grep and grepl in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the grep() function do in R?

grep() searches for patterns in a character vector and returns the indices of the elements that match the pattern.

beginner

How is grepl() different from grep()?

grepl() returns a logical vector (TRUE or FALSE) indicating if the pattern was found in each element, while grep() returns the indices of matching elements.

beginner

What will grep("cat", c("cat", "dog", "caterpillar")) return?

It will return 1 3 because "cat" is found in the 1st and 3rd elements.

beginner

What does grepl("dog", c("cat", "dog", "caterpillar")) return?

It returns FALSE TRUE FALSE showing which elements contain "dog".

intermediate

Can grep() and grepl() use regular expressions?

Yes, both functions support regular expressions to find complex patterns in text.

What does grep() return when it finds matches?

A The matching text itself

B Logical vector of TRUE/FALSE

C Number of matches

D Indices of matching elements

grep() returns the positions (indices) of elements that contain the pattern.

Which function returns a logical vector indicating matches?

A grepl()

B grep()

C gsub()

D sub()

grepl() returns TRUE or FALSE for each element depending on whether the pattern is found.

What will grepl("a", c("apple", "banana", "cherry")) return?

A [TRUE, TRUE, FALSE]

B [FALSE, TRUE, TRUE]

C [TRUE, FALSE, TRUE]

D [FALSE, FALSE, FALSE]

"a" is in "apple" and "banana" but not in "cherry".

If you want to get the actual matching text, which function should you use?

A grep()

B grepl()

C regmatches() with regexpr()

D grepl() with invert = TRUE

To extract matching text, use regmatches() with regexpr() or gregexpr().

Can grep() and grepl() handle case-insensitive searches?

A No, they are case-sensitive only

B Yes, by setting <code>ignore.case = TRUE</code>

C Only <code>grep()</code> can

D Only <code>grepl()</code> can

Both functions accept ignore.case = TRUE to ignore case differences.

Explain the difference between grep() and grepl() in R.

Think about what each function returns when you search text.

How can you use grep() or grepl() to find if a word exists in a list of strings?

Consider what output you want: positions or logical answers.