Functions in C: Built-in

C is a powerful programming language that offers a wide range of built-in functions to simplify programming tasks and provide ready-to-use functionality. These built-in functions are part of the standard library and cover various areas such as input/output, mathematical operations, string manipulation, and more. In this article, we will explore some of the important and commonly used built-in functions in C.

1. <cctype>:

This library provides functions for character classification and manipulation.

Function	Purpose	Example
isalnum()	Checks if a character is alphanumeric.	isalnum('A') returns true.
isalpha()	Checks if a character is alphabetic.	isalpha('7') returns false.
isdigit()	Checks if a character is a decimal digit.	isdigit('3') returns true.
islower()	Checks if a character is lowercase.	islower('x') returns true.
isupper()	Checks if a character is uppercase.	isupper('Z') returns true.
isspace()	Checks if a character is whitespace.	isspace(' ') returns true.
tolower()	Converts a character to lowercase.	tolower('H') returns 'h'.
toupper()	Converts a character to uppercase.	toupper('e') returns 'E'.

2. <math.h>:

This library provides mathematical functions and constants.

Function	Purpose	Example
sqrt()	Calculates the square root of a number.	sqrt(25) returns 5.
pow()	Calculates the power of a number.	pow(2, 3) returns 8.
abs()	Returns the absolute value of a number.	abs(-10) returns 10.
sin()	Calculates the sine of an angle (in radians).	sin(M_PI / 2) returns 1.
cos()	Calculates the cosine of an angle (in radians).	cos(0) returns 1.
tan()	Calculates the tangent of an angle (in radians).	tan(M_PI / 4) returns 1.

3. <string.h>:

This library provides functions for string manipulation and memory operations.

Function	Purpose	Example
strlen()	Returns the length of a C-style string.	strlen("Hello") returns 5.
strcpy()	Copies a C-style string to another.	char dest[10]; strcpy(dest, "Hello");
strcat()	Concatenates two C-style strings.	char str1[10] = "Hello"; char str2[10] = "World"; strcat(str1, str2);
strcmp()	Compares two C-style strings lexicographically.	strcmp("apple", "banana") returns a negative value.

4. <string>:

This library provides the string class for more convenient string manipulation compared to C-style strings.

Member Function	Purpose	Example
length()	Returns the length of the string.	string str = "Hello"; str.length() returns 5.
substr()	Returns a substring of the string.	string str = "Hello World"; str.substr(6, 5) returns "World".
find()	Finds the first occurrence of a substring within the string.	string str = "Hello World"; str.find("World") returns 6.
replace()	Replaces a portion of the string with another string.	string str = "Hello World"; str.replace(6, 5, "Universe") replaces "World" with "Universe".
operator+	Concatenates two strings.	string str1 = "Hello"; string str2 = "World"; string result = str1 + str2;
operator==	Compares two strings for equality.	string str1 = "Hello"; string str2 = "World"; if (str1 == str2) ...
size()	Returns the size of the string (same as length()).	string str = "Hello"; str.size() returns 5.

5. <cstdlib>:

This library provides general-purpose functions.

Function/Macro	Purpose	Example
atoi()	Converts a string to an integer.	int num = atoi("123");
atof()	Converts a string to a floating-point number.	float num = atof("3.14");
rand()	Generates a random integer.	int randomNum = rand();
srand()	Seeds the random number generator.	srand(time(NULL));
exit()	Terminates the program.	exit(0);

6. <ctime>:

This library provides functions for working with time and dates.

Function/Structure	Purpose	Example
time()	Obtains the current calendar time.	time_t currentTime = time(NULL);
ctime()	Converts a time value to a string representation.	time_t currentTime = time(NULL); char* timeString = ctime(&currentTime);
difftime()	Calculates the difference in seconds between two time values.	time_t startTime = time(NULL); // Perform some operations... time_t endTime = time(NULL); double duration = difftime(endTime, startTime);

7. <algorithm>:

This library provides functions for performing operations on sequences of elements.

Function	Purpose	Example
sort()	Sorts a sequence of elements.	int arr[] = {3, 1, 4, 1, 5}; sort(arr, arr + 5);
binary_search()	Checks if an element exists in a sorted sequence.	int arr[] = {1, 2, 3, 4, 5}; bool exists = binary_search(arr, arr + 5, 3);
find()	Finds the first occurrence of an element in a sequence.	vector<int> vec = {10, 20, 30, 40, 50}; auto it = find(vec.begin(), vec.end(), 30);

8. <stdlib.h>:

This library provides functions for memory allocation and other utility functions.

Function	Purpose	Example
malloc()	Allocates a block of memory.	int* ptr = (int*)malloc(5 * sizeof(int));
calloc()	Allocates a block of memory and initializes it to zero.	int* ptr = (int*)calloc(5, sizeof(int));
realloc()	Changes the size of a previously allocated block of memory.	int* ptr = (int*)malloc(5 * sizeof(int)); ptr = (int*)realloc(ptr, 10 * sizeof(int));
free()	Frees a block of memory previously allocated by malloc, calloc, or realloc.	free(ptr);