Count Words with Given Prefix in DSA Javascript - Execution Trace

Concept Flow - Count Words with Given Prefix

Start with word list

↓

Check each word

↓

Does word start with prefix?

No→Skip word

Yes↓

Increment count

↓

Repeat for all words

↓

Return count

We check each word one by one to see if it starts with the prefix. If yes, we add to count. Finally, we return the total count.

Execution Sample

DSA Javascript

function countPrefix(words, prefix) {
  let count = 0;
  for (const word of words) {
    if (word.startsWith(prefix)) count++;
  }
  return count;
}

This code counts how many words in the list start with the given prefix.

Execution Table

Step	Operation	Current Word	Prefix Check	Count	Visual State
1	Initialize count	-	-	0	count=0
2	Check word	apple	apple.startsWith('ap') = true	1	count=1
3	Check word	app	app.startsWith('ap') = true	2	count=2
4	Check word	banana	banana.startsWith('ap') = false	2	count=2
5	Check word	apricot	apricot.startsWith('ap') = true	3	count=3
6	Check word	bat	bat.startsWith('ap') = false	3	count=3
7	End loop	-	-	3	Final count=3

💡 All words checked, loop ends and count returned

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	Final
count	0	1	2	2	3	3	3	3
current word	-	apple	app	banana	apricot	bat	-	-

Key Moments - 3 Insights

Why does count only increase when prefix check is true?

Because we only want to count words that start with the prefix. See execution_table rows 2,3,5 where prefix check is true and count increases.

What happens if no words start with the prefix?

Count stays zero because the condition word.startsWith(prefix) is never true. This is shown in execution_table rows where prefix check is false and count remains unchanged.

Why do we check each word individually instead of all at once?

Because each word must be tested separately to see if it starts with the prefix. The loop in execution_table rows 2-6 shows this step-by-step checking.

Visual Quiz - 3 Questions

Look at the execution_table, what is the count after checking the word 'app'?

A 1

B 2

C 3

D 0

💡 Hint

Check the 'Count' column at Step 3 in execution_table

At which step does the prefix check return false for the first time?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Prefix Check' column in execution_table rows

If the prefix was changed to 'ba', what would be the count after checking all words?

A 1

B 3

C 2

D 0

💡 Hint

Look at words starting with 'ba' in the variable_tracker and execution_table

Concept Snapshot

Count Words with Given Prefix:
- Loop through each word in list
- Check if word starts with prefix
- If yes, increase count by 1
- Return total count after loop
- Uses string.startsWith(prefix) method

Full Transcript

This concept shows how to count words that start with a given prefix. We start with a count of zero. Then for each word, we check if it starts with the prefix using the startsWith method. If yes, we add one to the count. We repeat this for all words. Finally, we return the count. The execution table shows each step with the current word, prefix check result, and count. The variable tracker shows how count and current word change over time. Key moments clarify why count only increases on true prefix checks, what happens if no words match, and why we check words one by one. The quiz tests understanding of count values and prefix checks at different steps.