plot() function basics in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic purpose of the plot() function in MATLAB?

The plot() function is used to create 2D line plots to visualize data points connected by lines.

beginner

How do you plot the points (1,2), (2,4), and (3,6) using plot()?

Use plot([1 2 3], [2 4 6]) to plot x-coordinates [1 2 3] against y-coordinates [2 4 6].

intermediate

What does the command plot(x, y, 'r--') do?

It plots the data points with a red dashed line connecting them. 'r' means red color, '--' means dashed line style.

beginner

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

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

intermediate

What happens if you call plot() multiple times without hold on?

Each new plot() call clears the previous plot, so only the last plot is visible unless you use hold on.

Which command plots a blue solid line connecting points (1,1), (2,4), and (3,9)?

A plot([1 2 3], [1 4 9], 'k.')

B plot([1 2 3], [1 4 9], 'r--')

C plot([1 2 3], [1 4 9], 'b-')

D plot([1 2 3], [1 4 9], 'g:')

The 'b-' option means blue solid line. Other options specify different colors or line styles.

What does plot(x, y) do if x is omitted and only plot(y) is called?

A Plots y against its index values starting at 1

B Throws an error

C Plots y against zeros

D Plots y against random values

If only y is given, MATLAB plots y values against their indices (1, 2, 3, ...).

How do you keep multiple plots visible on the same figure?

A Use <code>hold on</code>

B Use <code>hold off</code>

C Call <code>plot()</code> multiple times without any command

D Use <code>clear</code>

hold on keeps the current plot so new plots add to it instead of replacing.

Which command adds a title to a plot?

A xlabel('My Plot')

B legend('My Plot')

C ylabel('My Plot')

D title('My Plot')

title() adds a title above the plot.

What is the output of plot([1 2 3], [4 5 6], 'ko')?

A Red dashed line connecting points

B Black circle markers at points without connecting lines

C Black solid line connecting points

D Blue dotted line connecting points

'k' means black color, 'o' means circle markers, no line style means no connecting lines.

Explain how to create a simple 2D line plot in MATLAB using the plot() function.

Think about how you give x and y points to <code>plot()</code>.

Describe how to customize the color and style of a plot line using the plot() function.

Use a string like 'r--' to set red dashed lines.

Practice

1. What does the plot() function do in MATLAB?

easy

A. It calculates the sum of two numbers.

B. It clears the command window.

C. It saves data to a file.

D. It creates a graph to visually display data points.

Solution

1. Step 1: Understand the purpose of plot()

   The plot() function is used to draw graphs in MATLAB, which helps visualize data points.

2. Step 2: Compare with other options

   Options A, C, and D describe unrelated functions like summing numbers, saving files, or clearing the window, which plot() does not do.

3. Final Answer:

   It creates a graph to visually display data points. -> Option D

4. Quick Check:

   plot() = graph display [OK]

Hint: Remember: plot() always draws graphs [OK]

Common Mistakes:

• Confusing plot() with data calculation functions
• Thinking plot() saves files
• Assuming plot() clears the screen

2. Which of the following is the correct syntax to plot y versus x in MATLAB?

easy

A. plot(x:y)

B. plot(y, x)

C. plot(x, y)

D. plot(x + y)

Solution

1. Step 1: Recall the syntax of plot()

   The correct syntax to plot y versus x is plot(x, y), where x and y are vectors of the same length.

2. Step 2: Check other options

   plot(y, x) reverses the order, plotting x versus y, which is incorrect. plot(x:y) uses a colon operator incorrectly. plot(x + y) tries to plot the sum, which is invalid.

3. Final Answer:

   plot(x, y) -> Option C

4. Quick Check:

   plot(x, y) = correct syntax [OK]

Hint: Always put x first, then y in plot() [OK]

Common Mistakes:

• Swapping x and y arguments
• Using colon operator inside plot()
• Trying to plot sums instead of vectors

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

x = 1:3;
y = [2 4 6];
plot(x, y)

medium

A. A plot with points (1,2), (2,4), (3,6) connected by lines.

B. A plot with points (2,1), (4,2), (6,3) connected by lines.

C. An error because x and y have different lengths.

D. A plot with points (1,1), (2,2), (3,3) connected by lines.

Solution

1. Step 1: Analyze vectors x and y

   x is [1 2 3] and y is [2 4 6], both have length 3, so they can be plotted against each other.

2. Step 2: Understand plot output

   The plot will connect points (1,2), (2,4), and (3,6) with lines.

3. Final Answer:

   A plot with points (1,2), (2,4), (3,6) connected by lines. -> Option A

4. Quick Check:

   plot(x, y) = points (1,2), (2,4), (3,6) [OK]

Hint: Match x and y points by position in vectors [OK]

Common Mistakes:

• Mixing up x and y coordinates
• Assuming error due to vector length mismatch
• Confusing point coordinates

4. Identify the error in this MATLAB code:

x = 1:5;
y = [2 4 6];
plot(x, y)

medium

A. Vectors x and y have different lengths causing an error.

B. plot() function is misspelled.

C. x and y must be column vectors, not row vectors.

D. No error; the plot will display correctly.

Solution

1. Step 1: Check lengths of x and y

   x has length 5, y has length 3. plot() requires vectors of equal length.

2. Step 2: Understand consequence

   Because lengths differ, MATLAB will produce an error when trying to plot.

3. Final Answer:

   Vectors x and y have different lengths causing an error. -> Option A

4. Quick Check:

   Equal length vectors needed [OK]

Hint: Ensure x and y vectors have same length before plotting [OK]

Common Mistakes:

• Ignoring vector length mismatch
• Thinking row vs column vector matters here
• Assuming plot() spelling error

5. You want to plot the function y = x^2 for x from -3 to 3 with a red dashed line. Which code correctly does this?

hard

A. x = -3:0.1:3; y = x.^2; plot(x, y, 'blue-')

B. x = -3:0.1:3; y = x.^2; plot(x, y, 'r--')

C. x = -3:0.1:3; y = x^2; plot(y, x, 'r--')

D. x = -3:3; y = x^2; plot(x, y, 'red-dash')

Solution

1. Step 1: Define x and y correctly

   x must be a vector from -3 to 3 with small steps (e.g., 0.1). y = x squared element-wise uses .^2.

2. Step 2: Use correct plot style

   The style string 'r--' means red dashed line. x = -3:0.1:3; y = x.^2; plot(x, y, 'r--') uses this correctly.

3. Step 3: Check other options

   x = -3:3; y = x^2; plot(x, y, 'red-dash') uses invalid syntax for y and style. x = -3:0.1:3; y = x^2; plot(y, x, 'r--') swaps x and y. x = -3:0.1:3; y = x.^2; plot(x, y, 'blue-') uses blue solid line, not red dashed.

4. Final Answer:

   x = -3:0.1:3; y = x.^2; plot(x, y, 'r--') -> Option B

5. Quick Check:

   Element-wise power + 'r--' = red dashed line [OK]

Hint: Use .^ for element-wise power and 'r--' for red dashed line [OK]

Common Mistakes:

• Using ^ instead of .^ for vectors
• Swapping x and y in plot()
• Wrong line style string