plot() function basics in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this plot command?

Consider the following MATLAB code snippet:

x = 1:5;
y = x.^2;
plot(x, y)

What does this plot show?

MATLAB

x = 1:5;
y = x.^2;
plot(x, y)

A A line graph with points (1,1), (2,4), (3,9), (4,16), (5,25) connected by straight lines

B A scatter plot with points (1,1), (2,4), (3,9), (4,16), (5,25) but no lines connecting them

C A bar chart with bars at x=1 to 5 with heights 1,4,9,16,25

D A pie chart showing the values 1,4,9,16,25 as slices

💡 Hint

The plot() function connects points with lines by default.

The plot(x, y) command in MATLAB creates a line plot connecting the points defined by vectors x and y. Here, y = x squared, so the points are (1,1), (2,4), (3,9), (4,16), (5,25).

❓ Predict Output

intermediate

What color and marker style does this plot use?

Look at this MATLAB code:

x = 0:0.5:2;
y = sin(pi*x);
plot(x, y, 'ro--')

What does the plot look like?

MATLAB

x = 0:0.5:2;
y = sin(pi*x);
plot(x, y, 'ro--')

A Black stars connected by dash-dot lines

B Blue squares connected by solid lines

C Green triangles connected by dotted lines

D Red circles connected by dashed lines

💡 Hint

The format string 'ro--' means red color, circle marker, dashed line.

In MATLAB plot format strings, 'r' means red color, 'o' means circle marker, and '--' means dashed line. So the plot shows red circles connected by dashed lines.

🔧 Debug

advanced

Why does this plot command cause an error?

Examine this MATLAB code:

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

What error occurs and why?

MATLAB

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

A No error, plot shows points with missing values

B Error because y contains even numbers only

C Error because x and y vectors have different lengths

D Error because x is not a column vector

💡 Hint

Check if x and y have the same number of elements.

The plot function requires x and y to have the same length. Here, x has 5 elements but y has only 3, so MATLAB throws an error about dimension mismatch.

🧠 Conceptual

advanced

What happens if you call plot with a single vector?

In MATLAB, what does the command plot([3 1 4 1 5]) do?

MATLAB

plot([3 1 4 1 5])

A Plots the vector values on the y-axis against their indices on the x-axis

B Plots the vector values on the x-axis against their indices on the y-axis

C Raises an error because two arguments are required

D Plots a horizontal line at y=3

💡 Hint

When only one vector is given, MATLAB uses indices as x values.

If plot is called with one vector, MATLAB plots the vector values on the y-axis and uses the indices (1,2,3,...) as x-axis values.

❓ Predict Output

expert

What is the output of this multi-line plot command?

Consider this MATLAB code:

x = 0:pi/4:pi;
y1 = sin(x);
y2 = cos(x);
plot(x, y1, '-b', x, y2, ':r')

What does the plot show?

MATLAB

x = 0:pi/4:pi;
y1 = sin(x);
y2 = cos(x);
plot(x, y1, '-b', x, y2, ':r')

A Only sin(x) plotted with a blue dashed line

B A blue solid line for sin(x) and a red dotted line for cos(x)

C A red solid line for sin(x) and a blue dotted line for cos(x)

D Only cos(x) plotted with a red solid line

💡 Hint

The format '-b' means blue solid line, ':r' means red dotted line.

The plot command plots two lines: sin(x) with a blue solid line and cos(x) with a red dotted line, using the specified format strings.

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