Legend control in R Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a legend to the plot.

R Programming

plot(x, y, type = 'l')
legend('topright', legend = [1], col = 'blue', lty = 1)

Drag options to blanks, or click blank then click option'

Ac('Data Line')

B'Data Line'

CData Line

D['Data Line']

Attempts:

3 left

💡 Hint

Common Mistakes

Using an unquoted string causes an error.

Using square brackets is not valid in R for vectors.

The legend function expects the legend argument as a character vector, so c('Data Line') is correct.

2fill in blank

medium

Complete the code to set the legend title.

R Programming

plot(x, y, type = 'l')
legend('topright', legend = c('Data'), title = [1], col = 'red', lty = 1)

Drag options to blanks, or click blank then click option'

Ac('Data Title')

Bc(Data Title)

C'Data Title'

DData Title

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes causes an error.

Using c() creates a vector, but title expects a single string.

The title argument expects a string, so it must be in quotes like 'Data Title'.

3fill in blank

hard

Fix the error in the legend position argument.

R Programming

plot(x, y, type = 'l')
legend([1], legend = c('Line'), col = 'green', lty = 1)

Drag options to blanks, or click blank then click option'

A'topright'

Btop right

Ctopright

Drighttop

Attempts:

3 left

💡 Hint

Common Mistakes

Using unquoted position names causes errors.

Using invalid position names causes the legend not to appear.

The position argument must be a string in quotes, so 'topright' is correct.

4fill in blank

hard

Fill both blanks to create a legend with two items and different colors.

R Programming

plot(x, y, type = 'l')
legend('bottomleft', legend = [1], col = [2], lty = 1)

Drag options to blanks, or click blank then click option'

Ac('Line1', 'Line2')

Bc('Line1')

Cc('red', 'blue')

Dc('green', 'yellow')

Attempts:

3 left

💡 Hint

Common Mistakes

Using only one label or one color causes mismatch errors.

Not using c() creates invalid arguments.

The legend needs two labels and two colors, so c('Line1', 'Line2') and c('red', 'blue') are correct.

5fill in blank

hard

Fill all three blanks to customize the legend with line types and a title.

R Programming

plot(x, y, type = 'l')
legend('top', legend = [1], col = [2], lty = [3], title = 'Legend')

Drag options to blanks, or click blank then click option'

Ac('A', 'B')

Bc('black', 'orange')

Cc(1, 2)

Dc('red', 'blue')

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatched vector lengths cause errors.

Using color names not recognized by R causes warnings.

The legend labels are c('A', 'B'), colors c('black', 'orange'), and line types c(1, 2) for solid and dashed lines.

Practice

1. What is the main purpose of the legend() function in R plotting?

easy

A. To explain the meaning of colors or symbols used in the plot

B. To change the plot background color

C. To add titles to the plot axes

D. To save the plot as an image file

Solution

1. Step 1: Understand the role of legend in plots

   A legend helps viewers understand what different colors or symbols represent in a plot.

2. Step 2: Identify the function purpose

   The legend() function in R adds this explanatory box to the plot.

3. Final Answer:

   To explain the meaning of colors or symbols used in the plot -> Option A

4. Quick Check:

   legend() explains plot symbols/colors [OK]

Hint: Legend explains plot symbols and colors clearly [OK]

Common Mistakes:

• Confusing legend() with title() or axis labeling functions
• Thinking legend() changes plot colors
• Assuming legend() saves plots

2. Which of the following is the correct syntax to add a legend at the top right corner with labels "A" and "B" in R?

easy

A. legend(topright, labels = c("A", "B"))

B. legend("topright", legend = c("A", "B"))

C. legend(position = "topright", c("A", "B"))

D. legend(c("A", "B"), position = "topright")

Solution

1. Step 1: Recall legend() function syntax

   The first argument is the position as a string, then legend= specifies labels.

2. Step 2: Check each option

   legend("topright", legend = c("A", "B")) uses correct syntax: legend("topright", legend = c("A", "B")). Others misuse argument names or order.

