C Notes

Hub Applications of C Programming Language Features of C Programming Language History of C Programming Language What is C Language?Why Learn C Programming in 2024?First C Program - Hello World in C with Explanation GCC Compiler - Complete Guide to GNU C Compiler Install C Compiler on Linux - GCC Setup Guide for All Distros Install C Compiler on macOS - Xcode Command Line Tools and Homebrew GCC Install C Compiler on Windows - MinGW, MSYS2, and WSL Guide Setup VS Code for C Programming - Complete Configuration Guide C Syntax Rules — Complete Guide Comments in C Programming Constants in C Programming Data Types in C Identifiers in C Programming Keywords in C Programming Structure of a C Program Variables in C Programming Arithmetic Operators in C Programming Assignment Operators in C Programming Bitwise Operators in C Programming Increment and Decrement Operators in C Logical Operators in C Programming Relational Operators in C Programming Ternary Operator in C Programming Break Statement in C Programming Continue Statement in C Programming Do-While Loop in C Programming For Loop in C Programming Goto Statement in C Programming if-else Statement in C Programming if Statement in C Programming Nested if Statements in C Programming Switch Statement in C Programming While Loop in C Programming Function Arguments in C - Pass by Value vs Pass by Reference Function Call in C Programming Function Declaration (Prototype) in C Function Definition in C Programming Introduction to Functions in C Programming Recursion in C Programming Return Statement in C Programming Storage Classes in C Programming Array Operations in C - Searching, Sorting, Insert and Delete Array Programs in C - Reverse, Rotate, Duplicates, Merge Introduction to Arrays in C Programming Multidimensional Arrays in C Programming One Dimensional Arrays in C Programming Passing Arrays to Functions in C Two Dimensional Arrays in C Programming String Comparison in C - strcmp, strncmp, Case-Insensitive String Functions in C - strlen, strcpy, strcat, strcmp C String Handling Library - Complete string.h Reference String Manipulation in C - Reverse, Uppercase, Palindrome String Programs in C - Anagram, Word Count, Vowels, Frequency Strings in C Programming - Complete Introduction Double Pointers in C - Pointer to Pointer Pointer Arithmetic in C Programming Pointer Declaration and Initialization in C Pointer Programs in C - Swap, String Reversal, Dynamic Allocation Pointers and Arrays in C - The Relationship Explained Pointers and Functions in C - Pass by Reference Introduction to Pointers in C Programming Void Pointers in C - Generic Pointers Guide calloc() in C – Contiguous Memory Allocation Dynamic Arrays in C – Runtime-Sized Arrays free() in C – Releasing Dynamic Memory malloc() in C – Dynamic Memory Allocation Memory Management Best Practices in C realloc() in C – Resizing Dynamic Memory Binary Files in C – fread and fwrite File Handling in C – Introduction File Handling Projects in C – Student Records & Phone Book File Modes in C – Complete Guide fopen() and fclose() in C – Opening and Closing Files Reading Files in C – fgetc, fgets, fscanf, fread Writing Files in C – fputc, fputs, fprintf, fwrite Conditional Compilation in C – #ifdef, #ifndef, #if #define Macros in C – Constants and Macro Functions Header Files in C – Creating Custom Headers #include Directive in C – File Inclusion Macro Functions in C – Parameterized Macros C Preprocessor – Overview and Directives Graphs in C – Adjacency Matrix, Adjacency List, BFS, and DFS Hash Tables in C – Implementation, Collision Handling, and Chaining Linked List in C – Singly, Doubly, and Circular with Implementation Queue in C – Linear, Circular, and Priority Queue Implementation Stack in C – Array and Linked List Implementation with Applications Trees in C – Binary Search Tree Implementation with Traversals C Programming Best Practices – Coding Standards, Safety, and Performance Bit Fields in C – Definition, Use Cases, and Protocol Headers C Standard Library Overview – All Headers and Common Functions Command Line Arguments in C – argc and argv Explained Debugging Techniques in C – GDB, Valgrind, and Sanitizers Function Pointers in C – Complete Guide with Examples Memory Layout of a C Program – Text, Data, BSS, Heap, Stack Bank Management System in C – Complete Project with File Storage File Encryption Tool in C – XOR Cipher Implementation Library Management System in C – Complete Project with Source Code Mini Compiler in C – Lexer and Parser for Simple Expressions Snake Game in C – Console-Based Game with Source Code Student Management System in C – Complete Project with Source Code C Programming Cheatsheet – Complete Quick Reference Guide Top 50 C Programming Interview Questions with Answers (2025)Memory Management Interview Questions in C with Answers Output Prediction Questions in C – What Will This Program Print?Pointer Interview Questions in C – Tricky Problems with Answers Common C Programming Errors and Solutions – Debugging Guide Important C Programs – 50+ Frequently Asked Programs with Solutions Quick Revision Notes for C Programming – Complete C in 10 Minutes Recommended C Programming Books – From Beginner to Expert Format Specifiers in C - Complete Reference getchar() and putchar() in C Programming gets() and puts() in C Programming C Input Output Programs and Examples printf() Function in C Programming scanf() Function in C Programming Arrays of Structures in C – Managing Collections Enums in C – Enumerated Data Types Nested Structures in C – Struct Within Struct Pointers to Structures in C – Arrow Operator Structures in C – User-Defined Data Types typedef in C – Creating Type Aliases Unions in C – Shared Memory Data Types

