Code chunks and output in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a code chunk in R Markdown?

A code chunk is a section in R Markdown where you write R code that will be run when the document is knitted. It starts with ```{r} and ends with ```.

beginner

How do you show the output of a code chunk in the knitted document?

By default, the output of the code inside a chunk is shown below the code when you knit the document.

beginner

What does the chunk option `echo=FALSE` do?

It hides the R code in the output but still shows the results of running the code.

intermediate

How can you prevent both code and output from showing in the final document?

Use chunk options `echo=FALSE` and `results='hide'` together to hide both code and output.

intermediate

What is the purpose of the chunk option `eval=FALSE`?

It prevents the code inside the chunk from running, so no output or results are generated.

In R Markdown, which symbol starts a code chunk?

A ```{r}

B ~~~{r}

C <code>

D //{r}

Code chunks in R Markdown start with three backticks followed by {r}.

What happens if you set `echo=FALSE` in a code chunk?

A Code runs twice

B Code and output are both hidden

C Code is shown but output is hidden

D Code is hidden but output is shown

Setting echo=FALSE hides the code but still shows the output.

Which chunk option stops the code from running?

A eval=FALSE

B echo=FALSE

C results='hide'

D include=FALSE

eval=FALSE prevents the code from running, so no output is produced.

How do you hide both code and output in a chunk?

A echo=TRUE only

B echo=FALSE and results='hide'

C eval=FALSE only

D No option needed

Using echo=FALSE hides code and results='hide' hides output.

What is the default behavior of code chunks in R Markdown?

A Show output only

B Hide code and output

C Show code and output

D Show code only

By default, both code and output are shown in the knitted document.

Explain what a code chunk is and how its output appears in an R Markdown document.

Think about how you write and run R code inside a document.

Describe how to control the visibility of code and output using chunk options.

Chunk options let you decide what the reader sees.

Practice

1. What is the purpose of an R code chunk in an R Markdown document?

easy

A. To run R code and show its output inline with text

B. To write plain text without any code

C. To create a table of contents automatically

D. To add images to the document

Solution

1. Step 1: Understand R Markdown structure

   R Markdown uses code chunks to embed R code inside text documents.

2. Step 2: Purpose of code chunks

   Code chunks run R code and display results inline, mixing code and output.

3. Final Answer:

   To run R code and show its output inline with text -> Option A

4. Quick Check:

   Code chunk = run code + show output [OK]

Hint: Code chunks run code and show output together [OK]

Common Mistakes:

• Thinking code chunks only hold text
• Confusing code chunks with document sections
• Assuming code chunks add images automatically

2. Which of the following is the correct way to start an R code chunk in R Markdown?

easy

A. {r}```

B. ```r

C. ```{r}

D. {r}

Solution

1. Step 1: Recall R Markdown chunk syntax

   R code chunks start with three backticks followed by {r} exactly.

2. Step 2: Check each option

   Only ```{r} correctly starts a chunk; others are invalid syntax.

3. Final Answer:

   ```{r} -> Option C

4. Quick Check:

   Chunk start = ```{r} [OK]

Hint: Start chunks with three backticks and {r} [OK]

Common Mistakes:

• Omitting curly braces around r
• Using only two backticks
• Confusing HTML tags with chunk syntax

3. What will be the output of this R code chunk?

```{r}
x <- 5
x * 2
```

medium

A. [1] 10

B. 10

C. x * 2

D. Error: object 'x' not found

Solution

1. Step 1: Understand the code execution

   Variable x is assigned 5, then multiplied by 2, resulting in 10.

2. Step 2: R output format

   R prints numeric output with index [1] before the value.

3. Final Answer:

   [1] 10 -> Option A

4. Quick Check:

   5 * 2 = 10 with [1] prefix [OK]

Hint: R prints numeric output with [1] prefix [OK]

Common Mistakes:

• Ignoring the [1] prefix in output
• Expecting just '10' without index
• Thinking code prints the expression itself

4. Identify the error in this R code chunk:

```{r}
print("Hello)
```

medium

A. Missing closing curly brace for chunk

B. Missing closing quote in the string

C. Incorrect function name 'print'

D. No error, code runs fine

Solution

1. Step 1: Check the string syntax

   The string starts with a double quote but lacks the closing double quote.

2. Step 2: Consequence of missing quote

   This causes a syntax error because the string is not properly closed.

3. Final Answer:

   Missing closing quote in the string -> Option B

4. Quick Check:

   Unclosed string causes syntax error [OK]

Hint: Check all quotes are closed in strings [OK]

Common Mistakes:

• Assuming chunk braces cause error
• Thinking print is misspelled
• Ignoring missing quote errors

5. You want to run an R code chunk but hide the code and only show the output. Which chunk option should you use?

hard

A. ```{r, include=FALSE}

B. ```{r, eval=FALSE}

C. ```{r, message=FALSE}

D. ```{r, echo=FALSE}

Solution

1. Step 1: Understand chunk options

   echo=FALSE hides the code but runs it, showing output only.

2. Step 2: Compare other options

   eval=FALSE skips running code, include=FALSE hides both code and output, message=FALSE hides messages only.

3. Final Answer:

   ```{r, echo=FALSE} -> Option D

4. Quick Check:

   echo=FALSE hides code, shows output [OK]

Hint: Use echo=FALSE to hide code but show output [OK]

Common Mistakes:

• Using eval=FALSE hides output too
• Using include=FALSE hides everything
• Confusing message=FALSE with code hiding