String searching (contains, strfind) in MATLAB - Step-by-Step Execution

Concept Flow - String searching (contains, strfind)

Start with string S

↓

Search for substring sub

↓

Return indices of sub

↓

End

The program starts with a main string and a substring to find. It checks if the substring exists inside the main string. If yes, it returns the positions; if no, it returns empty.

Execution Sample

MATLAB

S = 'hello world';
sub = 'lo';
idx = strfind(S, sub);
found = contains(S, sub);

This code searches for 'lo' inside 'hello world' using strfind and contains.

Execution Table

Step	Variable	Value	Action	Result
1	S	'hello world'	Initialize main string	'hello world'
2	sub	'lo'	Initialize substring to find	'lo'
3	idx	[4]	Call strfind(S, sub)	Returns [4] because 'lo' starts at position 4
4	found	true	Call contains(S, sub)	Returns true because 'lo' is in S
5	found	true	Assign contains result to found	found = true

💡 Search complete: strfind found index 4, contains returned true

Variable Tracker

Variable	Start	After strfind	After contains	Final
S	'hello world'	'hello world'	'hello world'	'hello world'
sub	'lo'	'lo'	'lo'	'lo'
idx	[]	[4]	[4]	[4]
found	false	false	true	true

Key Moments - 2 Insights

Why does strfind return [4] but contains returns true?

strfind returns the starting index of the substring in the main string (4 means 'lo' starts at position 4). contains returns a logical true/false if the substring exists anywhere. See execution_table rows 3 and 4.

What happens if the substring is not found?

strfind returns an empty array [], and contains returns false. This is shown by the exit_note and would be reflected in idx and found variables.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of idx after step 3?

A [4]

B []

C [1]

D true

💡 Hint

Check the 'Value' column for idx at step 3 in the execution_table.

At which step does the variable 'found' become true?

A Step 2

B Step 5

C Step 4

D Step 3

💡 Hint

Look at the 'found' variable changes in execution_table rows 4 and 5.

If sub = 'xyz' (not in S), what would strfind return?

A [4]

B [1]

C []

D true

💡 Hint

Recall from key_moments that strfind returns empty array if substring not found.

Concept Snapshot

String searching in MATLAB:
- Use strfind(S, sub) to get indices where sub starts in S
- Use contains(S, sub) to get true/false if sub is in S
- strfind returns empty [] if not found
- contains returns false if not found
- Both help find substrings inside strings

Full Transcript

This visual execution shows how MATLAB searches for a substring inside a string using strfind and contains. We start with a main string S and a substring sub. strfind returns the starting index of sub in S or empty if not found. contains returns true if sub is anywhere in S, otherwise false. The execution table traces variable values step-by-step. Key moments clarify why strfind returns indices and contains returns logical values. The quiz tests understanding of these steps.