String Comparison in C - strcmp, strncmp, Case-Insensitive

Complete guide to comparing strings in C using strcmp, strncmp, case-insensitive comparison, and custom comparison functions with practical examples.

Comparing strings is one of the most frequent operations in any program — validating user input, sorting names alphabetically, matching passwords, or searching for keywords. Since C strings are character arrays, you cannot use == to compare their content (that only compares memory addresses). Instead, C provides dedicated comparison functions.

Why You Cannot Use == for Strings

This is perhaps the most common beginner mistake in C:

c example

#include <stdio.h>

int main() {
    char str1[] = "Hello";
    char str2[] = "Hello";
    
    // WRONG: Compares memory addresses, not content!
    if (str1 == str2) {
        printf("Equal (address)\n");
    } else {
        printf("Not equal (address)\n");  // This prints!
    }
    
    return 0;
}

Output

Not equal (address)

Even though both contain "Hello", they are stored at different memory locations, so == returns false.

str1	[H][e][l][l][o][\0] at address 0x1000
str2	[H][e][l][l][o][\0] at address 0x2000
str1 == str2	0x1000 == 0x2000 → FALSE!

strcmp() - Full String Comparison

How strcmp Works

strcmp compares strings character by character using ASCII values. It stops at the first difference or when both strings end.

c example

int strcmp(const char *s1, const char *s2);

Return values:

0 → strings are identical
Negative → s1 < s2 (s1 comes first lexicographically)
Positive → s1 > s2 (s1 comes after s2 lexicographically)

c example

#include <stdio.h>
#include <string.h>

void compareAndPrint(const char *s1, const char *s2) {
    int result = strcmp(s1, s2);
    
    if (result == 0)
        printf("\"%s\" == \"%s\"\n", s1, s2);
    else if (result < 0)
        printf("\"%s\" < \"%s\" (result: %d)\n", s1, s2, result);
    else
        printf("\"%s\" > \"%s\" (result: %d)\n", s1, s2, result);
}

int main() {
    compareAndPrint("apple", "apple");
    compareAndPrint("apple", "banana");
    compareAndPrint("banana", "apple");
    compareAndPrint("cat", "car");
    compareAndPrint("hello", "hello world");
    compareAndPrint("ABC", "abc");
    
    return 0;
}

Output

"apple" == "apple"
"apple" < "banana" (result: -1)
"banana" > "apple" (result: 1)
"cat" > "car" (result: 16)
"hello" < "hello world" (result: -32)
"ABC" < "abc" (result: -32)

Step-by-Step Comparison Process

Position 0: 'c' vs 'c'	equal, continue
Position 1: 'a' vs 'a'	equal, continue
Position 2: 't' vs 'r'	different!
't' (116) - 'r' (114) = +2	return positive
Result	"cat" > "car"

Practical Use: Sorting Strings

Example

#include <stdio.h> #include <string.h> void sortStrings(char arr[][20], int n) { char temp[20]; for (int i = 0; i < n - 1; i++) { for (int j = 0; j < n - i - 1; j++) { if (strcmp(arr[j], arr[j + 1]) > 0) { strcpy(temp, arr[j]); strcpy(arr[j], arr[j + 1]); strcpy(arr[j + 1], temp); } } } } int main() { char names[5][20] = {"Charlie", "Alice", "Eve", "Bob", "Diana"}; int n = 5; printf("Before sorting:\n"); for (int i = 0; i < n; i++) printf(" %s\n", names[i]); sortStrings(names, n); printf("\nAfter sorting:\n"); for (int i = 0; i < n; i++) printf(" %s\n", names[i]); return 0; }

