nchar and substring in R Programming - Step-by-Step Execution

Concept Flow - nchar and substring

Start with a string

↓

Use nchar to get length

↓

Use substring(text, start, end) to extract part

↓

Output substring

↓

End

We start with a string, find its length using nchar, then extract a part using substring by specifying start and end positions.

Execution Sample

R Programming

text <- "Hello, World!"
len <- nchar(text)
part <- substring(text, 8, 12)
print(len)
print(part)

This code finds the length of the string and extracts characters from position 8 to 12.

Execution Table

Step	Variable	Expression	Value	Action
1	text	Assign "Hello, World!"	"Hello, World!"	Store string
2	len	nchar(text)	13	Calculate length of text
3	part	substring(text, 8, 12)	"World"	Extract substring from position 8 to 12
4	print(len)	print(13)	13	Output length
5	print(part)	print("World")	World	Output substring

💡 All steps executed; program ends after printing substring.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
text	undefined	"Hello, World!"	"Hello, World!"	"Hello, World!"	"Hello, World!"
len	undefined	undefined	13	13	13
part	undefined	undefined	undefined	"World"	"World"

Key Moments - 2 Insights

Why does substring(text, 8, 12) return "World" and not include the comma or exclamation mark?

Because substring extracts characters from position 8 to 12 exactly. The comma is at position 6 and exclamation mark at 13, so they are outside this range (see execution_table step 3).

What happens if the end position in substring is greater than the string length?

R extracts characters up to the string's end without error. For example, substring(text, 8, 20) would return "World!" because the string ends at position 13.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the value of len?

A 13

B 12

C 11

D 14

💡 Hint

Check the 'Value' column for step 2 in execution_table.

At which step is the substring variable assigned its value?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for the step where 'part' is assigned in execution_table.

If substring(text, 1, 5) was used instead, what would be the output at step 3?

A ", Wor"

B "World"

C "Hello"

D "Hello,"

💡 Hint

Substring extracts characters from start to end positions; check positions 1 to 5 in the string.

Concept Snapshot

nchar(text) returns the number of characters in text.
substring(text, start, end) extracts characters from start to end positions.
Positions start at 1.
If end > length, substring returns up to string end.
Useful for slicing parts of strings easily.

Full Transcript

This example starts with a string variable text assigned "Hello, World!". The nchar function calculates its length, which is 13 characters. Then substring extracts characters from position 8 to 12, which is "World". The program prints the length and the extracted substring. Positions in substring start at 1, so the comma and exclamation mark are outside the extracted range. If the end position exceeds the string length, substring returns characters up to the end without error.

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