String slicing and indexing in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of negative index slicing

What is the output of this Ruby code?

Ruby

str = "programming"
puts str[-4, 3]

A "amm"

B "min"

C "mmi"

D "ing"

💡 Hint

Remember that negative index counts from the end, and the second number is length.

str[-4, 3] starts 4 characters from the end ('m') and takes 3 characters: 'm', 'm', 'i'.

❓ Predict Output

intermediate

Output of range slicing

What will this Ruby code print?

Ruby

text = "hello world"
puts text[2..5]

A "llo "

B "llo w"

C "lo w"

D "ello"

💡 Hint

Range includes both start and end indices.

text[2..5] extracts characters at indices 2,3,4,5: 'l','l','o',' '.

🔧 Debug

advanced

Identify the error in slicing

What error does this Ruby code raise?

Ruby

word = "example"
puts word[3, -2]

A nil output (no error)

B ArgumentError

C TypeError

D IndexError

💡 Hint

Negative length in slice returns nil instead of error.

In Ruby, a negative length in slice returns nil, no error is raised.

❓ Predict Output

advanced

Output of slice with range and negative indices

What is the output of this Ruby code?

Ruby

s = "abcdefg"
puts s[-5..-3]

A "cdef"

B "cde"

C "def"

D "bcd"

💡 Hint

Negative indices count from the end, inclusive range.

s[-5..-3] means indices 2 to 4: 'c','d','e'.

🧠 Conceptual

expert

Number of characters extracted by slice

How many characters does this Ruby slice extract?

Ruby

str = "university"
result = str[1, 4]

A 6

B 5

C 3

D 4

💡 Hint

The second argument is the length of the slice.

str[1,4] extracts 4 characters starting at index 1.

Practice

1. In Ruby, what does str[0] return when str = "hello"?

easy

A. The first character of the string, 'h'

B. The entire string 'hello'

C. The last character of the string, 'o'

D. An error because indexing starts at 1

Solution

1. Step 1: Understand Ruby string indexing

   Ruby strings are indexed starting at 0, so the first character is at index 0.

2. Step 2: Apply indexing to the string 'hello'

   Accessing str[0] returns the first character 'h'.

3. Final Answer:

   The first character of the string, 'h' -> Option A

4. Quick Check:

   Index 0 = 'h' [OK]

Hint: Remember Ruby indexes start at zero for strings [OK]

Common Mistakes:

• Thinking indexing starts at 1
• Expecting the whole string instead of one character
• Confusing last character with first

2. Which of the following is the correct way to get the substring 'ell' from str = "hello" using slicing?

easy

A. str[1,3]

B. str[0,3]

C. str[2..4]

D. str[1...3]

Solution

1. Step 1: Understand slicing syntax

   In Ruby, str[start, length] extracts a substring starting at start with length characters.

2. Step 2: Extract 'ell' from 'hello'

   'ell' starts at index 1 and has length 3, so str[1,3] returns 'ell'.

3. Final Answer:

   str[1,3] -> Option A

4. Quick Check:

   Substring from index 1, length 3 = 'ell' [OK]

Hint: Use [start, length] to slice substrings easily [OK]

Common Mistakes:

• Using range with wrong end index
• Confusing start and length parameters
• Using negative indices incorrectly

3. What is the output of this Ruby code?

str = "programming"
puts str[-4, 3]

medium

A. ing

B. ram

C. amm

D. min

Solution

1. Step 1: Understand negative indexing and slicing

   Negative index -4 means counting from the end: 'g'( -1), 'n'( -2), 'i'( -3), 'm'( -4). So index -4 points to 'm'.

2. Step 2: Extract substring starting at 'm' with length 3

   From 'm' take 3 characters: 'm', 'i', 'n' forming 'min'.

3. Final Answer:

   min -> Option D

4. Quick Check:

   str[-4,3] = 'min' [OK]

Hint: Negative index counts from end; length slices forward [OK]

Common Mistakes:

• Confusing negative index position
• Using range instead of start,length
• Off-by-one errors in slicing length

4. Identify the error in this Ruby code snippet:

str = "example"
puts str[2..5, 2]

medium

A. Returns nil because of invalid range

B. Returns 'am' as expected

C. SyntaxError due to wrong slicing syntax

D. Returns 'amp' substring

Solution

1. Step 1: Analyze slicing syntax used

   The code uses str[2..5, 2] which mixes range and length parameters incorrectly. Ruby does not support this syntax.

2. Step 2: Understand correct slicing methods

   Use either str[2..5] for range or str[2, 2] for start and length. Mixing both causes a syntax error.

3. Final Answer:

   SyntaxError due to wrong slicing syntax -> Option C

4. Quick Check:

   Invalid slice syntax = SyntaxError [OK]

Hint: Use either range or start,length, not both together [OK]

Common Mistakes:

• Mixing range and length in one slice
• Expecting multiple parameters in range slice
• Ignoring Ruby's slice method rules

5. Given str = "abracadabra", which code extracts every second character starting from index 0 to index 8 (inclusive)?

hard

A. str[0..8][::2]

B. str[0..8].chars.select.with_index { |_, i| i.even? }.join

C. str[0, 8, 2]

D. str[0..8].chars.each_slice(2).map(&:first).join

Solution

1. Step 1: Understand Ruby slicing limitations

   Ruby's slice method does not support step values like Python. So str[0, 8, 2] or str[0..8][::2] are invalid.

2. Step 2: Use chars with index filtering to pick every second character

   str[0..8] gets substring 'abracadab'. Then chars.select.with_index { |_, i| i.even? } picks characters at even indices: a (0), r (2), c (4), d (6), b (8). Joining gives 'ar cdb' without spaces: 'arcdb'.

3. Final Answer:

   str[0..8].chars.select.with_index { |_, i| i.even? }.join -> Option B

4. Quick Check:

   Use chars and select.with_index for stepping [OK]

Hint: Ruby slice has no step; use chars with index filtering [OK]

Common Mistakes:

• Trying Python-style slicing with step
• Using invalid slice parameters
• Not converting string to chars before filtering