String matching basics in Data Structures Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find if the substring exists in the main string.

Data Structures Theory

if "needle" [1] "haystack":
    print("Found")

Drag options to blanks, or click blank then click option'

A==

Bnot in

Cin

Dcontains

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of 'in' to check substring presence.

The keyword in checks if one string is inside another.

2fill in blank

medium

Complete the code to get the starting index of the substring in the main string.

Data Structures Theory

index = "haystack".[1]("needle")

Drag options to blanks, or click blank then click option'

Afind

BindexOf

Clocate

Dsearch

Attempts:

3 left

💡 Hint

Common Mistakes

Using methods that don't exist in Python like 'indexOf'.

The find method returns the first index of the substring or -1 if not found.

3fill in blank

hard

Fix the error in the code to correctly check if the substring is not in the main string.

Data Structures Theory

if "needle" [1] "haystack":
    print("Not found")

Drag options to blanks, or click blank then click option'

Ain

Bnot in

C!=

Dis not

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' which compares equality, not substring presence.

The not in keyword checks if a substring is absent from a string.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps words to their lengths only if the length is greater than 3.

Data Structures Theory

{word: [1] for word in words if [2] > 3}

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Alen(word)

Bword

Dword.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'word.length' which is not valid in Python.

The comprehension maps each word to its length using len(word), and filters words with length greater than 3.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps uppercase words to their lengths only if the length is less than 5.

Data Structures Theory

{ [1]: [2] for word in words if [3] < 5 }

Drag options to blanks, or click blank then click option'

Aword.upper()

Blen(word)

Dword

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'word' instead of 'word.upper()' for keys.

The comprehension uses word.upper() as keys, maps to len(word), and filters words with length less than 5.

Practice

1. What is the main goal of string matching in computer science?

easy

A. To sort a list of strings alphabetically

B. To find if a smaller text appears inside a bigger text

C. To count the number of characters in a string

D. To convert a string to uppercase

Solution

1. Step 1: Understand the definition of string matching

   String matching is about checking if a smaller text (pattern) exists inside a larger text (text).

2. Step 2: Compare options with the definition

   Only To find if a smaller text appears inside a bigger text describes this goal correctly; others describe different string operations.

3. Final Answer:

   To find if a smaller text appears inside a bigger text -> Option B

4. Quick Check:

   String matching = find smaller text inside bigger text [OK]

Hint: Remember: string matching means searching for a pattern inside text [OK]

Common Mistakes:

• Confusing string matching with sorting strings
• Thinking string matching counts characters
• Mixing string matching with case conversion

2. Which of the following is the correct way to check if the string pattern exists in text using Python?

easy

A. if pattern in text:

B. if text contains pattern:

C. if text has pattern:

D. if pattern inside text:

Solution

1. Step 1: Recall Python syntax for substring check

   In Python, the correct syntax to check if one string is inside another is using the in keyword.

2. Step 2: Evaluate each option

   Only if pattern in text: uses the correct Python syntax. Options A, B, and D are not valid Python statements.

3. Final Answer:

   if pattern in text: -> Option A

4. Quick Check:

   Python substring check uses 'in' keyword [OK]

Hint: Use 'in' keyword to check substring in Python [OK]

Common Mistakes:

• Using incorrect keywords like 'contains' or 'has'
• Writing invalid syntax like 'inside'
• Confusing method calls with keywords

3. What will be the output of the following code?

text = "hello world"
pattern = "world"
print(pattern in text)

medium

A. False

B. world

C. True

D. Error

Solution

1. Step 1: Understand the 'in' operator in Python

   The expression pattern in text checks if the string 'world' is inside 'hello world'.

2. Step 2: Check if 'world' is a substring of 'hello world'

   Since 'world' appears at the end of 'hello world', the expression returns True.

3. Final Answer:

   True -> Option C

4. Quick Check:

   'world' in 'hello world' = True [OK]

Hint: Check if substring is exactly inside the main string [OK]

Common Mistakes:

• Assuming output is the substring itself
• Confusing True/False output
• Expecting an error due to syntax

4. Identify the error in the following code snippet for string matching:

text = "data structures"
pattern = "structure"
if pattern in text:
print("Found")

medium

A. Indentation error in the print statement

B. Wrong variable names used

C. Incorrect string matching operator

D. Missing quotation marks around pattern

Solution

1. Step 1: Check Python syntax for if statement

   Python requires the code inside an if block to be indented properly.

2. Step 2: Inspect the print statement indentation

   The print statement is not indented, causing an indentation error.

3. Final Answer:

   Indentation error in the print statement -> Option A

4. Quick Check:

   Python blocks need indentation [OK]

Hint: Indent code inside if statements properly [OK]

Common Mistakes:

• Ignoring indentation rules in Python
• Assuming 'in' operator is wrong
• Changing variable names unnecessarily

5. You want to find all starting positions where the pattern "ana" appears in the text "banana". Which of the following lists shows the correct starting indices (0-based)?

hard

A. [1, 2, 3]

B. [0, 2, 4]

C. [2, 4]

D. [1, 3]

Solution

1. Step 1: Identify occurrences of 'ana' in 'banana'

   Check each substring of length 3 in 'banana': 'ban'(0), 'ana'(1), 'nan'(2), 'ana'(3), 'na'(4 incomplete).

2. Step 2: List starting indices where substring matches 'ana'

   'ana' appears starting at index 1 and index 3.

3. Final Answer:

   [1, 3] -> Option D

4. Quick Check:

   Positions of 'ana' in 'banana' = [1, 3] [OK]

Hint: Slide window over text to find all pattern starts [OK]

Common Mistakes:

• Counting overlapping matches incorrectly
• Using 1-based instead of 0-based indices
• Missing one of the occurrences