String matching basics in Data Structures Theory - Step-by-Step Execution

Concept Flow - String matching basics

Start with Text and Pattern

↓

Set index i=0 in Text

↓

Compare Pattern with Text substring at i

↓

Pattern found

↓

Check if i <= Text length - Pattern length

Yes/No↓

End

The process slides the pattern over the text, comparing characters at each position until a match is found or the text ends.

Execution Sample

Data Structures Theory

text = "hello"
pattern = "ll"
for i in range(len(text) - len(pattern) + 1):
    if text[i:i+len(pattern)] == pattern:
        print(f"Pattern found at index {i}")
        break

This code checks each position in 'hello' to find where 'll' appears.

Analysis Table

Step	i (index in text)	Substring compared	Comparison Result	Action
1	0	"he"	No	Increment i
2	1	"el"	No	Increment i
3	2	"ll"	Yes	Print match and break
4	-	-	-	Exit loop after match

💡 Loop ends after finding pattern at index 2

State Tracker

Variable	Start	After 1	After 2	After 3	Final
i	0	1	2	-	2
substring	"he"	"el"	"ll"	-	"ll"
match_found	False	False	True	-	True

Key Insights - 3 Insights

Why do we stop checking after i = len(text) - len(pattern)?

Because beyond this point, the remaining text is shorter than the pattern, so a full match is impossible (see execution_table row 4).

What happens if the pattern is not found in the text?

The loop completes all iterations without a match, and no break occurs, meaning no match is printed (not shown in this example but implied by the loop structure).

Why do we compare substrings of length equal to the pattern?

To check if the pattern exactly matches that part of the text, ensuring a correct match (see execution_table column 'Substring compared').

Visual Quiz - 3 Questions

Look at the execution table, what substring is compared at step 2?

A "he"

B "el"

C "ll"

D "lo"

💡 Hint

Check the 'Substring compared' column at step 2 in the execution_table.

At which step does the pattern match the text?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for 'Yes' in the 'Comparison Result' column in the execution_table.

If the pattern was "lo" instead of "ll", at which index would the match be found?

A 1

B 2

C 3

D 4

💡 Hint

Consider the substring of length 2 starting at each index in 'hello' and check where "lo" appears.

Concept Snapshot

String matching basics:
- Slide pattern over text from start to end
- At each position, compare substring of text with pattern
- If match found, stop and report index
- Stop when remaining text is shorter than pattern
- Simple and direct method for finding patterns

Full Transcript

String matching basics involve checking if a smaller string (pattern) appears inside a larger string (text). We start at the beginning of the text and compare the pattern with each substring of the same length. If they match, we report the position and stop. If not, we move one character forward and try again. We stop when there is no room left for the pattern to fit. This method is straightforward and helps find exact matches in text.