C strcmp Function

What is strcmp()?

The strcmp() function compares two strings lexicographically (dictionary order) character by character. It returns an integer indicating whether the strings are equal or which one is greater.

Header File

#include <string.h>

Syntax

int strcmp(const char *str1, const char *str2);

Return Values

Return Value	Meaning
0	Strings are equal
> 0 (positive)	`str1` is greater than `str2`
< 0 (negative)	`str1` is less than `str2`

How Comparison Works

Compares ASCII values character by character until:

A difference is found
Null terminator (\0) is reached

Basic Examples

Example 1: Equal Strings

#include <stdio.h>
#include <string.h>
int main() {
char str1[] = "Hello";
char str2[] = "Hello";
int result = strcmp(str1, str2);
if(result == 0) {
printf("Strings are EQUAL\n");
}
printf("Return value: %d\n", result);  // 0
return 0;
}

Example 2: Different Strings

#include <stdio.h>
#include <string.h>
int main() {
char str1[] = "Apple";
char str2[] = "Banana";
int result = strcmp(str1, str2);
printf("'Apple' vs 'Banana': %d\n", result);  // Negative (A < B)
printf("'Banana' vs 'Apple': %d\n", strcmp(str2, str1));  // Positive
if(result < 0) {
printf("'Apple' comes before 'Banana'\n");
}
return 0;
}

Example 3: Case Sensitivity

#include <stdio.h>
#include <string.h>
int main() {
char str1[] = "hello";
char str2[] = "HELLO";
int result = strcmp(str1, str2);
printf("'hello' vs 'HELLO': %d\n", result);
// 'h' (104) vs 'H' (72) → 104 - 72 = 32 (positive)
if(result != 0) {
printf("strcmp() is CASE-SENSITIVE!\n");
}
return 0;
}

Practical Examples

Example 4: User Input Validation

#include <stdio.h>
#include <string.h>
int main() {
char username[50];
char password[50];
// Correct credentials
char correct_user[] = "admin";
char correct_pass[] = "secret123";
printf("Username: ");
scanf("%s", username);
printf("Password: ");
scanf("%s", password);
// Compare strings
if(strcmp(username, correct_user) == 0 && 
strcmp(password, correct_pass) == 0) {
printf("Login Successful!\n");
} else {
printf("Invalid username or password!\n");
}
return 0;
}

Example 5: Sorting Strings

#include <stdio.h>
#include <string.h>
int main() {
char fruits[5][20] = {"Banana", "Apple", "Mango", "Cherry", "Date"};
char temp[20];
// Bubble sort using strcmp
for(int i = 0; i < 4; i++) {
for(int j = i + 1; j < 5; j++) {
if(strcmp(fruits[i], fruits[j]) > 0) {
// Swap if fruits[i] > fruits[j]
strcpy(temp, fruits[i]);
strcpy(fruits[i], fruits[j]);
strcpy(fruits[j], temp);
}
}
}
printf("Sorted fruits:\n");
for(int i = 0; i < 5; i++) {
printf("%s\n", fruits[i]);
}
return 0;
}
/* Output:
Apple
Banana
Cherry
Date
Mango
*/

Example 6: Menu System

#include <stdio.h>
#include <string.h>
int main() {
char choice[10];
while(1) {
printf("\nMenu:\n");
printf("1. Start\n");
printf("2. Stop\n");
printf("3. Exit\n");
printf("Enter choice: ");
scanf("%s", choice);
if(strcmp(choice, "1") == 0 || strcmp(choice, "start") == 0) {
printf("Starting...\n");
}
else if(strcmp(choice, "2") == 0 || strcmp(choice, "stop") == 0) {
printf("Stopping...\n");
}
else if(strcmp(choice, "3") == 0 || strcmp(choice, "exit") == 0) {
printf("Exiting...\n");
break;
}
else {
printf("Invalid choice!\n");
}
}
return 0;
}

Example 7: Command Line Arguments

#include <stdio.h>
#include <string.h>
int main(int argc, char *argv[]) {
if(argc != 3) {
printf("Usage: %s <name> <age>\n", argv[0]);
return 1;
}
// Compare first argument
if(strcmp(argv[1], "admin") == 0) {
printf("Welcome Admin!\n");
} else {
printf("Hello %s\n", argv[1]);
}
// Check if age is numeric (simple check)
for(int i = 0; argv[2][i] != '\0'; i++) {
if(argv[2][i] < '0' || argv[2][i] > '9') {
printf("Age must be a number!\n");
return 1;
}
}
printf("Age: %s\n", argv[2]);
return 0;
}

Example 8: Searching in Array

#include <stdio.h>
#include <string.h>
int main() {
char names[6][20] = {"Alice", "Bob", "Charlie", "David", "Emma", "Frank"};
char search[20];
int found = -1;
printf("Enter name to search: ");
scanf("%s", search);
for(int i = 0; i < 6; i++) {
if(strcmp(names[i], search) == 0) {
found = i;
break;
}
}
if(found != -1) {
printf("'%s' found at index %d\n", search, found);
} else {
printf("'%s' not found\n", search);
}
return 0;
}