Output

Before sorting:
  Charlie
  Alice
  Eve
  Bob
  Diana

After sorting:
  Alice
  Bob
  Charlie
  Diana
  Eve

strncmp() - Compare First N Characters

When you only need to compare a prefix — like checking file extensions or command prefixes:

c example

int strncmp(const char *s1, const char *s2, size_t n);

c example

#include <stdio.h>
#include <string.h>

int main() {
    char filename[] = "report_2024.pdf";
    
    // Check if file starts with "report"
    if (strncmp(filename, "report", 6) == 0) {
        printf("This is a report file.\n");
    }
    
    // Check file extension
    char *ext = filename + strlen(filename) - 4;
    if (strcmp(ext, ".pdf") == 0) {
        printf("It's a PDF file.\n");
    }
    
    // Compare different strings with limit
    printf("\nstrncmp(\"Hello\", \"Help\", 3) = %d\n", strncmp("Hello", "Help", 3));
    printf("strncmp(\"Hello\", \"Help\", 4) = %d\n", strncmp("Hello", "Help", 4));
    
    return 0;
}

Output

This is a report file.
It's a PDF file.

strncmp("Hello", "Help", 3) = 0
strncmp("Hello", "Help", 4) = -4

Case-Insensitive String Comparison

C standard library doesn't include a case-insensitive comparison function, but most systems provide one (non-standard):

POSIX: strcasecmp

c example

#include <stdio.h>
#include <strings.h>  // Note: strings.h (with 's')

int main() {
    // Available on Linux/macOS (POSIX)
    printf("%d\n", strcasecmp("Hello", "HELLO"));  // 0
    printf("%d\n", strcasecmp("Apple", "banana")); // negative
    return 0;
}

Custom Implementation (Portable)

Example

#include <stdio.h> #include <ctype.h> int strcmpIgnoreCase(const char *s1, const char *s2) { while (*s1 != '\0' && *s2 != '\0') { int diff = tolower((unsigned char)*s1) - tolower((unsigned char)*s2); if (diff != 0) return diff; s1++; s2++; } return tolower((unsigned char)*s1) - tolower((unsigned char)*s2); } int main() { printf("Compare \"Hello\" vs \"HELLO\": %d\n", strcmpIgnoreCase("Hello", "HELLO")); printf("Compare \"Apple\" vs \"apple\": %d\n", strcmpIgnoreCase("Apple", "apple")); printf("Compare \"ABC\" vs \"abd\": %d\n", strcmpIgnoreCase("ABC", "abd")); // Practical use: user input validation char input[] = "YES"; if (strcmpIgnoreCase(input, "yes") == 0) { printf("\nUser confirmed!\n"); } return 0; }

Output

Compare "Hello" vs "HELLO": 0
Compare "Apple" vs "apple": 0
Compare "ABC" vs "abd": -1

User confirmed!

Lexicographic Order Explained

String comparison follows lexicographic order — dictionary ordering based on character codes:

'0'-'9'	48-57
'A'-'Z'	65-90
'a'-'z'	97-122
Therefore	digits < uppercase < lowercase

Comparison with memcmp

memcmp compares raw bytes without stopping at null terminators:

c example

#include <stdio.h>
#include <string.h>

int main() {
    char s1[] = "Hello\0World";  // Contains embedded null
    char s2[] = "Hello\0Earth";
    
    // strcmp stops at first '\0'
    printf("strcmp: %d\n", strcmp(s1, s2));    // 0 (both = "Hello")
    
    // memcmp compares all specified bytes
    printf("memcmp(11): %d\n", memcmp(s1, s2, 11));  // Non-zero
    
    return 0;
}

Output

strcmp: 0
memcmp(11): 1

Comparison Function Summary

Function	Compares	Stops at '\0'?	Case-sensitive?	Standard?
`strcmp`	Full strings	Yes	Yes	C89+
`strncmp`	First n chars	Yes	Yes	C89+
`strcasecmp`	Full strings	Yes	No	POSIX
`strncasecmp`	First n chars	Yes	No	POSIX
`memcmp`	n bytes	No	Yes	C89+

Interview Questions

Q1: What does strcmp return when comparing "abc" with "abcd"?

It returns a negative value. When all characters of the shorter string match the beginning of the longer string, strcmp compares the null terminator '\0' (value 0) of "abc" with 'd' (value 100) in "abcd", resulting in 0 - 100 = -100.

Q2: Why should you use strncmp instead of strcmp in security-sensitive code?

strncmp bounds the comparison length, preventing potential issues with unterminated strings or timing attacks. It's also useful when you only need to check a prefix (like a command name) without caring about the rest.

