Basic histogram with plt.hist in Matplotlib - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a histogram?

A histogram is a type of chart that shows how data is spread out by grouping values into bins and counting how many values fall into each bin.

beginner

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

plt.hist() creates a histogram plot by dividing data into bins and showing the frequency of data points in each bin.

beginner

Which parameter in plt.hist() controls the number of bins?

The bins parameter controls how many bins the data is divided into in the histogram.

beginner

How can you add labels to the x-axis and y-axis in a histogram?

Use plt.xlabel('label') for the x-axis and plt.ylabel('label') for the y-axis to add labels.

beginner

What does the height of each bar in a histogram represent?

The height of each bar shows how many data points fall into that bin range.

What does the bins parameter in plt.hist() control?

A Color of the bars

B Width of the bars

C Number of bars in the histogram

D Title of the plot

The bins parameter sets how many groups (bars) the data is split into.

Which matplotlib function is used to create a histogram?

A <code>plt.bar()</code>

B <code>plt.hist()</code>

C <code>plt.plot()</code>

D <code>plt.scatter()</code>

plt.hist() is the function to create histograms.

What does the height of a histogram bar represent?

A The number of data points in the bin

B The maximum value in the bin

C The average value in the bin

D The bin width

The height shows how many data points fall into that bin.

How do you add a label to the x-axis in matplotlib?

A <code>plt.xlabel()</code>

B <code>plt.title()</code>

C <code>plt.ylabel()</code>

D <code>plt.legend()</code>

plt.xlabel() adds a label to the x-axis.

If you want more detailed bins in your histogram, what should you do?

A Decrease the number of bins

B Add a title

C Change the color

D Increase the number of bins

Increasing bins divides data into smaller groups for more detail.

Explain how to create a basic histogram using matplotlib's plt.hist() function.

Think about the steps from data to visual bars.

Describe what information a histogram provides about a dataset.

Imagine explaining to a friend how data is spread out.

Practice

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

easy

A. It creates a scatter plot of two variables.

B. It plots a line graph connecting data points.

C. It creates a histogram to show data frequency distribution.

D. It generates a pie chart to show proportions.

Solution

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

   The function plt.hist() groups data into bins and shows how often values fall into each bin.

2. Step 2: Compare with other plot types

   Line graphs connect points, scatter plots show points, and pie charts show parts of a whole, which are different from histograms.

3. Final Answer:

   It creates a histogram to show data frequency distribution. -> Option C

4. Quick Check:

   Histogram = frequency bars [OK]

Hint: Histograms show frequency counts in bars [OK]

Common Mistakes:

• Confusing histogram with line plot
• Thinking plt.hist() creates scatter plots
• Mixing histogram with pie chart

2. Which of the following is the correct way to create a histogram of data stored in a list named data using matplotlib?

easy

A. plt.histogram(data); plt.show()

B. plt.plot.hist(data); plt.show()

C. plt.bar(data); plt.show()

D. plt.hist(data); plt.show()

Solution

1. Step 1: Recall the correct function name

   The correct function to create a histogram is plt.hist(), not plt.plot.hist() or plt.histogram().

2. Step 2: Confirm usage with plt.show()

   After plotting, plt.show() displays the figure. Using plt.bar() creates bar charts, not histograms.

3. Final Answer:

   plt.hist(data); plt.show() -> Option D

4. Quick Check:

   Use plt.hist() for histograms [OK]

Hint: Use plt.hist() to plot histograms, then plt.show() [OK]

Common Mistakes:

• Using plt.plot.hist() which is invalid
• Calling plt.histogram() which doesn't exist
• Using plt.bar() instead of plt.hist()

3. What will be the output of the following code?

import matplotlib.pyplot as plt

values = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]
plt.hist(values, bins=4)
plt.show()

medium

A. A histogram with 4 bars showing frequencies: 1, 2, 3, and 4

B. A histogram with 10 bars, one for each value

C. A line plot connecting the values 1 to 4

D. An error because bins must be greater than 4

Solution

1. Step 1: Understand bins parameter

   Setting bins=4 divides the data range into 4 intervals (bins).

2. Step 2: Count frequencies in each bin

   The values 1, 2, 3, and 4 fall into these bins with frequencies 1, 2, 3, and 4 respectively, so the histogram shows 4 bars with these heights.

3. Final Answer:

   A histogram with 4 bars showing frequencies: 1, 2, 3, and 4 -> Option A

4. Quick Check:

   Bins split data; bars show counts [OK]

Hint: Bins split data; bar height = count in bin [OK]

Common Mistakes:

• Thinking bins=4 means 10 bars
• Confusing histogram with line plot
• Assuming bins must be greater than data length

4. Identify the error in this code snippet that tries to plot a histogram:

import matplotlib.pyplot as plt

values = [5, 7, 8, 5, 6]
plt.hist(values, bins='five')
plt.show()

medium

A. plt.show() is missing and needed to display the plot.

B. The bins parameter should be an integer, not a string.

C. plt.hist() requires a color argument.

D. The values list contains invalid data types.

Solution

1. Step 1: Check bins parameter type

   The bins argument must be an integer or sequence of numbers, not a string like 'five'.

2. Step 2: Verify other parts

   The data list is valid, color is optional, and plt.show() is present, so no other errors.

3. Final Answer:

   The bins parameter should be an integer, not a string. -> Option B

4. Quick Check:

   Bins must be int or list, not string [OK]

Hint: Bins must be number, not text [OK]

Common Mistakes:

• Passing bins as a string instead of integer
• Thinking color is required
• Forgetting plt.show() (not the case here)

5. You have a dataset of exam scores ranging from 0 to 100. You want to create a histogram with 5 equal-width bins and color the bars blue. Which code snippet correctly achieves this?

hard

A. plt.hist(scores, bins=5, color='blue'); plt.show()

B. plt.hist(scores, bins=[0,20,50,70,90,100], color='blue'); plt.show()

C. plt.hist(scores, bins=5, colors='blue'); plt.show()

D. plt.hist(scores, bins=5, color=blue); plt.show()

Solution

1. Step 1: Use bins=integer for equal-width

   bins=5 automatically divides the range (0-100) into 5 equal-width bins of 20 units each: [0-20, 20-40, 40-60, 60-80, 80-100].

2. Step 2: Color the bars blue

   The color='blue' parameter sets the bar color correctly.

3. Step 3: Eliminate distractors

   B specifies explicit bins [0,20,50,70,90,100] with unequal widths (20,30,20,20,10); C uses invalid 'colors'; D has unquoted 'blue' causing NameError.

4. Final Answer:

   plt.hist(scores, bins=5, color='blue'); plt.show() -> Option A

5. Quick Check:

   bins=5 (equal width) + color='blue' [OK]

Hint: bins=5 for equal-width over data range, color='blue' [OK]

Common Mistakes:

• Using explicit bins with unequal widths
• Using 'colors' instead of 'color'
• Using color=blue without quotes