Detailed ASCII Comparison Examples

#include <stdio.h>
#include <string.h>
int main() {
// ASCII values for reference
printf("ASCII 'A': %d\n", 'A');  // 65
printf("ASCII 'a': %d\n", 'a');  // 97
printf("ASCII '0': %d\n", '0');  // 48
printf("\nComparison results:\n");
// Same string
printf("'cat' vs 'cat': %d\n", strcmp("cat", "cat"));  // 0
// Different at first character
printf("'cat' vs 'dog': %d\n", strcmp("cat", "dog"));  // 'c'(99)-'d'(100) = -1
// Different at later position
printf("'cat' vs 'car': %d\n", strcmp("cat", "car"));  // 't'(116)-'r'(114) = 2
// One string is prefix of another
printf("'cat' vs 'catalog': %d\n", strcmp("cat", "catalog"));  // '\0'(0)-'a'(97) = -97
printf("'catalog' vs 'cat': %d\n", strcmp("catalog", "cat"));   // 'a'(97)-'\0'(0) = 97
// Uppercase vs lowercase
printf("'Apple' vs 'apple': %d\n", strcmp("Apple", "apple"));  // 'A'(65)-'a'(97) = -32
return 0;
}

strcmp() vs Other Comparison Functions

Function	Purpose	Case-Sensitive	Length Limited
`strcmp()`	Full string compare	Yes	No
`strncmp()`	Compare n characters	Yes	Yes
`strcasecmp()`	Case-insensitive (non-standard)	No	No
`memcmp()`	Compare memory blocks	N/A	Yes

Example: strcmp vs strncmp

#include <stdio.h>
#include <string.h>
int main() {
char str1[] = "Computer";
char str2[] = "Computing";
// strcmp compares entire string
printf("strcmp: %d\n", strcmp(str1, str2));  // 'e'(101)-'i'(105) = -4
// strncmp compares only first 4 chars
printf("strncmp (4 chars): %d\n", strncmp(str1, str2, 4));  // 0 (Comp == Comp)
// Check prefix
if(strncmp(str1, str2, 4) == 0) {
printf("Both start with 'Comp'\n");
}
return 0;
}

Example: Manual Implementation of strcmp

#include <stdio.h>
int my_strcmp(const char *s1, const char *s2) {
while(*s1 && *s2 && *s1 == *s2) {
s1++;
s2++;
}
return *s1 - *s2;
}
int main() {
char str1[] = "Hello";
char str2[] = "Hello";
char str3[] = "World";
printf("Custom strcmp('Hello','Hello'): %d\n", my_strcmp(str1, str2));  // 0
printf("Custom strcmp('Hello','World'): %d\n", my_strcmp(str1, str3));  // Negative
return 0;
}

Common Patterns and Best Practices

Pattern 1: Check Equality

if(strcmp(str1, str2) == 0) {
// Strings are equal
}

Pattern 2: Check Not Equal

if(strcmp(str1, str2) != 0) {
// Strings are different
}

Pattern 3: Alphabetical Order

if(strcmp(str1, str2) < 0) {
// str1 comes before str2 alphabetically
}

Pattern 4: Safe Comparison with Length Check

#include <stdio.h>
#include <string.h>
int safe_strcmp(const char *s1, const char *s2, size_t max_len) {
return strncmp(s1, s2, max_len);
}
int main() {
char input[20];
printf("Enter 'quit' to exit: ");
fgets(input, sizeof(input), stdin);
// Remove newline
input[strcspn(input, "\n")] = '\0';
// Safe comparison
if(strcmp(input, "quit") == 0) {
printf("Exiting...\n");
}
return 0;
}

Common Mistakes

Mistake	Wrong Code	Correct Code
Using = instead of ==	`if(strcmp(a,b) = 0)`	`if(strcmp(a,b) == 0)`
Forgetting string.h	No include	`#include <string.h>`
Expecting 1 for equal	`if(strcmp(a,b))`	`if(strcmp(a,b) == 0)`
Comparing incompatible types	`strcmp("hi", 'h')`	Both must be strings

Quick Reference

Condition	Code
Strings equal	`strcmp(s1, s2) == 0`
Strings not equal	`strcmp(s1, s2) != 0`
s1 < s2 (alphabetically)	`strcmp(s1, s2) < 0`
s1 > s2 (alphabetically)	`strcmp(s1, s2) > 0`
s1 comes before s2	`strcmp(s1, s2) < 0`

Key Points

Returns 0 when strings are identical
Case-sensitive - 'A' and 'a' are different
Compares ASCII values character by character
Stops at null terminator or first difference
Does NOT check buffer sizes - ensure strings are null-terminated
Use strncmp() for comparing only first n characters
Use strcasecmp() for case-insensitive comparison (if available)

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