3. Final Answer:

   legend("topright", legend = c("A", "B")) -> Option B

4. Quick Check:

   Correct legend() syntax = legend("topright", legend = c("A", "B")) [OK]

Hint: First argument is position string, use legend= for labels [OK]

Common Mistakes:

• Omitting quotes around position
• Using wrong argument names like labels instead of legend
• Passing labels before position

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

plot(1:3, col = c("red", "green", "blue"), pch = 16)
legend("bottomleft", legend = c("Red", "Green", "Blue"), col = c("red", "green", "blue"), pch = 16)

medium

A. A plot with colored points but legend shows empty boxes

B. A plot with default black points and no legend

C. A plot with three points colored red, green, blue and a legend at bottom left showing these colors with filled circles

D. Syntax error due to missing parameters

Solution

1. Step 1: Understand plot() parameters

   Points 1,2,3 are plotted with colors red, green, blue and filled circle symbol (pch=16).

2. Step 2: Understand legend() parameters

   Legend placed at bottom left with matching labels, colors, and symbols (filled circles).

3. Final Answer:

   A plot with three points colored red, green, blue and a legend at bottom left showing these colors with filled circles -> Option C

4. Quick Check:

   plot colors and legend match correctly [OK]

Hint: Legend colors and symbols must match plot points [OK]

Common Mistakes:

• Assuming legend uses default black color
• Forgetting to set pch in legend
• Confusing legend position strings

4. Identify the error in this R code snippet that tries to add a legend:

plot(1:5, col = 1:5, pch = 15)
legend("top", labels = c("One", "Two", "Three", "Four", "Five"), col = 1:5, pch = 15)

medium

A. The argument 'labels' should be 'legend' in the legend() function

B. The position 'top' is invalid in legend()

C. The colors 1:5 are not valid color codes

D. pch = 15 is not a valid plotting character

Solution

1. Step 1: Check legend() argument names

   The correct argument for labels is 'legend', not 'labels'. Using 'labels' causes no legend to appear.

2. Step 2: Verify position and other parameters

   Position 'top' is valid, colors 1:5 are valid R colors, pch=15 is a valid filled square symbol.

3. Final Answer:

   The argument 'labels' should be 'legend' in the legend() function -> Option A

4. Quick Check:

   Use legend=, not labels= in legend() [OK]

Hint: Use legend= for labels inside legend() [OK]

Common Mistakes:

• Using 'labels' instead of 'legend' argument
• Thinking 'top' is invalid position
• Confusing color codes with integers

5. You want to create a plot with two groups of points: group A in red circles and group B in blue triangles. How do you add a legend that clearly shows these groups with correct symbols and colors?

hard

A. legend("topright", legend = c("Group A", "Group B"), col = c(1, 2), pch = c(16, 17))

B. legend("topright", labels = c("Group A", "Group B"), colors = c("red", "blue"), symbols = c(1, 2))

C. legend("topright", legend = c("Group A", "Group B"), color = c("red", "blue"), pch = c("circle", "triangle"))

D. legend("topright", legend = c("Group A", "Group B"), col = c("red", "blue"), pch = c(1, 2))

Solution

1. Step 1: Identify correct legend arguments

   Use 'legend' for labels, 'col' for colors, and 'pch' for symbols (numeric codes).

2. Step 2: Match symbols and colors to groups

   Red circles use pch=1, blue triangles use pch=2, colors as "red" and "blue" strings.

3. Step 3: Check options

   legend("topright", legend = c("Group A", "Group B"), col = c("red", "blue"), pch = c(1, 2)) uses correct argument names and values. legend("topright", labels = c("Group A", "Group B"), colors = c("red", "blue"), symbols = c(1, 2)) uses wrong argument names. legend("topright", legend = c("Group A", "Group B"), color = c("red", "blue"), pch = c("circle", "triangle")) uses invalid pch values. legend("topright", legend = c("Group A", "Group B"), col = c(1, 2), pch = c(16, 17)) uses numeric colors 1 and 2 which are black and red, not blue.

4. Final Answer:

   legend("topright", legend = c("Group A", "Group B"), col = c("red", "blue"), pch = c(1, 2)) -> Option D

5. Quick Check:

   Correct legend args and matching colors/symbols [OK]

Hint: Use legend=, col= colors, pch= numeric symbols [OK]

Common Mistakes:

• Using wrong argument names like labels or colors
• Using string names for pch instead of numbers
• Using numeric colors that don't match intended colors