Q3: How does locale affect string comparison?

strcmp uses raw byte values (ASCII/UTF-8) and is locale-independent. For locale-aware comparison (proper alphabetical ordering in different languages), use strcoll() which respects the current locale settings.

Q4: Implement a version of strcmp that compares strings in reverse order.

Compare from the last characters moving backward. First find both lengths, then compare from len-1 toward 0. If lengths differ and all compared characters match, the longer string is "greater".

Summary

Never use == to compare strings — it compares addresses, not content
strcmp returns 0 for equality, negative if s1 < s2, positive if s1 > s2
strncmp limits comparison to first N characters — useful for prefix matching
Case-insensitive comparison requires strcasecmp (POSIX) or a custom function
Lexicographic order is based on ASCII values: digits < uppercase < lowercase
memcmp compares raw bytes and doesn't stop at null terminators

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for String Comparison in C - strcmp, strncmp, Case-Insensitive.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.

Search Terms

c-programming, c programming, programming, strings, string, comparison, string comparison in c - strcmp, strncmp, case-insensitive

Continue learning this concept

StringsString Functions in C - strlen, strcpy, strcat, strcmpMaster the essential string functions in C including strlen, strcpy, strncpy, strcat, strncat, strcmp, and strncmp with examples, custom implementations, and safety tips.StringsC String Handling Library - Complete string.h ReferenceComplete reference guide to the C string.h library covering all string and memory functions including strchr, strstr, strtok, memcpy, memset, and more.StringsString Manipulation in C - Reverse, Uppercase, PalindromeLearn string manipulation techniques in C including reversing strings, converting case, checking palindromes, removing characters, and extracting substrings.StringsString Programs in C - Anagram, Word Count, Vowels, FrequencyPractice essential string programs in C including anagram check, word count, vowel count, character frequency, remove duplicates, and more coding problems.StringsStrings in C Programming - Complete IntroductionLearn about strings in C programming including character arrays, null terminator, string declaration, initialization methods, and how strings are stored in memory.Advanced CC Programming Best Practices – Coding Standards, Safety, and PerformanceLearn essential C programming best practices covering coding standards, memory safety, error handling, performance optimization, and writing maintainable C code.

PreviousTwo Dimensional Arrays in C Programming NextString Functions in C - strlen, strcpy, strcat, strcmp

C Notes

String Comparison in C - strcmp, strncmp, Case-Insensitive

Complete guide to comparing strings in C using strcmp, strncmp, case-insensitive comparison, and custom comparison functions with practical examples.

Why You Cannot Use == for Strings

This is perhaps the most common beginner mistake in C:

c example

#include <stdio.h>

int main() {
    char str1[] = "Hello";
    char str2[] = "Hello";
    
    // WRONG: Compares memory addresses, not content!
    if (str1 == str2) {
        printf("Equal (address)\n");
    } else {
        printf("Not equal (address)\n");  // This prints!
    }
    
    return 0;
}

Output

Not equal (address)

Even though both contain "Hello", they are stored at different memory locations, so == returns false.

str1	[H][e][l][l][o][\0] at address 0x1000
str2	[H][e][l][l][o][\0] at address 0x2000
str1 == str2	0x1000 == 0x2000 → FALSE!

strcmp() - Full String Comparison

How strcmp Works

strcmp compares strings character by character using ASCII values. It stops at the first difference or when both strings end.

c example

int strcmp(const char *s1, const char *s2);

Return values:

0 → strings are identical
Negative → s1 < s2 (s1 comes first lexicographically)
Positive → s1 > s2 (s1 comes after s2 lexicographically)

c example

#include <stdio.h>
#include <string.h>

void compareAndPrint(const char *s1, const char *s2) {
    int result = strcmp(s1, s2);
    
    if (result == 0)
        printf("\"%s\" == \"%s\"\n", s1, s2);
    else if (result < 0)
        printf("\"%s\" < \"%s\" (result: %d)\n", s1, s2, result);
    else
        printf("\"%s\" > \"%s\" (result: %d)\n", s1, s2, result);
}

int main() {
    compareAndPrint("apple", "apple");
    compareAndPrint("apple", "banana");
    compareAndPrint("banana", "apple");
    compareAndPrint("cat", "car");
    compareAndPrint("hello", "hello world");
    compareAndPrint("ABC", "abc");
    
    return 0;
}

Output

"apple" == "apple"
"apple" < "banana" (result: -1)
"banana" > "apple" (result: 1)
"cat" > "car" (result: 16)
"hello" < "hello world" (result: -32)
"ABC" < "abc" (result: -32)

