String formatting with sprintf in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the sprintf function do in R?

It creates a formatted string by inserting values into a template, similar to filling blanks in a sentence.

beginner

How do you format a number to show exactly 2 decimal places using sprintf?

Use %.2f in the format string, like sprintf("%.2f", 3.14159) which outputs "3.14".

beginner

What does the format specifier %d mean in sprintf?

It means to format the value as an integer (whole number).

intermediate

How can you include a literal percent sign (%) in the output of sprintf?

Use %% in the format string to print a single percent sign.

intermediate

What happens if you provide more values than format specifiers in sprintf?

Extra values are ignored; only values matching the format specifiers are used.

Which sprintf format specifier would you use to format a floating-point number with 3 decimal places?

A %.3d

B %d

C %s

D %.3f

The %.3f specifier formats a number as a float with 3 decimal places.

What will sprintf("Hello %s", "world") output?

A Hello %s

B Hello world

C world Hello

D Error

The %s is replaced by the string "world", so the output is "Hello world".

How do you print a percent sign (%) using sprintf?

A Use %%

B Use %p

C Use %s

D Use %percent

Double percent signs (%%) print a single percent sign in the output.

What does sprintf("%d", 3.7) output?

A 4

B 3.7

C 3

D Error

The %d specifier converts the number to an integer by truncating the decimal part.

If you have fewer values than format specifiers in sprintf, what happens?

A Error or warning

B It fills missing values with zero

C It ignores missing values

D It repeats last value

Missing values for format specifiers cause an error or warning because sprintf expects matching values.

Explain how to use sprintf to format a string with a name and a number showing two decimals.

Think about placeholders for text and numbers.

Describe how to print a literal percent sign (%) using sprintf and why it requires special syntax.

Percent signs are used to mark placeholders.

Practice

1. What does the sprintf function in R primarily do?

easy

A. It reads data from a file and stores it in a string.

B. It splits a string into a list of words.

C. It converts strings to uppercase letters.

D. It creates a formatted string by inserting values into placeholders.

Solution

1. Step 1: Understand the purpose of sprintf

   The sprintf function formats strings by replacing placeholders with values.

2. Step 2: Compare options with the function's purpose

   Only It creates a formatted string by inserting values into placeholders. describes this behavior correctly; others describe unrelated string operations.

3. Final Answer:

   It creates a formatted string by inserting values into placeholders. -> Option D

4. Quick Check:

   sprintf formats strings = B [OK]

Hint: Remember sprintf builds strings by filling placeholders [OK]

Common Mistakes:

• Confusing sprintf with string splitting functions
• Thinking sprintf changes case of letters
• Assuming sprintf reads files

2. Which of the following is the correct syntax to format an integer using sprintf in R?

easy

A. sprintf("%d", 42)

B. sprintf("%s", 42)

C. sprintf("%f", 42)

D. sprintf("%c", 42)

Solution

1. Step 1: Identify the placeholder for integers

   The placeholder %d is used for integers in sprintf.

2. Step 2: Match the placeholder with the value type

   Since 42 is an integer, %d is correct; %s is for strings, %f for floats, and %c for characters.

3. Final Answer:

   sprintf("%d", 42) -> Option A

4. Quick Check:

   Integer placeholder = %d [OK]

Hint: Use %d for integers, %s for strings, %f for decimals [OK]

Common Mistakes:

• Using %s for numbers instead of %d
• Using %f for integers without decimals
• Confusing %c with integer formatting

3. What is the output of this R code?

sprintf("Name: %s, Age: %d", "Alice", 30)

medium

A. "Name: %s, Age: %d"

B. "Name: Alice, Age: 30"

C. "Name: Alice, Age: %d"

D. "Name: %s, Age: 30"

Solution

1. Step 1: Match placeholders with arguments

   The first placeholder %s is replaced by "Alice" (string), the second %d by 30 (integer).

2. Step 2: Construct the final string

   Replacing placeholders gives "Name: Alice, Age: 30" as the output string.

3. Final Answer:

   "Name: Alice, Age: 30" -> Option B

4. Quick Check:

   Placeholders replaced correctly = A [OK]

Hint: Placeholders %s and %d replaced by arguments in order [OK]

Common Mistakes:

• Not replacing placeholders with values
• Mixing up %s and %d placeholders
• Outputting the format string literally

4. Identify the error in this R code:

sprintf("Value: %d", "ten")

medium

A. Incorrect use of double quotes around the format string.

B. Missing closing parenthesis in sprintf function.

C. Using %d placeholder with a string causes a type mismatch error.

D. No error; code runs correctly.

Solution

1. Step 1: Check placeholder and argument types

   The placeholder %d expects an integer, but "ten" cannot be coerced to a number, resulting in NA and a warning.

2. Step 2: Determine if it causes an error

   The code runs but produces a warning and outputs "Value: NA"; this is a type mismatch issue.

3. Final Answer:

   Using %d placeholder with a string causes a type mismatch error. -> Option C

4. Quick Check:

   Type mismatch warning occurs = A [OK]

Hint: sprintf coerces types, may produce NA and warning [OK]

Common Mistakes:

• Confusing coercion warning with type mismatch error
• Looking for missing parenthesis (it's present)
• Assuming invalid quotes (they're correct)

5. You want to format a floating-point number with exactly 2 decimal places using sprintf. Which code snippet achieves this?

hard

A. sprintf("%.2f", 3.14159)

B. sprintf("%2f", 3.14159)

C. sprintf("%f.2", 3.14159)

D. sprintf("%f", round(3.14159, 2))

Solution

1. Step 1: Understand the format for decimal precision

   The format %.2f tells sprintf to print a float with 2 digits after the decimal point.

2. Step 2: Compare options for correct syntax

   sprintf("%.2f", 3.14159) uses %.2f, which is correct. sprintf("%2f", 3.14159) misses the dot, A has incorrect syntax, and D rounds before formatting but does not guarantee 2 decimals in output.

3. Final Answer:

   sprintf("%.2f", 3.14159) -> Option A

4. Quick Check:

   Use %.2f for 2 decimals [OK]

Hint: Use %.2f to fix two decimal places in sprintf [OK]

Common Mistakes:

• Omitting the dot before precision number
• Using round() but forgetting format specifier
• Incorrect order of precision and type specifier