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	Chapter 15 - Power Programming: Tapping Important C and C++ Libraries
	Visual C++ 6: The Complete Reference
	Chris H. Pappas and William H. Murray, III

Copyright 1998 The McGraw-Hill Companies

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	The Math Functions (MATH.H)

	The functions prototyped in the MATH.H header file permit a great variety of mathematical, algebraic, and trigonometric operations.

	The math functions are relatively easy to use and to understand for those familiar with algebraic and trigonometric concepts. The most popular math functions are shown in Table 15-10.

	Table 15-10: Popular Math Functions

	Function

Description

	abs( )

Returns absolute value of integer argument

	acos( ), acosl( )

Arc cosine

	asin( ), asinl( )

Arc sine

	atan( ), atanl( )

Arc tangent

	atan2( ), atan2l( )

Arc tangent of two numbers

	ceil( ), ceill( )

Greatest integer

	cos( ), cosl( )

Cosine

	cosh( ), coshl( )

Hyberbolic cosine

	exp( ), expl( )

Exponential value

	fabs( ), fabsl( )

Absolute value

	floor( ), floorl( )

Smallest value

	fmod( ), fmodl( )

Modulus operator

	frexp( ), frexpl( )

Split mantissa and exponent

	hypot( ), hypotl( )

Hypotenuse

	labs( )

Returns absolute value of long argument

	ldexp( ), ldexpl( )

x times 2 to the exp power

	log( ), logl( )

Natural log

	log10( ), log10l( )

Common log

	modf( ), modfl( )

Mantissa and exponent

	pow( ), powl( )

x to y power

	pow10( ), pow10l( )

x raised by power of 10

	sin( ), sinl( )

Sine

	sinh( ), sinhl( )

Hyperbolic sine

	sqrt( ), sqrtl( )

Square root

	tan( ), tanl( )

Tangent

	tanh( ), tanhl( )

Hyperbolic tangent

	Note		Functions accept and return double values except where noted.

	Note		Functions ending in “l” accept and return long double values.

	Many of these functions were demonstrated in earlier chapters. When using trigonometric functions, remember that angle arguments are always specified in radians.

	Programmers desiring complex number arithmetic must resort to using struct complex and the _cabs( ) function described in MATH.H. Following is the only structure available for complex arithmetic in Visual C++:

	struct complex {double x,double y}

	This structure is used by the _cabs( ) function. The _cabs( ) function returns the absolute value of a complex number.

	Building a Table of Trigonometric Values

	Since math functions have already been used extensively in this book, the only example for this section involves an application that will generate a table of sine, cosine, and tangent values for the angles from zero to 45 degrees.

	This application also takes advantage of the special C++ formatting abilities. Study the following listing to determine how the output will be sent to the screen:

	// //  math.cpp //  A program that demonstrates the use of several //  math functions. //  Copyright (c) Chris H. Pappas and William H. Murray, 1998 // #include <iostream.h> #include <iomanip.h> #include <math.h> #define PI 3.14159265359

	main( ) {  int i;  double x,y,z,ang;  for (i=0;i<=45;i++) {    ang=PI*i/180;  // convert degrees to radians    x=sin(ang);    y=cos(ang);    z=tan(ang);    // formatting output columns    cout << setiosflags(ios::left) << setw(8)         << setiosflags(ios::fixed) << setprecision(6);    // data to print    cout << i << “\t” << x << “\t” <<            y << “\t” << z << “\n”;  }  return (0); }

	This application uses the sin( ), cos( ), and tan( ) functions to produce a formatted trigonometric table. The angles are stepped from zero to 45 degrees and are converted to radians before being sent to each function. This particular C++ formatting is discussed in more detail in Chapter 18.

