Chapter Summary

This chapter introduces students to four essential C standard libraries that provide pre-built functions for common programming tasks. Understanding these libraries enables more efficient and safer code development.

string.h Library

The string.h library provides functions for manipulating C-strings (null-terminated character arrays). Key functions include:

strcpy() - String Copy

• Purpose: Copies one string to another, including the null terminator
• Syntax: strcpy(destination, source)
• Important: Programmer must ensure destination is large enough to hold source
• Example:

  char dest[10];
  strcpy(dest, "hello");  // dest now contains "hello"

strcmp() - String Compare

• Purpose: Compares two strings lexically (alphabetically)
• Return Values:
  • Returns $0$ if strings are identical
  • Returns negative value if first string is lexically earlier
  • Returns positive value if first string is lexically later
• Critical: Always compare return value against $0$, not strings directly
• Correct Usage:

  if (strcmp(s1, s2) == 0)      // s1 equals s2
  if (strcmp(s1, s2) < 0)       // s1 is lexically earlier
  if (strcmp(s1, s2) > 0)       // s1 is lexically later

strlen() - String Length

• Purpose: Returns number of characters before null terminator
• Performance Consideration: Function must scan entire string to find \0, making it slow for long strings
• Best Practice: Call strlen() once before loops, not inside loop conditions
• Example - Efficient:

  int length = strlen(str);
  for (i = 0; i < length; i++) { ... }

• Example - Inefficient (AVOID):

  for (i = 0; i < strlen(str); i++) { ... }  // strlen() called every iteration

strcat() - String Concatenate

• Purpose: Appends second string to end of first string
• Syntax: strcat(destination, source)
• Important: Destination must be large enough to hold both strings
• Example:

  char s1[20] = "hello";
  strcat(s1, "world");  // s1 now contains "helloworld"

Additional string.h Functions

• strncpy(): Copies up to $n$ characters
• strncat(): Concatenates up to $n$ characters
• strncmp(): Compares up to $n$ characters
• strchr(): Finds first occurrence of character
• strrchr(): Finds last occurrence of character
• strstr(): Finds first occurrence of substring

ctype.h Library

The ctype.h library provides functions to test character properties and perform case conversion.

Character Testing Functions

All return truthy (non-zero) if condition is true, falsey (zero) if false. Always test against $0$:

• isalpha(c): Tests if alphabetic (A-Z, a-z)
• isdigit(c): Tests if digit (0-9)
• isalnum(c): Tests if alphanumeric (letter or digit)
• isspace(c): Tests if whitespace (space, tab, newline)
• isupper(c): Tests if uppercase letter
• islower(c): Tests if lowercase letter
• isprint(c): Tests if printable character
• isgraph(c): Tests if printable but not space
• ispunct(c): Tests if punctuation
• iscntrl(c): Tests if control character
• isxdigit(c): Tests if hexadecimal digit (0-9, A-F, a-f)

Case Conversion Functions

• toupper(c): Returns uppercase equivalent; returns original if not lowercase
• tolower(c): Returns lowercase equivalent; returns original if not uppercase

Critical Concept: Pass-by-Value

toupper() and tolower() use pass-by-value, meaning they do not modify the original character. The return value must be captured:

char lowerA = 'a';
char upperA = toupper(lowerA);  // Must capture return value
// lowerA is still 'a', upperA is now 'A'

math.h Library

The math.h library provides common mathematical functions:

Rounding Functions

• ceil(x): Returns smallest integer ≥ $x$
• floor(x): Returns largest integer ≤ $x$
• round(x): Rounds $x$ to nearest integer

Absolute Value and Roots

• fabs(x): Returns absolute value of $x$
• sqrt(x): Returns square root of $x$

Exponential and Logarithmic Functions

• pow(x, y): Returns $x^y$
• exp(x): Returns $e^x$
• log(x): Returns natural logarithm of $x$
• log10(x): Returns base-10 logarithm of $x$

Trigonometric Functions

• sin(x): Returns sine of $x$ (in radians)
• cos(x): Returns cosine of $x$ (in radians)
• tan(x): Returns tangent of $x$ (in radians)

Important: All trigonometric functions use radians, not degrees. Convert degrees to radians by multiplying by $\pi/180$.

stdlib.h Library

The stdlib.h library contains general-purpose functions. This chapter focuses on string conversion and random number generation.

String to Number Conversion

• atoi(s): Converts string $s$ to integer
• atol(s): Converts string $s$ to long integer
• atof(s): Converts string $s$ to double (floating-point)

Example:

int x = atoi("42");        // x = 42
double y = atof("3.14");   // y = 3.14

Pseudo-Random Number Generation

rand() Function

• Purpose: Returns pseudo-random integer between $0$ and RAND_MAX (inclusive)
• RAND_MAX: Compiler-dependent constant, guaranteed ≥ $32767$
• Behavior: Without seeding, generates same sequence every program run

srand() Function - Seeding

• Purpose: Sets starting point (seed) for random number sequence
• Syntax: srand(seed_value)
• Key Points:
  • Call srand() exactly once at program start
  • Different seeds produce different sequences
  • Same seed always produces same sequence
  • Use srand(time(NULL)) for different sequence each run

Example - Reproducible (for debugging):

srand(42);  // Fixed seed for consistent results
for (i = 0; i < 5; i++) {
    printf("%d\n", rand());
}

Example - Different Each Run:

#include <time.h>
srand(time(NULL));  // Seed with current time
for (i = 0; i < 5; i++) {
    printf("%d\n", rand());
}

Understanding Pseudo-Random Generators

• Random number generators are not truly random but follow a deterministic sequence
• The seed determines which point in the sequence to start
• Calling srand() multiple times is unnecessary and can produce unexpected results
• For debugging, use fixed seed; for production, use time(NULL)