Saving plots (ggsave) in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output file type saved by this ggsave command?

Consider the following R code that creates a plot and saves it using ggsave. What file type will be created?

R Programming

library(ggplot2)
p <- ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point()
ggsave(filename = "myplot.png", plot = p)

A A PDF file named 'myplot.png' is saved

B A PNG file named 'myplot.png' is saved

C A JPEG file named 'myplot.png' is saved

D No file is saved because the extension is incorrect

💡 Hint

Check the file extension in the filename argument.

ggsave uses the file extension to determine the output format. Since the filename ends with '.png', it saves a PNG file.

❓ Predict Output

intermediate

What will be the dimensions of the saved plot?

What are the width and height in inches of the saved plot from this code?

R Programming

library(ggplot2)
p <- ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point()
ggsave(filename = "plot.pdf", plot = p, width = 5, height = 4)

A Width 5 inches, Height 4 inches

B Width 4 inches, Height 5 inches

C Width and height default to 7 inches each

D Width and height are ignored and plot size depends on screen size

💡 Hint

Look at the width and height arguments in ggsave.

The width and height arguments set the size of the saved plot in inches. Here, width=5 and height=4 inches.

❓ Predict Output

advanced

What error does this ggsave command produce?

What error will this code produce when run?

R Programming

library(ggplot2)
p <- ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point()
ggsave(filename = "plot", plot = p)

A Error: Unknown graphics device 'plot'

B No error, saves a file named 'plot' with default format

C Error: No file extension provided in filename

D Error: plot object is missing

💡 Hint

ggsave needs a file extension to know the format.

ggsave requires a file extension to determine the output format. Without it, it throws an error about missing extension.

❓ Predict Output

advanced

What is the resolution (dpi) of the saved image?

What is the dpi (dots per inch) of the saved plot from this code?

R Programming

library(ggplot2)
p <- ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point()
ggsave(filename = "plot.png", plot = p, dpi = 300)

A 300 dpi

B 150 dpi

C 72 dpi

D Default dpi depends on device and is not set here

💡 Hint

Check the dpi argument in ggsave.

The dpi argument sets the resolution of the saved image. Here it is explicitly set to 300 dpi.

❓ Predict Output

expert

What is the output of this code saving multiple plots in a loop?

What files will be created and what will be their contents after running this code?

R Programming

library(ggplot2)
for(i in 1:3) {
  p <- ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point() + ggtitle(paste("Plot", i))
  ggsave(filename = paste0("plot", i, ".png"), plot = p, width = 4, height = 3)
}

A No files are saved because ggsave cannot be used inside a loop

B One PNG file named plot3.png with the last plot titled 'Plot 3'

C Three PNG files named plot1.png, plot2.png, plot3.png but all plots have the title 'Plot 3'

D Three PNG files named plot1.png, plot2.png, plot3.png each with a scatter plot titled 'Plot 1', 'Plot 2', and 'Plot 3' respectively

💡 Hint

Look at the filename and title inside the loop.

The loop creates three separate plots with different titles and saves each to a uniquely named file.

Practice

1. What does the ggsave() function do in R's ggplot2 package?

easy

A. Saves the current or specified plot as an image file

B. Displays the plot on the screen

C. Creates a new plot object

D. Deletes the last plot created

Solution

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

   The ggsave() function is designed to save plots created by ggplot2 as image files like PNG, PDF, or JPEG.

2. Step 2: Differentiate from other plot functions

   Unlike functions that display or create plots, ggsave() specifically saves the plot to a file on disk.

3. Final Answer:

   Saves the current or specified plot as an image file -> Option A

4. Quick Check:

   ggsave() = Save plot [OK]

Hint: Remember: ggsave saves plots to files, not just shows them [OK]

Common Mistakes:

• Thinking ggsave displays the plot
• Confusing ggsave with plot creation
• Assuming ggsave deletes plots

2. Which of the following is the correct syntax to save the last plot as a PNG file named "plot.png" with default size using ggsave()?

easy

A. ggsave("plot.png", plot)

B. ggsave(plot.png)

C. ggsave("plot.png")

D. ggsave(plot, "plot.png")

Solution

1. Step 1: Check the basic ggsave syntax

   The simplest way to save the last plot is to call ggsave() with the filename as a string, e.g., ggsave("plot.png").