	Following is a partial output from this application:

	0 0.000000 1.000000 0.000000 1 0.017452 0.999848 0.017455 2 0.034899 0.999391 0.034921 .   .      .        . .   .      .        . .   .      .        . 28 0.469472 0.882948 0.531709 29 0.484810 0.874620 0.554309 30 0.500000 0.866025 0.577350 31 0.515038 0.857167 0.600861 32 0.529919 0.848048 0.624869 .    .      .        . .    .      .        . .    .      .        . 43 0.681998 0.731354 0.932515 44 0.694658 0.719340 0.965689 45 0.707107 0.707107 1.000000

	The Time Functions (TIME.H)

	Table 15-11 shows some of the time and date function found in TIME.H.

	Table 15-11: Time and Date Functions

	Names

Description

	asctime( )

Converts date and time to an ASCII string and uses tm structure

	ctime( )

Converts date and time to a string

	difftime( )

Calculates the difference between two times

	gmtime( )

Converts date and time to GMT using tm structure

	localtime( )

Converts date and time to tm structure

	strftime( )

Allows formatting of date and time data for output

	time( )

Obtains current time (system)

	tzset( )

Sets time variables for environment variable TZ

	These functions offer a variety of ways to obtain time and/or date formats for programs. A discussion of the syntax for each function is included in the next section.

	Time and Date Structures and Syntax

	Many of the date and time functions described in the previous section use the tm structure defined in TIME.H. This structure is shown here:

	struct tm  {  int  tm_sec;  int  tm_min;  int  tm_hour;  int  tm_mday;  int  tm_mon;  int  tm_year;  int  tm_wday;  int  tm_yday;  int  tm_isdst; };

	The syntax for calling each date and time function differs according to the function’s ability. The syntax and parameters for each function are shown in Table 15-12.

	Table 15-12: Time and Date Function Parameters

	Function

Description

	char * ASCTIME(CONST STRUCT       tm * tblock)

Converts the structure into a 26-character string

For example: Sun June 1, 10:18:20 1998\n\0

	char * CTIME(CONST TIME_T      *time)

Converts a time value, pointed to by * time into a 26-char string (see asctime( ))

	double difftime(time_t time2,       time_t time1)

Calculates the difference between time2 and time1 and returns a double

	struct tm * GMTIME(CONST TIME_T         * TIMER)

Accepts address of a value returned by the function time( ) and returns a pointer to the structure with GMT information

	struct tm * LOCALTIME(CONST TIME_T          * TIMER)

Accepts address of a value returned by the function time( ) and returns a pointer to the structure with local time information

	size_t strftime (char * s,        size_t maxsize,        const char * fmt,        const struct tm * t)

Formats date and time information for output. s points to the string information, maxsize is maximum string length, fmt represents the format, and t points to a structure of type tm. The formatting options include:

%a Abbreviate weekday name

%A Full weekday name

%b Abbreviate month name

%B Full month name

%c Date and time information

%d Day of month (01 to 31)

%H Hour (00 to 23)

%I Hour (00 to 12)

%j Day of year (001 to 366)

%m Month (01 to 12)

%M Minutes (00 to 59)

%p AM or PM

%S Seconds (0 to 59)

%U Week number (00 to 51), Sunday is first day

%w Weekday (0 to 6)

%W Week number (00 to 51), Monday is first day

%x Date

%X Time

%y Year, without century (00 to 99)

%Y Year, with century

%Z Time zone name

%% Character %

	time_t time(time_t *timer)

Returns the time in seconds since 00:00:00 GMT, January 1, 1998

	void _tzset (void)

Sets the global variables daylight, timezone0, and tzname0 based on the environment string

	The TZ environment string uses the following syntax:

	TZ = zzz[+/-]d[d]{lll}

	Here, zzz represents a three-character string with the local time zone—for example, “EST” for eastern standard time. The [+/]d[d] argument contains an adjustment for the local time zone’s difference from GMT. Positive numbers are a westward adjustment, while negative numbers are an eastward adjustment. For example, a five (5) would be used for EST. The last argument, {lll}, represents the local time zone’s daylight savings time—for example, EDT for eastern daylight savings time.

