Contemporary programming systems provide a rich set of library functions that make design and coding easier. Use of standard library functions offer several advantages:
C++ programs may use either C++ libraries (such as those dealing with input/output) or C standard libraries. The C standard libraries contain some well-known, well-worn gems like string manipulation, math functions, etc. This tech note is a brief overview of the C standard libraries that will assist development of our class projects. This overview is not exhaustive or complete! Please don't hesitate to explore on your own. Use the UNIX apropos or man command to search for and read about other library functions.
Namespaces are a C++ feature that assist the construction of very large software systems. When a system gets to several thousand lines of code, it's easy to get clashes between the names (variables, functions, etc.) used by different modules in the system. Programmers sometimes use naming conventions to avoid such clashes, but this approach requires personal initiative and discipline, and is not conducive to automatic checking by the compiler. A namespace allows scope and name management.
We will not use namespaces in our C++ programs because the programs will be rather small and clashes will be easy to control. However, you must be aware of one important use of namespaces -- the separation of current standard and pre-standard library functions. To prevent misuse of current standard library functions (by inadvertently accessing pre-standard library functions), current standard library functions are contained in the std namespace. Thus, you will need to add the C++ declaration:
using namespace std ;to correctly select the current standard library. To further reduce the likelyhood of error, current standard library names begin with the letter "c" and do not end in ".h".
Here is a table of some useful standard C libraries.
cassert | Check assumption/correctness by asserting a logical condition |
cctype | Character functions |
cfloat | Maximum and minimum values of floating-point types |
climits | Maximum and minimum values of integer types |
cmath | Mathematical library functions |
cstdlib | Standard C library |
cstring | Standard C string library |
The subsections that follow describe many of the functions in these standard C libraries. The descriptions below are terse. Please use the UNIX man command to get more information about a function before using it. Just enter a command like:
man isalphato get more information about a function. In this command, we are asking for more information about the isalpha function. The man page will contain information about restrictions on argument values, return values and error conditions. Be sure to program defensively -- detect and handle error conditions before your program tilts over and dies.
The assert macro tests the given expression and if it is false, the program is terminated. A diagnostic message is written to the standard error output stream.
assert(expression) | Expression verification macro |
The functions in the cctype library operate on individual characters. There are functions for detecting certain character types (e.g., letter, digit) and for converting between upper and lower case.
isalnum(c) | Returns true if c is a letter or digit; otherwise, false |
isalpha(c) | Returns true if c is a letter; otherwise, false |
iscntrl(c) | Returns true if c is a control character; otherwise, false |
isdigit(c) | Returns true if c is a decimal digit; otherwise, false |
isgraph(c) | Returns true if c is printing character (except space); otherwise, false |
islower(c) | Returns true if c is lower case; otherwise, false |
isprint(c) | Returns true if c is a printing character; otherwise, false |
ispunct(c) | Returns true if c is a punctuation character; otherwise, false |
isspace(c) | Returns true if c is a white space character; otherwise, false |
isupper(c) | Returns true if c is upper case; otherwise, false |
isxdigit(c) | Returns true if c is a hexadecimal digit; otherwise, false |
tolower(c) | Returns lower case equivalent of c |
toupper(c) | Returns upper case equivalent of c |
The standard C string functions operate on the common, one-dimensional character array representation of strings. In general, these strings must be terminated by a null character, '\0'.
strcat(s1, s2) | Append (concatenate) s2 to s1 |
strncat(s1, s2, n) | Append (concatenate) at most n characters of s2 to s1 |
strcpy(s1, s2) | Copy s2 to s1; Note the direction of the copy! |
strncpy(s1, s2, n) | Copy first n characters of s2 to s1; Note the direction of the copy! |
strlen(s) | Return the length of string s, not counting the terminating null character. |
strcmp(s1, s2) | Compare s1 with s2; Return an integer less than zero, equal to zero, or greater than zero if s1 is less than, equal to, or greater than s2 |
strncmp(s1, s2, n) | Like strcmp above, but compare only the first n characters of s1 and s2 |
strchr(s, c) | Return a pointer to the first occurence of character c in string s; return null (zero) if not found |
strrchr(s, c) | Return a pointer to the last occurence of character c in string s; return null (zero) if not found |
strstr(s1, s2) | Return a pointer to the first occurence of string s1 in string s2; return null (zero) if not found |
strpbrk(s1, s2) | Return a pointer to the first occurence in string s1 of any character contained in string s2; return null (zero) if not found (The characters in s2 are sometimes called "break" characters.) |
strspn(s1, s2) | Span the first part of string s1 using the characters in string s2; That is, return the number of characters in s1 before the occurrence of any character in string s2 |
strcspn(s1, s2) | Span the first part of string s1 using the complement of the set of characters in string s2; That is, return the number of characters in s1 before the occurrence of any character not in string s2 |
The functions in the cstdlib library are an eclectic and varied lot. It includes functions to convert from a string representation of a number to its internal representation, to take the absolute value of an integr, to generate a pseudo-random integer number, and to explicitly terminate a program, among many others.
