Useful functions provided with C++

C++ includes a number of useful functions which you can use in your programs by simply including more #include lines at the top. This page lists a selection of these.

stdlib.h
math.h
ctype.h

`stdlib.h`

This holds miscellaneous useful functions.

int rand(): returns a random (ok, pseudorandom) integer.
int srand(int seed): initializes the pseudorandom generator using the value of seed (which should be a reasonably random number). If you use the same seed every time you run the program, you may get the same ``random'' sequence every time you run the program. Often programmers use the return value of time(NULL) as the seed.
int abs(int n): returns the absolute value of the integer n.

`math.h`

This holds many of the more useful mathematical functions.

double sin(double x): returns the sine of x (where x is in radians).
double cos(double x): returns the cosine of x (where x is in radians).
double tan(double x): returns the tangent of x (where x is in radians).
double asin(double x): returns the arcsine of x in radians between -pi/2 and pi/2, for x between -1 and 1.
double acos(double x): returns the arccosine of x in radians between 0 and pi, for x between -1 and 1.
double atan(double x): returns the arctangent of x in radians between -pi/2 and pi/2.
double atan2(double y, double x): returns the angle (in radians between -pi and pi) of a line with rise y for each run of x.
double sinh(double x): returns the hyperbolic sine of x.
double cosh(double x): returns the hyperbolic cosine of x.
double tanh(double x): returns the hyperbolic tangent of x.
double exp(double x): returns the e^x (the exponential function, with e around 2.71828).
double log(double x): returns the natural logarithm of x.
double log10(double x): returns the base-10 logarithm of x.
double pow(double x, double y): returns x^y.
double sqrt(double x): returns the square root of x.
double ceil(double x): returns x rounded up to the next integer (the smallest integer that is at least x).
double floor(double x): returns x rounded down to an integer (the largest integer that is at most x).
double fabs(double x): returns the absolute value for x.
double fmod(double x, double y): returns a number r between -y and y so that x = i*y + r for some integer i.

`ctype.h`

This holds functions for working with characters.

int isalnum(char c): returns 1 if c is a letter or a digit, 0 otherwise.
int isalpha(char c): returns 1 if c is a letter, 0 otherwise.
int isdigit(char c): returns 1 if c is a digit, 0 otherwise.
int islower(char c): returns 1 if c is a lower-case letter, 0 otherwise.
int isupper(char c): returns 1 if c is a upper-case letter, 0 otherwise.
int isspace(char c): returns 1 if c is a blank-space character (space, tab, end-of-line), 0 otherwise.
int isupper(char c): returns 1 if c is a upper-case letter, 0 otherwise.
int isxdigit(char c): returns 1 if c is a hexadecimal digit, 0 otherwise.
char tolower(char c): returns the lower-case equivalent of c (which will be c itself if c is not an upper-case letter).
char toupper(char c): returns the upper-case equivalent of c (which will be c itself if c is not an lower-case letter).