	Several of these functions are used in example programs in the next section.

	Working with the localtime( ) and asctime( ) Functions

	Many times it is necessary to obtain the time and date in a programming application. The next program returns these values by using the localtime( ) and asctime( ) functions:

	/* *   asctim.c *   Demonstrating the use of the localtime( ) and asctime( ) *   functions. *   Copyright (c) Chris H. Pappas and William H. Murray, 1998 */ #include <time.h> #include <stdio.h> struct tm *date_time; time_t timer; main( ) {  time(&timer);  date_time=localtime(&timer);  printf(“The present date and time is: %s\n”,  asctime(date_time));  return (0); }

	This program formats the time and date information in the manner shown here:

	The present date and time is: Sat May 31 13:16:20 1998

	Working with the gmtime( ) and asctime( ) Functions

	There are other functions that you can also use to return time and date information. The next program is similar to the last example, except that the gmtime( ) function is used.

	/* *   cmtime.c *   Demonstrating the use of the gmtime( ) and asctime( ) *   functions. *   Copyright (c) Chris H. Pappas and William H. Murray, 1998 */ #include <time.h> #include <stdio.h> main( ) {  struct tm *date_time;  time_t timer;  time(&timer);  date_time=gmtime(&timer);  printf(“%.19s\n”,asctime(date_time));  return (0); }

	The following date and time information was returned by this program:

	Sat May 31 14:13:25

	Working with the strftime( ) Function

	The strftime( ) function provides the greatest formatting flexibility of all the date and time functions. The following program illustrates several formatting options.

	/* *   strtm.c *   Demonstrating the use of the strftime( ) function. *   Copyright (c) Chris H. Pappas and William H. Murray, 1998 */ #include <time.h> #include <stdio.h> main( ) {  struct tm *date_time;  time_t timer;  char str[80];  time(&timer);  date_time=localtime(&timer);  strftime(str,80,"It is %X on %A, %x",           date_time);  printf(“%s\n”,str);  return (0); }

	Here is a sample of the output for this program:

	It is 17:18:45 on Saturday, 05/31/98

	You may find that the strftime( ) function is not portable from one system to another. Use it with caution if portability is a consideration.

	Working with the ctime( ) Function

	The following C++ program illustrates how to make a call to the ctime( ) function. This program shows how easy it is to obtain date and time information from the system.

	// //  ctime.cpp //  Demonstrating the use of the ctime( ) function. //  Copyright (c) Chris H. Pappas and William H. Murray, 1998 // #include <time.h> #include <iostream.h> time_t longtime; main( ) {  time(&longtime);  cout << “The time and date are ” <<          ctime(&longtime) << “\n”;  return (0); }

	The output, sent to the screen, would appear in the following format:

	The time and date are Sat May 31 14:23:27 1998

	Creating a Time Delay Routine

	Usually it is desirable for programs to execute as quickly as possible. However, there are times when slowing down information makes it easier for the user to view and understand. The time_delay( ) function in the following application delays program execution. The delay variable is in seconds. For this example, there is a two-second delay between each line of output to the screen.

	/* *   tdelay.c *   A C program that demonstrates how to create a delay *   function for slowing program output. *   Copyright (c) Chris H. Pappas and William H. Murray, 1998 */ #include <stdio.h> #include <time.h> void time_delay(int); main( ) {  int i;  for (i=0;i<25;i++) {    time_delay(2);    printf(“The count is %d\n”,i);  }  return (0); } void time_delay(int t) {  long initial,final;  long ltime;  initial=time(&ltime);  final=initial+t;  while (time(&ltime) < final);  return; }

	What other uses might the time_delay( ) function have? One case might be where the computer is connected to an external data-sensing device, such as a thermocouple or strain gauge. The function could be used to take readings every minute, hour, or day.

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