plot() function basics in MATLAB - Step-by-Step Execution

Concept Flow - plot() function basics

Define x and y data

↓

Call plot(x, y)

↓

MATLAB creates a figure window

↓

Draw line connecting points (x(i), y(i))

↓

Display plot on screen

The plot() function takes x and y data, creates a figure, draws lines connecting points, and shows the plot.

Execution Sample

MATLAB

x = 1:4;
y = [10 20 15 25];
plot(x, y);

This code plots points (1,10), (2,20), (3,15), (4,25) connected by lines.

Execution Table

Step	Action	x	y	Plot State	Output
1	Define x	[1 2 3 4]	-	No plot yet	No output
2	Define y	-	[10 20 15 25]	No plot yet	No output
3	Call plot(x,y)	[1 2 3 4]	[10 20 15 25]	Figure created, line drawn connecting points	Plot window opens showing line graph
4	Display plot	-	-	Plot visible with points connected by lines	Graph shown on screen

💡 Plot displayed; no further code to execute

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
x	undefined	[1 2 3 4]	[1 2 3 4]	[1 2 3 4]	[1 2 3 4]
y	undefined	undefined	[10 20 15 25]	[10 20 15 25]	[10 20 15 25]

Key Moments - 3 Insights

Why does the plot() function need both x and y vectors?

Because plot() connects points using pairs (x(i), y(i)). Without both, it cannot know where to place points. See execution_table step 3 where plot(x,y) uses both vectors.

What happens if x and y have different lengths?

MATLAB will give an error because it cannot pair points properly. In the execution_table, x and y have the same length, so plot works.

Does plot() show the graph immediately after calling it?

Yes, plot() creates the figure and draws the line immediately, as shown in execution_table step 3 and 4.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the value of y?

A undefined

B [10 20 15 25]

C [1 2 3 4]

D empty

💡 Hint

Check the 'y' column at step 2 in execution_table; it is defined after step 2.

At which step does the plot window open?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Output' column in execution_table; plot window opens at step 3.

If x was [1 2 3] but y was [10 20 15 25], what would happen?

A Plot shows all 4 points ignoring x

B Plot shows only first 3 points

C MATLAB throws an error

D Plot shows no points

💡 Hint

Recall key_moments about vector length mismatch causing errors.

Concept Snapshot

plot(x, y) draws a line graph connecting points (x(i), y(i))
x and y must be vectors of the same length
MATLAB opens a figure window to display the plot
Use plot() to visualize data points simply

Full Transcript

The plot() function in MATLAB draws a line graph from x and y data vectors. First, you define x and y arrays of the same length. Then calling plot(x, y) creates a figure window and draws lines connecting each point pair. The execution table shows step-by-step how variables x and y are defined, then plot() is called, and finally the graph appears. Key moments include understanding why x and y must match in length and that plot() immediately displays the graph. The visual quiz tests understanding of variable values and plot behavior at each step.

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