LESSON 5

CSCI 15A                                               Jim Murphy

Let's look at the code on page 92 for the case study
on height and distance traveled by a projectile
starting at a given velocity and angle in degrees

// Given an initial velocity, a takeoff angle, and using
// an input value for seconds, this program determines
// the height and distance (in meters) of a projectile.
#include <iostream.h>   // for cout and cin
#include <math.h>         // for sin, cos, and pow
#include "ourstuff.h"     // for decimals(cout, 1)

int main( )
{  // Acceleration due to gravity in meters/sec/sec
const double gravity = 9.8;
// Conversion factor from degrees to radians
const double degToRad = 3.1415926 / 180.0;

// Declare Objects
double velocity;   // Takeoff speed in meters/second
double aDegrees;  // Angle as input
double aRadians;  // Angle in its radian equivalent
double seconds;    // Time in flight in seconds
double height, distance; // Compute and display

// Input:
cout << "Takeoff angle in degrees: ";
cin >> aDegrees;
cout << "   Initial velocity (m/s): ";
cin >> velocity;
cout << "  Seconds into the flight: ";
cin  >> seconds;

// Process: Compute height and distance
aRadians = aDegrees * degToRad;
distance = (velocity * cos(aRadians)) * seconds;
height = -0.5 * gravity * pow(seconds, 2)
         + (velocity * sin(aRadians)) * seconds;

// Output: Display height and distance
// First comment out 
// decimals(cout, 1);
cout << endl
 << "After " << seconds
 << " seconds, the projectile is at (in meters)"<<endl
 << " height = " << height                             << endl
 << "distance = " << distance                        << endl;

return 0;
}
For input 45, 35 and 5.05 we get

After 5.05 seconds, the projectile is at (in meters)
  height = 0.018872
distance = 124.981125

Then remove the comment // on decimals and try it
We have seen the classes float, int, double, char,
ostream, and istream

We will soon see ifstream for input from a file on disk
and from Mercer string for "text data"& bankAccount

Classes contain not only data members but also
member functions which are associated with each
object of the particular class

For example the "decimals" function from ourstuff.h
makes use of ostream's member functions flags,
precision, and setf  - but we may prefer to use them
with decimals rather than use them directly ourselves

// Used by decimals to reestablish initial default 
characteristics
// of the system after decimals has been used to alter the 
format
static long defaultFlags = cout.flags();
static int defaultPrecision = cout.precision();
static long int default_x_flags = cout.setf(0);

long decimals(ostream & cout, int n )
{ // Note: This function affects any ostream object
  long oldFlags = cout.flags();
  if(n==0) {
    // Reset format to default
    cout.flags(defaultFlags);
    cout.precision(defaultPrecision);
    cout.setf(default_x_flags);
  }
  else if(n > 0 ) {
    // Change the output stream to show decimal point and
    // n decimal places for any subsequent float or double.

    // Change the output stream to show decimal point and
    // n decimal places for any subsequent float or double.
    cout.setf(ios::fixed, ios::floatfield);
    cout.setf(ios::showpoint);
    cout.precision(n);
  }
  // Allow user to get a value to return
  // output stream to original state
  return oldFlags;
}


two other examples of ostream member functions
include 
int ostream::width(int  n);
sets the next cout to use a minimum of n columns as
x = cout.width(12);
x then contains the old width
The default width is 0
cout << 3.5 << “next” << endl;
3.5next

x=cout.width(8);
cout << 3.5 << “next” << endl;
     3.5next
cout.width(0);


char  ch = cout.fill ( '*' );
sets the leading fill character to * and saves the
previous fill character in ch
The default fill character is the space
Some member functions are operators such as << and
>> with different versions for each data type
cin is in the istream class which gets input from the
keyboard - but it is also possible to get data from a
file on disk using the ifstream class which use the
same operators as cin and is in the include file
#include <fstream.h>
We can then construct such an object with
cout << “try this \n\n” << “and this” << endl;
try this

and this

ifstream inFile = "b:\\mydat\\bank.dat";
or in UNIX
ifstream inFile = "~/15a/good.dat";
If the file is in the current directory then 
ifstream inFile = "good.dat";   //  is sufficient

Then we can use something like:

inFile >> amount >> rate >> length;
good.dat
106000  8.5  30

rather than the interactive into we have seen so much
The file could have been prepared with an editor like
pico good.dat

spaces and newlines separate the data items

Try modifying a previous program to use file input
rather than the keyboard - notice your input section
will not need cout prompts for the data and also notice
that the data in the file must be in the same order as
that expected by the inFile statement in the program