String comparison - Step-by-Step Execution

Concept Flow - String comparison

Start

↓

Read two strings

↓

Compare characters one by one

↓

Characters equal?

No→Return difference

Yes↓

End of strings?

No→Move to next char

Yes↓

Strings equal

→Return 0

↓

End

The program compares two strings character by character until a difference is found or both strings end.

Execution Sample

C

int result = strcmp("cat", "car");
if (result == 0) {
  printf("Equal\n");
} else {
  printf("Not equal\n");
}

This code compares two strings "cat" and "car" and prints if they are equal or not.

Execution Table

Step	Index	Char in str1	Char in str2	Comparison	Result	Action
1	0	'c'	'c'	'c' == 'c'	0	Continue to next character
2	1	'a'	'a'	'a' == 'a'	0	Continue to next character
3	2	't'	'r'	't' != 'r'	't' - 'r' = 2	Return 2 (strings differ)

💡 At index 2, characters differ ('t' vs 'r'), so strcmp returns positive value 2.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
index	0	1	2	2	2
char_str1	'c'	'a'	't'	't'	't'
char_str2	'c'	'a'	'r'	'r'	'r'
result	undefined	0	0	2	2

Key Moments - 2 Insights

Why does strcmp return a positive number instead of just 1 or -1?

strcmp returns the difference between the first differing characters' ASCII codes, not just 1 or -1, as shown in step 3 of the execution_table where 't' - 'r' equals 2.

What happens if both strings are exactly the same?

If all characters match and both strings end at the same time, strcmp returns 0, indicating equality, as explained in the concept_flow and would be shown if no difference was found.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'result' after step 2?

A 1

B 0

C 2

D -1

💡 Hint

Check the 'result' column in execution_table row for step 2.

At which index do the two strings first differ according to the execution_table?

A 2

B 1

C 0

D 3

💡 Hint

Look at the 'Index' column where 'Comparison' shows characters are not equal.

If the first differing characters were 'r' in str1 and 't' in str2, what would strcmp return?

A 0

B 2

C -2

D 1

💡 Hint

strcmp returns the difference: char_str1 - char_str2, so 'r' - 't' is negative.

Concept Snapshot

strcmp(str1, str2) compares two strings character by character.
Returns 0 if equal.
Returns positive if first differing char in str1 > str2.
Returns negative if first differing char in str1 < str2.
Stops at first difference or end of strings.

Full Transcript

This visual execution shows how the C function strcmp compares two strings. It checks each character from the start, comparing them one by one. If characters are equal, it moves to the next. When it finds a difference, it returns the difference of their ASCII codes. If no difference is found and both strings end, it returns zero meaning the strings are equal. The example compares "cat" and "car". At index 2, 't' and 'r' differ, so strcmp returns 2, a positive number. This means "cat" is greater than "car" in lexicographical order.