C Language-summary of commonly used built-in functions

Ctype.h
1.isalpha (): Tests whether a character is a letter, or returns 1, otherwise returns 0.
2.isdigit (): Tests whether a character is a number, or returns 1, otherwise returns 0.
3.isupper (): Tests whether a character is uppercase.
4.islower (): Tests whether a character is a lowercase letter.
5.toupper (): The lowercase letter is converted to uppercase and returned.
6.tolower (): Turns uppercase letters into lowercase letters and returns.


String.h
1.strcpy (): Copies the contents of a string into a character array.
2.strlen (): Calculates the length of a string in a character array. Does not include NULL 0
3.strcat (): Merges a two-character array.

Stdio.h
1.puts (): Outputs a string. The default is to add a line break at the end.
Example: Puts ("www.baidu.com");
puts (table); Talbe is a character array
2.gets (): Gets a string that ends with enter.
3.fopen (): Gets a file entry address
4.fclose (): Closes a file.
5.fprintf (): Enter string information into the file.
6.fgets (): reads a row of data.
7.fputs (): Output data to file.
8.feof (): Detects whether the end of the file is reached.
9.fseek (): Random Access file

Math.h
1.ceil (): Rounding up, return value is floating point
2.floor (): Rounding down, return value is floating point
3.fabs (): Returns the floating point absolute value
4.cos (x): Returns the cosine of the angle X
5.sin (x): Returns the sinusoidal value of the angle X
6.tan (x): Returns the tangent of the angle X
7.pow (): Calculates the power of a value
8.SQRT (): Returns the square root of a value
9.exp (x): Returns the value of E^x
10.log (x): Returns the natural logarithm of x, which is mathematically written in ln (x).
11.LOG10 (x): Returns the base 10 logarithm.

Stdlib.h
1.rand (): Randomly generates a value from 0 to 32767. If Rand () is not set to generate a random number of seeds, then it always generates the same random number.
2.srand (): Sets the random number generation seed.

An example of a positive and true random number:
#include <stdlib.h>
#include <math.h>

#define NUM_CARDS54

time_t T;
Srand (Time (&t));
Sub_draw = (rand ()% (num_cards));
3.malloc (): Allocating heap memory
Cases:
int *temps;
temps = (int *) malloc (10*sizeof (int));

4.free (): Free heap Memory

C Language-summary of commonly used built-in functions