2. Step 2: Identify incorrect syntax

   ggsave(plot.png) lacks quotes around filename, causing an error. Options A and D reference an undefined plot or have wrong argument order.

3. Final Answer:

   ggsave("plot.png") -> Option C

4. Quick Check:

   Filename as string = ggsave("plot.png") [OK]

Hint: Filename must be a quoted string in ggsave() [OK]

Common Mistakes:

• Forgetting quotes around filename
• Passing plot object without naming argument
• Swapping argument order

3. What will be the result of this code?

library(ggplot2)
p <- ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point()
ggsave("myplot.pdf", plot = p, width = 5, height = 4)

medium

A. Saves the plot 'p' as a 5x4 inch PDF file named 'myplot.pdf'

B. Throws an error because width and height are missing units

C. Saves the last plot instead of 'p' ignoring the plot argument

D. Creates a PNG file instead of PDF

Solution

1. Step 1: Analyze the ggsave parameters

   The code specifies filename "myplot.pdf", plot object p, and size 5 by 4 inches. This is valid syntax.

2. Step 2: Understand default units and file type

   Width and height default to inches if units not specified. The file extension '.pdf' sets the output format to PDF.

3. Final Answer:

   Saves the plot 'p' as a 5x4 inch PDF file named 'myplot.pdf' -> Option A

4. Quick Check:

   ggsave() saves plot with given size and format [OK]

Hint: File extension sets format; width/height default to inches [OK]

Common Mistakes:

• Assuming width/height need units explicitly
• Thinking plot argument is ignored
• Confusing file format with extension

4. Identify the error in this code snippet:

library(ggplot2)
p <- ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point()
ggsave(filename = myplot.png, plot = p)

medium

A. Missing library call for ggsave

B. Plot object 'p' is not defined

C. ggsave() cannot save PNG files

D. Missing quotes around filename 'myplot.png'

Solution

1. Step 1: Check the filename argument

   The filename argument must be a string in quotes. Here, myplot.png is unquoted, causing an error.

2. Step 2: Verify other parts

   The plot object p is defined, PNG is supported, and library(ggplot2) is loaded, so no other errors.

3. Final Answer:

   Missing quotes around filename 'myplot.png' -> Option D

4. Quick Check:

   Filename must be quoted string [OK]

Hint: Always put quotes around filenames in ggsave() [OK]

Common Mistakes:

• Forgetting quotes around filename
• Assuming ggsave needs extra libraries
• Thinking PNG format is unsupported

5. You want to save two different ggplot objects, p1 and p2, as PNG files with different sizes. Which code correctly saves p1 as 6x4 inches and p2 as 8x6 inches?

hard

A. ggsave(p1, "p1.png", width = 6, height = 4)
ggsave(p2, "p2.png", width = 8, height = 6)

B. ggsave("p1.png", plot = p1, width = 6, height = 4)
ggsave("p2.png", plot = p2, width = 8, height = 6)

C. ggsave("p1.png", p1, 6, 4)
ggsave("p2.png", p2, 8, 6)

D. ggsave("p1.png", width = 6, height = 4)
ggsave("p2.png", width = 8, height = 6)

Solution

1. Step 1: Check correct argument order and naming

   ggsave("p1.png", plot = p1, width = 6, height = 4) ggsave("p2.png", plot = p2, width = 8, height = 6) uses correct syntax: filename as first argument (string), plot named with plot=, and width/height named.

2. Step 2: Identify errors in other options

   ggsave(p1, "p1.png", width = 6, height = 4) ggsave(p2, "p2.png", width = 8, height = 6) swaps plot and filename positionally without naming plot=, causing plot object to be assigned to filename (error). ggsave("p1.png", p1, 6, 4) ggsave("p2.png", p2, 8, 6) passes sizes positionally to wrong parameters (device/path). ggsave("p1.png", width = 6, height = 4) ggsave("p2.png", width = 8, height = 6) omits plot argument, so last plot saved instead.

3. Final Answer:

   Correctly saves both plots with specified sizes -> Option B

4. Quick Check:

   Filename first, plot named, size named = ggsave("p1.png", plot = p1, width = 6, height = 4) ggsave("p2.png", plot = p2, width = 8, height = 6) [OK]

Hint: Always name plot= and specify filename as first argument [OK]

Common Mistakes:

• Swapping filename and plot arguments
• Not naming plot argument
• Omitting plot argument saves last plot instead