int abs(int j) | Return the absolute value of integer j. |
double atof(s) | Convert the external floating-point representation of the string s to the internal numeric representation of its value and return the double value. |
int atoi(s) | Convert the external integer representation of the string s to the internal numeric representation of its value and return the int value. |
long int atof(s) | Convert the external integer representation of the string s to the internal numeric representation of its value and return the long int value. |
void exit(int status) | Terminate program execution; Return exit status to the operating system; By convention, a status value of zero indicates normal termination while non-zero values indicate an error condition; A program may also return EXIT_SUCCESS or EXIT_FAILURE |
int rand() | Return a pseudo-random integer in the range from 0 to RAND_MAX |
void srand(int seed) | Sets the seed (initial value) for pseudo-random number generation; A sequence of pseudo-random numbers is repeatable by setting the same seed value |
The pseudo-random number generator implemented by the functions above is a rather poor one. It has a short period and the sequence of pseudo-random numbers quickly repeats. A better pseudo-random number generator is implemented by the functions in the table below.
long random(void) | Returns a pseudo-random number in the range 1 to 2^{31}-1 |
void srandom(unsigned int seed) | Initializes the pseudo-random number generator with seed |
char initstate(...) | Initializes the pseudo-random number generator in great and gory detail (advanced feature) |
char setstate(...) | Allows switching between pseudo-random number generator states (advanced feature) |
The standard C math library defines a large number of useful mathematical functions. The table below lists just a few of these functions.
double cos(double x) | Return the cosine of x (radians) |
double exp(double x) | Return e^{x} |
double fabs(double x) | Return absolute value of x |
double log(double x) | Return the natural logarithm of x |
double log10(double x) | Return the base-10 logarithm of x |
double sin(double x) | Return the sine of x (radians) |
double sqrt(double x) | Return the square root of x (x greater than or equal to zero) |
double tan(double x) | Return the tangent of x (radians) |
This library defines the following symbolic constants. These constants specify the smallest and largest value that can be held by a particular C/C++ character or integer variable.
SCHAR_MIN | Minimum value a signed char can hold |
SCHAR_MAX | Maximum value signed char can hold |
UCHAR_MAX | Maximum value unsigned char can hold |
CHAR_MIN | Minimum value char can hold |
CHAR_MAX | Maximum value char can hold |
SHORT_MIN | Minimum value short int can hold |
SHORT_MAX | Maximum value short int can hold |
USHRT_MAX | Maximum value unsigned short int can hold |
INT_MIN | Minimum value int can hold |
INT_MAX | Maximum value int can hold |
UINT_MAX | Maximum value unsigned int can hold |
LONG_MIN | Minimum value long int can hold |
LONG_MAX | Maximum value long int can hold |
ULONG_MAX | Maximum value unsigned long can hold |
This library defines the following symbolic constants. These constants specify the smallest and floating-point value that can be held by a particular C/C++ float or double variable.
FLT_MIN | Minimum value a float can hold |
FLT_MAX | Maximum value float can hold |
FLT_EPSILON | Smallest positive value representable as a float |
DBL_MIN | Minimum value a double can hold |
DBL_MAX | Maximum value double can hold |
DBL_EPSILON | Smallest positive value representable as a double |
LDBL_MIN | Minimum value a long double can hold |
LDBL_MAX | Maximum value long doublet can hold |
LDBL_EPSILON | Smallest positive value representable as a long double |