Count Words with Given Prefix in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if a word starts with the given prefix.

DSA Go

if strings.[1](word, prefix) { return true } else { return false }

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

AHasPrefix

BContains

CSplit

DIndex

Attempts:

3 left

💡 Hint

Common Mistakes

Using strings.Contains instead of strings.HasPrefix

Using strings.Index which returns position, not boolean

The strings.HasPrefix function checks if a string starts with the given prefix.

2fill in blank

medium

Complete the code to iterate over all words in the list.

DSA Go

for [1] := 0; i < len(words); i++ { word := words[i] }

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

Ai

Bx

Ck

Dj

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable different from the one in the loop condition

Using a variable not declared in the loop

The variable i is commonly used as the index in loops.

3fill in blank

hard

Fix the error in the code to correctly count words with the prefix.

DSA Go

count := 0
for _, word := range words {
    if strings.HasPrefix(word, prefix) {
        count [1] 1
    }
}
return count

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

A=

B-=

C+=

D*=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' which overwrites count

Using '-=' which decreases count

Use count += 1 to increase the count by one each time a word matches.

4fill in blank

hard

Fill both blanks to create a map of words and their prefix match status.

DSA Go

result := make(map[string]bool)
for _, [1] := range words {
    result[word] = strings.[2](word, prefix)
}
return result

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

Aword

Bi

CHasPrefix

DContains

Attempts:

3 left

💡 Hint

Common Mistakes

Using index variable instead of word

Using strings.Contains instead of strings.HasPrefix

Use word to iterate and strings.HasPrefix to check prefix.

5fill in blank

hard

Fill all three blanks to build a prefix count map for a list of words.

DSA Go

prefixCount := make(map[string]int)
for _, word := range words {
    for i := 1; i <= len(word); i++ {
        prefix := word[:[1]]
        prefixCount[prefix] [2] [3] 1
    }
}
return prefixCount

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

Ai

B+=

C+

Dlen(word)

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong slice index

Using '=' instead of '+=' for map update

Using '-' instead of '+' for increment

Use i to slice prefixes, += to add counts, and + to increment.

Practice

1. What does the problem "Count Words with Given Prefix" ask you to do?

easy

A. Count the total number of characters in all words

B. Find the longest word in a list

C. Count how many words in a list start with a specific prefix

D. Sort the words alphabetically

Solution

1. Step 1: Understand the problem statement

   The problem asks to find how many words begin with a given prefix string.

2. Step 2: Identify the key operation

   We need to check each word and count those that start with the prefix.

3. Final Answer:

   Count how many words in a list start with a specific prefix -> Option C

4. Quick Check:

   Counting words by prefix = Count how many words in a list start with a specific prefix [OK]

Hint: Look for words starting exactly with the prefix [OK]

Common Mistakes:

• Confusing prefix with substring anywhere in word
• Counting total words instead of filtered ones
• Ignoring case sensitivity if required

2. Which of the following Go code snippets correctly checks if a word starts with a given prefix pref?

easy

A. strings.HasPrefix(word, pref)

B. strings.Contains(word, pref)

C. word[:len(pref)] == pref

D. word == pref

Solution

1. Step 1: Recall Go standard library functions

   strings.HasPrefix(word, pref) returns true if word starts with pref.

2. Step 2: Compare other options

   strings.Contains checks anywhere, slicing may panic if prefix longer, equality checks full word.

3. Final Answer:

   strings.HasPrefix(word, pref) -> Option A

4. Quick Check:

   Use HasPrefix for prefix check [OK]

Hint: Use strings.HasPrefix for safe prefix check [OK]

Common Mistakes:

• Using strings.Contains which checks anywhere in word
• Slicing without length check causing runtime panic
• Comparing whole word instead of prefix

3. What is the output of the following Go code?

package main
import (
  "fmt"
  "strings"
)
func countPrefix(words []string, prefix string) int {
  count := 0
  for _, w := range words {
    if strings.HasPrefix(w, prefix) {
      count++
    }
  }
  return count
}
func main() {
  words := []string{"apple", "app", "application", "banana", "apply"}
  fmt.Println(countPrefix(words, "app"))
}

medium

A. 4

B. 2

C. 3

D. 5

Solution

1. Step 1: Identify words starting with "app"

   Words: "apple", "app", "application", "banana", "apply". "banana" does not start with "app".

2. Step 2: Count matching words

   Matching words: "apple", "app", "application", "apply" -> 4 words.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Count of words starting with "app" = 4 [OK]

Hint: Count words starting exactly with prefix using HasPrefix [OK]

Common Mistakes:

• Counting "banana" incorrectly
• Off-by-one error in counting
• Misreading prefix length

4. The following Go code is intended to count words starting with a prefix but causes a runtime error. What is the error?

func countPrefix(words []string, prefix string) int {
  count := 0
  for _, w := range words {
    if w[:len(prefix)] == prefix {
      count++
    }
  }
  return count
}

medium

A. No error, code works fine

B. Index out of range panic if word shorter than prefix

C. Logical error: counts words containing prefix anywhere

D. Syntax error due to missing import

Solution

1. Step 1: Analyze slicing operation

   The code slices w[:len(prefix)] without checking if w is long enough.

2. Step 2: Identify runtime error cause

   If w is shorter than prefix, slicing causes index out of range panic.

3. Final Answer:

   Index out of range panic if word shorter than prefix -> Option B

4. Quick Check:

   Slicing without length check causes panic [OK]

Hint: Check word length before slicing prefix [OK]

Common Mistakes:

• Ignoring length check before slicing
• Assuming code compiles without imports
• Confusing prefix check with substring check

5. You have a list of words and want to count how many start with each prefix from a list of prefixes. Which approach is best for efficient counting in Go?

hard

A. Use strings.Contains to check prefix in each word

B. For each prefix, loop over all words and count matches

C. Sort words and use binary search for each prefix

D. Build a trie from words, then query counts for each prefix

Solution

1. Step 1: Understand the problem scale

   Counting for many prefixes over many words needs efficiency.

2. Step 2: Compare approaches

   Looping over all words for each prefix is slow (O(P*W)). Sorting + binary search helps but trie allows prefix counting efficiently.

3. Step 3: Choose best data structure

   Trie stores words by prefix, enabling fast prefix count queries.

4. Final Answer:

   Build a trie from words, then query counts for each prefix -> Option D

5. Quick Check:

   Trie enables efficient prefix counting [OK]

Hint: Use trie for fast prefix count queries [OK]

Common Mistakes:

• Using nested loops causing slow performance
• Using strings.Contains which is incorrect for prefix
• Ignoring data structure benefits