Count Words with Given Prefix in DSA C

Mental Model

We want to find how many words start with a certain beginning part. We check each word and count if it matches the start.

Analogy: Imagine you have a box of books and you want to count how many titles start with the word 'The'. You look at each title and count those that begin with 'The'.

Words: [apple, app, ape, bat, ball]
Prefix: 'ap'
Count how many words start with 'ap'

List:
apple -> app -> ape -> bat -> ball -> null
Prefix: ap

Dry Run Walkthrough

Input: words: ["apple", "app", "ape", "bat", "ball"], prefix: "ap"

Goal: Count how many words start with the prefix "ap"

Step 1: Check word 'apple' if it starts with 'ap'

apple (starts with 'ap') -> count = 1

Why: We found one word starting with the prefix

Step 2: Check word 'app' if it starts with 'ap'

apple -> app (starts with 'ap') -> count = 2

Why: Another word matches the prefix

Step 3: Check word 'ape' if it starts with 'ap'

apple -> app -> ape (starts with 'ap') -> count = 3

Why: Third word matches the prefix

Step 4: Check word 'bat' if it starts with 'ap'

apple -> app -> ape -> bat (does not start with 'ap') -> count = 3

Why: No match, count stays the same

Step 5: Check word 'ball' if it starts with 'ap'

apple -> app -> ape -> bat -> ball (does not start with 'ap') -> count = 3

Why: No match, count stays the same

Result:

Final count = 3

Annotated Code

DSA C

#include <stdio.h>
#include <string.h>

int countWordsWithPrefix(char words[][20], int n, char prefix[]) {
    int count = 0;
    int prefixLen = strlen(prefix);
    for (int i = 0; i < n; i++) {
        // Check if current word starts with prefix
        if (strncmp(words[i], prefix, prefixLen) == 0) {
            count++;
        }
    }
    return count;
}

int main() {
    char words[5][20] = {"apple", "app", "ape", "bat", "ball"};
    char prefix[] = "ap";
    int n = 5;
    int result = countWordsWithPrefix(words, n, prefix);
    printf("Count of words starting with '%s': %d\n", prefix, result);
    return 0;
}

for (int i = 0; i < n; i++) {

iterate over each word in the list

if (strncmp(words[i], prefix, prefixLen) == 0) {

check if current word starts with the prefix

count++;

increment count for matching word

Output

Count of words starting with 'ap': 3

Complexity Analysis

Time: O(n * m) because we check each of the n words and compare up to m characters of the prefix

Space: O(1) because we only use a few extra variables regardless of input size

vs Alternative: Compared to building a trie for prefix search, this approach is simpler but slower for large datasets

Edge Cases

Empty list of words

Returns 0 because no words to check

DSA C

for (int i = 0; i < n; i++) {

Prefix longer than some words

strncmp safely returns non-zero, so those words don't count

DSA C

if (strncmp(words[i], prefix, prefixLen) == 0) {

No words match the prefix

Count remains 0 and is returned

DSA C

count++

When to Use This Pattern

When asked to count or find words starting with a certain prefix, use prefix comparison on each word to solve simply.

Common Mistakes

Mistake: Comparing entire word instead of only prefix length
Fix: Use strncmp with prefix length to compare only the start of each word

Summary

Counts how many words in a list start with a given prefix.

Use when you need to find how many words share the same beginning letters.

The key is to compare only the prefix length of each word to the prefix.

Practice

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

easy

A. Count how many words in a list start with a given prefix

B. Count the total number of characters in all words

C. Find the longest word in the list

D. Sort the words alphabetically

Solution

1. Step 1: Understand the problem statement

   The problem asks to find how many words start with a specific prefix string.

2. Step 2: Identify the key operation

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

3. Final Answer:

   Count how many words in a list start with a given prefix -> Option A

4. Quick Check:

   Count words starting with prefix = Count how many words in a list start with a given prefix [OK]

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

Common Mistakes:

• Counting total characters instead of words
• Looking for words containing prefix anywhere
• Sorting words instead of counting

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

easy

A. strcmp(word, prefix) == 0

B. strncmp(word, prefix, strlen(prefix)) == 0

C. strstr(word, prefix) != NULL

D. strlen(word) == strlen(prefix)

Solution

1. Step 1: Understand string comparison functions

   strncmp compares first n characters; here n = length of prefix.

2. Step 2: Check which code confirms prefix match

   strncmp(word, prefix, strlen(prefix)) == 0 means word starts with prefix exactly.

3. Final Answer:

   strncmp(word, prefix, strlen(prefix)) == 0 -> Option B

4. Quick Check:

   strncmp with prefix length = 0 means prefix match [OK]

Hint: Use strncmp with prefix length to check start [OK]

Common Mistakes:

• Using strcmp which requires full string match
• Using strstr without checking start position
• Comparing lengths only

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

char *words[] = {"apple", "app", "application", "banana"};
char prefix[] = "app";
int count = 0;
for (int i = 0; i < 4; i++) {
  if (strncmp(words[i], prefix, strlen(prefix)) == 0) {
    count++;
  }
}
printf("%d\n", count);

medium

A. 4

B. 2

C. 1

D. 3

Solution

1. Step 1: Check each word against prefix "app"

   "apple" starts with "app" (yes), "app" (yes), "application" (yes), "banana" (no).

2. Step 2: Count matching words

   Three words match the prefix, so count = 3.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Words starting with "app" = 3 [OK]

Hint: Count words where strncmp returns 0 for prefix length [OK]

Common Mistakes:

• Counting words that contain prefix anywhere
• Off-by-one errors in loop
• Incorrect prefix length in strncmp

4. Find the bug in this C code that counts words starting with prefix prefix:

int countWords(char *words[], int n, char *prefix) {
  int count = 0;
  for (int i = 0; i <= n; i++) {
    if (strncmp(words[i], prefix, strlen(prefix)) == 0) {
      count++;
    }
  }
  return count;
}

medium

A. Initialize count to 1 instead of 0

B. Use strcmp instead of strncmp

C. Loop condition should be i < n, not i <= n

D. strlen(prefix) should be replaced with strlen(words[i])

Solution

1. Step 1: Analyze loop boundary

   Loop runs from i = 0 to i <= n, which accesses words[n] (out of bounds).

2. Step 2: Correct loop condition

   Change to i < n to avoid accessing beyond array size.

3. Final Answer:

   Loop condition should be i < n, not i <= n -> Option C

4. Quick Check:

   Array index must be less than size [OK]

Hint: Use i < n for array loops to avoid overflow [OK]

Common Mistakes:

• Using <= instead of < in loops
• Confusing strcmp and strncmp usage
• Wrong length argument in strncmp

5. You have a list of words and want to count how many start with prefix pre. Which approach is best to optimize counting if you must do many prefix queries efficiently?

hard

A. Build a trie (prefix tree) from words and count nodes matching prefix

B. Use strncmp on each word for every query

C. Sort words and use binary search for prefix range

D. Store words in a hash set and check prefix membership

Solution

1. Step 1: Understand multiple prefix queries

   Checking each word with strncmp repeatedly is slow for many queries.

2. Step 2: Choose data structure for fast prefix counting

   Trie stores words by prefixes, allowing quick count of words with given prefix.

3. Step 3: Compare alternatives

   Binary search helps but trie is more direct for prefix counts; hash set doesn't support prefix queries well.

4. Final Answer:

   Build a trie (prefix tree) from words and count nodes matching prefix -> Option A

5. Quick Check:

   Trie = fast prefix count [OK]

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

Common Mistakes:

• Using linear search for many queries
• Ignoring trie benefits for prefix problems
• Using hash set which doesn't support prefix search