Step-by-Step Comparison Process

Position 0: 'c' vs 'c'	equal, continue
Position 1: 'a' vs 'a'	equal, continue
Position 2: 't' vs 'r'	different!
't' (116) - 'r' (114) = +2	return positive
Result	"cat" > "car"

Practical Use: Sorting Strings

Example

Output

Before sorting:
  Charlie
  Alice
  Eve
  Bob
  Diana

After sorting:
  Alice
  Bob
  Charlie
  Diana
  Eve

strncmp() - Compare First N Characters

When you only need to compare a prefix — like checking file extensions or command prefixes:

c example

int strncmp(const char *s1, const char *s2, size_t n);

c example

#include <stdio.h>
#include <string.h>

int main() {
    char filename[] = "report_2024.pdf";
    
    // Check if file starts with "report"
    if (strncmp(filename, "report", 6) == 0) {
        printf("This is a report file.\n");
    }
    
    // Check file extension
    char *ext = filename + strlen(filename) - 4;
    if (strcmp(ext, ".pdf") == 0) {
        printf("It's a PDF file.\n");
    }
    
    // Compare different strings with limit
    printf("\nstrncmp(\"Hello\", \"Help\", 3) = %d\n", strncmp("Hello", "Help", 3));
    printf("strncmp(\"Hello\", \"Help\", 4) = %d\n", strncmp("Hello", "Help", 4));
    
    return 0;
}

Output

This is a report file.
It's a PDF file.

strncmp("Hello", "Help", 3) = 0
strncmp("Hello", "Help", 4) = -4

Case-Insensitive String Comparison

C standard library doesn't include a case-insensitive comparison function, but most systems provide one (non-standard):

POSIX: strcasecmp

c example

#include <stdio.h>
#include <strings.h>  // Note: strings.h (with 's')

int main() {
    // Available on Linux/macOS (POSIX)
    printf("%d\n", strcasecmp("Hello", "HELLO"));  // 0
    printf("%d\n", strcasecmp("Apple", "banana")); // negative
    return 0;
}

Custom Implementation (Portable)

Example

Output

Compare "Hello" vs "HELLO": 0
Compare "Apple" vs "apple": 0
Compare "ABC" vs "abd": -1

User confirmed!

Lexicographic Order Explained

String comparison follows lexicographic order — dictionary ordering based on character codes:

'0'-'9'	48-57
'A'-'Z'	65-90
'a'-'z'	97-122
Therefore	digits < uppercase < lowercase

Comparison with memcmp

memcmp compares raw bytes without stopping at null terminators:

c example

#include <stdio.h>
#include <string.h>

int main() {
    char s1[] = "Hello\0World";  // Contains embedded null
    char s2[] = "Hello\0Earth";
    
    // strcmp stops at first '\0'
    printf("strcmp: %d\n", strcmp(s1, s2));    // 0 (both = "Hello")
    
    // memcmp compares all specified bytes
    printf("memcmp(11): %d\n", memcmp(s1, s2, 11));  // Non-zero
    
    return 0;
}

Output

strcmp: 0
memcmp(11): 1

Comparison Function Summary

Function	Compares	Stops at '\0'?	Case-sensitive?	Standard?
`strcmp`	Full strings	Yes	Yes	C89+
`strncmp`	First n chars	Yes	Yes	C89+
`strcasecmp`	Full strings	Yes	No	POSIX
`strncasecmp`	First n chars	Yes	No	POSIX
`memcmp`	n bytes	No	Yes	C89+

Interview Questions

Q1: What does strcmp return when comparing "abc" with "abcd"?

Q2: Why should you use strncmp instead of strcmp in security-sensitive code?

Q3: How does locale affect string comparison?

Q4: Implement a version of strcmp that compares strings in reverse order.

Compare from the last characters moving backward. First find both lengths, then compare from len-1 toward 0. If lengths differ and all compared characters match, the longer string is "greater".

Summary

Never use == to compare strings — it compares addresses, not content
strcmp returns 0 for equality, negative if s1 < s2, positive if s1 > s2
strncmp limits comparison to first N characters — useful for prefix matching
Case-insensitive comparison requires strcasecmp (POSIX) or a custom function
Lexicographic order is based on ASCII values: digits < uppercase < lowercase
memcmp compares raw bytes and doesn't stop at null terminators

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for String Comparison in C - strcmp, strncmp, Case-Insensitive.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.

Search Terms

c-programming, c programming, programming, strings, string, comparison, string comparison in c - strcmp, strncmp, case-insensitive

Continue learning this concept

PreviousTwo Dimensional Arrays in C Programming NextString Functions in C - strlen, strcpy, strcat, strcmp