C Case Sensitivity: strcmp, tolower & Common Mistakes

1. Introduction

2. C language is a case-sensitive language

3. Determining Uppercase vs Lowercase (Single Character)

1. Range Check Using Character Codes

if ('A' <= c && c <= 'Z') {
    // c is uppercase
}
if ('a' <= c && c <= 'z') {
    // c is lowercase
}

2. Using Functions from the Standard Library (ctype.h)

• isupper(c) … returns true (non‑zero) if c is an uppercase alphabetic character
• islower(c) … returns true if c is a lowercase alphabetic character

#include <stdio.h>
#include <ctype.h>

int main(void) {
    char c;
    printf("Please enter a single alphabetic character: ");
    scanf("%c", &c);

    if (isupper(c)) {
        printf("The entered character is uppercase.
");
    } else if (islower(c)) {
        printf("The entered character is lowercase.
");
    } else {
        printf("The entered character is not an alphabetic letter.
");
    }
    return 0;
}

4. Methods for Comparing Strings Case‑Sensitive and Case‑Insensitive

1. Case‑Sensitive Comparison Using the strcmp Function

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

int main(void) {
    char s1[100], s2[100];
    printf("Please enter the first string: ");
    scanf("%s", s1);
    printf("Please enter the second string: ");
    scanf("%s", s2);

    if (strcmp(s1, s2) == 0) {
        printf("The two strings match.
");
    } else {
        printf("The two strings do not match.
");
    }
    return 0;
}

2. Case‑Insensitive Comparison (strcasecmp or stricmp)

• strcasecmp(s1, s2) … compare ignoring case
• On Windows, use _stricmp(s1, s2) or stricmp(s1, s2)

#include <stdio.h>
#include <strings.h> // Linux or macOS
// #include <string.h> // Use stricmp or _stricmp on Windows

int main(void) {
    char s1[100], s2[100];
    printf("Please enter the first string: ");
    scanf("%s", s1);
    printf("Please enter the second string: ");
    scanf("%s", s2);

    if (strcasecmp(s1, s2) == 0) {
        printf("The two strings match (ignoring case).
");
    } else {
        printf("The two strings do not match.
");
    }
    return 0;
}

3. Comparing Only the First n Characters with strncmp or strncasecmp

if (strncasecmp(s1, s2, 5) == 0) {
    // The first 5 characters match
}

4. You Can Implement Your Own When Unavailable

5. Case Conversion (Uppercase ⇔ Lowercase Conversion)

1. Standard Library toupper() and tolower() Functions

• toupper(c) — Returns the uppercase version if c is lowercase; otherwise returns c unchanged
• tolower(c) — Returns the lowercase version if c is uppercase; otherwise returns c unchanged

Example: Converting Lowercase to Uppercase or Uppercase to Lowercase

#include <stdio.h>
#include <ctype.h>

int main(void) {
    char c;
    printf("Please enter a single alphabetic character: ");
    scanf(" %c", &c);

    printf("Uppercase: %cn", toupper(c));
    printf("Lowercase: %cn", tolower(c));

    return 0;
}

2. Converting an Entire String

Example: Converting a String to All Lowercase

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

int main(void) {
    char str[100];
    printf("Please enter a string: ");
    scanf("%s", str);

    for (int i = 0; i < strlen(str); i++) {
        str[i] = tolower(str[i]);
    }
    printf("Lowercase conversion result: %sn", str);

    return 0;
}

3. Conversion Using Arithmetic Operations

// Lowercase to Uppercase
if ('a' <= c && c <= 'z') {
    c = c - ('a' - 'A');
}

// Uppercase to Lowercase
if ('A' <= c && c <= 'Z') {
    c = c + ('a' - 'A');
}

6. Common Mistakes, Pitfalls, and Error Cases

1. Misusing strcmp

if (strcmp("Hello", "hello") == 0) {
    // condition does not hold
}

2. Forgetting Case Conversion

if (strcmp(user_input, "password") == 0) {
    // If the input value is "Password" or "PASSWORD", it will not match
}

3. strcasecmp and stricmp Availability and Platform Dependence

• Linux/UNIX systems … strcasecmp
• Windows … stricmp or _stricmp
• Older environments may support neither

4. Compile‑time and Run‑time Error Examples

• Missing header file includes (ctype.h, string.h)
• Typo in function name (e.g., toloower, toLower etc.)
• Buffer overflow due to insufficient array size

printf("%c", tolower('A'));

5. Bug Example: Authentication Failure Due to Case Mismatch

• The system treats “Password” and “password” as different
• Users often don’t care about case
• Consequently, even with the correct password, authentication fails

• Normalize all input to lowercase
• Specify clearly in the specification whether case matters and inform users

7. Mini column on differences with other languages (Python, Java, etc.)

1. Distinction of identifiers (variable names, function names)

• C Example: value and Value are different
• Java Fully distinguished. It is strict about case in class and method names as well.
• Python Also case‑sensitive. Python’s naming conventions are clearly defined in PEP 8, so consistent use of case is recommended.
• JavaScript / C++ / Go, etc. All are case‑sensitive.

2. Differences in string comparison

• C String comparison uses functions such as strcmp, and case distinction is the default.
• Python You can compare with the == operator, and it is case‑sensitive. To perform a case‑insensitive comparison, convert using lower() or casefold() before comparing.

  if s1.lower() == s2.lower():
      # case-insensitive comparison

• Java The equals() method is case‑sensitive, while equalsIgnoreCase() provides a case‑insensitive comparison.
• JavaScript The === operator is case‑sensitive. For case‑insensitive comparison, use toLowerCase() and similar methods to normalize before comparing.

3. User‑focused cautions

4. Differences in coding conventions and practices

8. Summary Table of Uppercase and Lowercase Comparisons

Quick Reference Table by Use Case – Main Functions/Methods

Key Points for Usage Examples

• When comparing strings, choose functions based on case‑sensitivity
• Verify that non‑standard functions like strcasecmp are available in your development environment
• For portability and maintainability, converting with tolower() or toupper() followed by strcmp is reliable

9. Summary

Common Implementation Patterns (Recommended Examples)

• Normalize the case of input values before comparing
• Check whether environment-dependent functions are available, and consider custom implementations if not
• Be mindful of case rules when naming and coding

10. FAQ (Frequently Asked Questions and Answers)

if (isupper(c)) { /* uppercase */ }
if (islower(c)) { /* lowercase */ }

11. Related Links & References

Useful External References & Official Documentation

• ASCII Code Table (external site) A handy list that shows the code values of alphabets and symbols at a glance.
• Linux man page – strcasecmp(3) Detailed reference for case‑insensitive comparisons when needed.

Conclusion