nchar and substring in R Programming

Introduction

We use nchar to find out how many characters are in a string. We use substring to get a part of a string.

When you want to count the letters in a word or sentence.

When you need to cut out a piece of a text, like a name or code.

When checking if a string is too long or too short.

When extracting a specific part from a string, like area code from a phone number.

When cleaning or formatting text data by taking only needed parts.

Syntax

R Programming

nchar(x)
substring(text, first, last = 1000000)

nchar(x) returns the number of characters in x.

substring(text, first, last) extracts characters from position first to last in text.

Examples

Counts the letters in the word "hello".

R Programming

nchar("hello")
# Returns 5

Empty string has zero characters.

R Programming

nchar("")
# Returns 0

Gets the first 4 letters from "programming".

R Programming

substring("programming", 1, 4)
# Returns "prog"

Gets substring from position 3 to the end.

R Programming

substring("data", 3)
# Returns "ta"

Sample Program

This program counts the letters in the word "education" and extracts letters from position 2 to 5.

R Programming

word <- "education"
length_of_word <- nchar(word)
part_of_word <- substring(word, 2, 5)

cat("Word:", word, "\n")
cat("Number of characters:", length_of_word, "\n")
cat("Substring from 2 to 5:", part_of_word, "\n")

Important Notes

Time complexity: Both nchar and substring run quickly, usually in constant time relative to string length.

Space complexity: substring creates a new string, so it uses extra space proportional to the substring length.

Common mistake: Forgetting that string positions start at 1 in R, not 0.

Use nchar when you need string length; use substring when you want part of a string.

Summary

nchar tells you how many characters are in a string.

substring lets you take a piece out of a string by giving start and end positions.

Positions in strings start at 1 in R, so count carefully.

Practice

1. What does the nchar function do in R?

easy

A. Returns the number of characters in a string

B. Extracts a substring from a string

C. Converts a string to uppercase

D. Counts the number of words in a string

Solution

1. Step 1: Understand the purpose of nchar

   The nchar function counts how many characters are in a string, including spaces and punctuation.

2. Step 2: Compare with other options

   Other options describe different functions: substring extracts parts, toupper converts case, and word count is not done by nchar.

3. Final Answer:

   Returns the number of characters in a string -> Option A

4. Quick Check:

   nchar("hello") = 5 [OK]

Hint: Remember: nchar counts characters, substring extracts parts [OK]

Common Mistakes:

• Confusing nchar with substring
• Thinking nchar counts words
• Assuming nchar changes the string

2. Which of the following is the correct way to extract the substring "cat" from the string "concatenate" in R?

easy

A. substring("concatenate", 3, 5)

B. substring("concatenate", 4, 6)

C. substring("concatenate", 5, 7)

D. substring("concatenate", 2, 4)

Solution

1. Step 1: Identify the position of "cat" in "concatenate"

   "concatenate" is c(1)o(2)n(3)c(4)a(5)t(6)e(7)n(8)a(9)t(10)e(11). The substring "cat" starts at position 4 and ends at 6.

2. Step 2: Use substring with correct start and end

   Using substring("concatenate", 4, 6) extracts characters 4 to 6, which is "cat".

3. Final Answer:

   substring("concatenate", 4, 6) -> Option B

4. Quick Check:

   substring("concatenate", 4, 6) = "cat" [OK]

Hint: Count characters starting at 1 to find substring positions [OK]

Common Mistakes:

• Starting count at 0 instead of 1
• Mixing up start and end positions
• Using incorrect substring indices

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

text <- "Programming"
len <- nchar(text)
sub <- substring(text, 4, 7)
print(len)
print(sub)

medium

A. 11 and "gram"

B. 11 and "gramm"

C. 10 and "gram"

D. 10 and "gramm"

Solution

1. Step 1: Calculate length of "Programming" using nchar

   "Programming" has 11 characters: P(1) r(2) o(3) g(4) r(5) a(6) m(7) m(8) i(9) n(10) g(11). So, len = 11.

2. Step 2: Extract substring from position 4 to 7

   Characters 4 to 7 are g(4) r(5) a(6) m(7), so substring is "gram".

3. Final Answer:

   11 and "gram" -> Option A

4. Quick Check:

   nchar("Programming") = 11, substring(4,7) = "gram" [OK]

Hint: Count characters carefully; substring includes both start and end [OK]

Common Mistakes:

• Counting length as 10 instead of 11
• Extracting one extra character in substring
• Confusing substring end position as exclusive

4. The following code is intended to extract "data" from the string "database" but it gives wrong output. What is the error?

text <- "database"
result <- substring(text, 1, 3)
print(result)

medium

A. The string variable is misspelled

B. substring function syntax is wrong

C. nchar should be used instead of substring

D. Start and end positions are incorrect; should be 1 to 4

Solution

1. Step 1: Check the substring positions used

   substring(text, 1, 3) extracts characters 1 to 3: "dat" from "database".

2. Step 2: Identify correct positions for "data"

   "data" is characters 1 to 4, so end position should be 4, not 3.

3. Final Answer:

   Start and end positions are incorrect; should be 1 to 4 -> Option D

4. Quick Check:

   substring("database", 1, 4) = "data" [OK]

Hint: Remember substring end position is inclusive [OK]

Common Mistakes:

• Using end position one less than needed
• Confusing substring with nchar usage
• Assuming substring excludes end position

5. You have a vector of strings: words <- c("apple", "banana", "cherry"). Write R code using substring and nchar to create a new vector containing the last 3 characters of each word. Which code snippet does this correctly?

hard

A. substring(words, nchar(words), nchar(words)+2)

B. substring(words, 1, 3)

C. substring(words, nchar(words)-2, nchar(words))

D. substring(words, nchar(words)-3, nchar(words)-1)

Solution

1. Step 1: Understand how to get last 3 characters

   For each word, last 3 characters start at position (length - 2) and end at length. Use nchar(words) to get length.

2. Step 2: Use substring with vectorized start and end

   substring(words, nchar(words)-2, nchar(words)) extracts last 3 characters for each word correctly.

3. Step 3: Check other options

   substring(words, 1, 3) extracts first 3 characters, C uses invalid end position, D extracts wrong positions.

4. Final Answer:

   substring(words, nchar(words)-2, nchar(words)) -> Option C

5. Quick Check:

   Last 3 chars of "banana" = "ana" [OK]

Hint: Use nchar(word)-2 as start for last 3 chars [OK]

Common Mistakes:

• Using fixed start positions instead of dynamic
• Off-by-one errors in start or end positions
• Assuming substring end is exclusive