar() [OK]

Common Mistakes:

• Swapping categories and values order
• Passing multiple separate arguments instead of lists
• Omitting categories or values

3. What will be the height of the bar labeled 'B' in this code?

import matplotlib.pyplot as plt
categories = ['A', 'B', 'C']
values = [4, 9, 2]
plt.bar(categories, values)
plt.show()

medium

A. 9

B. Cannot determine

C. 2

D. 4

Solution

1. Step 1: Match categories with values

   The list categories is ['A', 'B', 'C'] and values is [4, 9, 2]. The bar for 'B' corresponds to the second value, 9.

2. Step 2: Confirm bar height for 'B'

   Since 'B' is the second category, its bar height is the second value, which is 9.

3. Final Answer:

   9 -> Option A

4. Quick Check:

   Bar 'B' height = 9 [OK]

Hint: Match category index to value index [OK]

Common Mistakes:

• Mixing up category and value positions
• Assuming bar height is category label
• Ignoring list order

4. Identify the error in this code that tries to plot a vertical bar chart:

import matplotlib.pyplot as plt
categories = ['X', 'Y', 'Z']
values = [3, 6]
plt.bar(categories, values)
plt.show()

medium

A. The categories list has fewer items than values list.

B. plt.bar() requires numeric categories, not strings.

C. The values list has fewer items than categories list.

D. plt.show() is missing.

Solution

1. Step 1: Compare lengths of categories and values

   The categories list has 3 items ['X', 'Y', 'Z'], but values list has only 2 items [3, 6]. This mismatch causes an error.

2. Step 2: Understand plt.bar() requirements

   Each category must have a corresponding value. Since values are fewer, matplotlib will raise a ValueError.

3. Final Answer:

   The values list has fewer items than categories list. -> Option C

4. Quick Check:

   Lengths must match: values shorter than categories [OK]

Hint: Check categories and values list lengths match [OK]

Common Mistakes:

• Ignoring length mismatch between categories and values
• Thinking categories must be numeric
• Forgetting plt.show()

5. You want to create a vertical bar chart showing sales for products ['Pen', 'Notebook', 'Eraser'] with sales [10, 15, 10]. You also want to highlight the product with the highest sales in red and others in blue. Which code correctly achieves this?

hard

A. plt.bar(['Pen', 'Notebook', 'Eraser'], [10, 15, 10], color='red')
plt.show()

B. colors = ['blue', 'red', 'blue']
plt.bar(['Pen', 'Notebook', 'Eraser'], [10, 15, 10], color=colors)
plt.show()

C. colors = ['red', 'blue', 'blue']
plt.bar(['Pen', 'Notebook', 'Eraser'], [10, 15, 10], color=colors)
plt.show()

D. plt.bar(['Pen', 'Notebook', 'Eraser'], [10, 15, 10])
plt.show()

Solution

1. Step 1: Identify highest sales product

   The sales are [10, 15, 10], so 'Notebook' has the highest sales (15).

2. Step 2: Assign colors to highlight highest sales

   We want 'Notebook' bar in red and others in blue. colors = ['blue', 'red', 'blue'] plt.bar(['Pen', 'Notebook', 'Eraser'], [10, 15, 10], color=colors) plt.show() sets colors as ['blue', 'red', 'blue'], matching the order of products.

3. Step 3: Check other options

   plt.bar(['Pen', 'Notebook', 'Eraser'], [10, 15, 10], color='red') plt.show() colors all bars red, not highlighting only highest. colors = ['red', 'blue', 'blue'] plt.bar(['Pen', 'Notebook', 'Eraser'], [10, 15, 10], color=colors) plt.show() colors 'Pen' red incorrectly. plt.bar(['Pen', 'Notebook', 'Eraser'], [10, 15, 10]) plt.show() uses default colors without highlight.

4. Final Answer:

   colors = ['blue', 'red', 'blue'] plt.bar(['Pen', 'Notebook', 'Eraser'], [10, 15, 10], color=colors) plt.show() -> Option B

5. Quick Check:

   Color list matches bars, red on highest sales [OK]

Hint: Use color list matching bars, red for highest [OK]

Common Mistakes:

• Coloring all bars same color
• Assigning wrong color order
• Not using color parameter