Drill #1 Chapter 4 (Programming Principles..)

[mwaddell]

*Below is a an exercise that I am practicing on. Does my code make sense?

1. Write a program that consists of a while-loop that (each time around the loop) reads in two ints and then prints them.
Exit the program when a terminating '|' is entered.

int main()
{
int val1 =0;
int val2 =0;
cout<<"Please enter two positive integers.\n";
cin>>val1>>val2;

while (val1<100 && val2<100) //I conditioned my number range less than 100.
{
cout<<val1<<" "<<val2<<'\n';
val1++;
val2++;

if (val1 == '|'|| val2 == '|')
{
cout<<"'|' terminates.\n";
}
return 0;
}

[keskiverto]

First, please use the code tags. See http://www.cplusplus.com/articles/jEywvCM9/

Your code in tags, with some whitespace:

int main()
{
  int val1 =0;
  int val2 =0;
  cout<<"Please enter two positive integers.\n";
  cin>>val1>>val2;  // you read values only once
  // instructions: reads in two ints each time around the loop

  // The cin >> to int variable can fail, if stream does not contain integral content
  // e.g. | is not anumber. On fail, the stream goes to error state and you can read no more.
  while (val1<100 && val2<100)  // no need to limit. Instructions: two ints ... big, small, negative
  {
    cout<<val1<<" "<<val2<<'\n';
    val1++; // Not in instructions
    val2++; // Not in instructions

    if (val1 == '|'|| val2 == '|')
    // One of these is true, if user typed a number that happens to be ASCII code for |
    {
      cout<<"'|' terminates.\n"; // Says so, but does not actually terminate
    }
    return 0; // this return is inside the loop, so program always exits on first iteration
  }

[mwaddell]

Okay, so "read-in" and the "display" of the values happens within the loop. How how would I structure code to meet this?

while ()
{
cin>>val1>>val2;

if (val1 == " " && val2 == " ") //no idea on how to condition this with a integer value. 
cout<<"val1<<" "<<val2<<'\n';

else if (val1 == int('|'))
cout<<"termination character." 
}
 

 

[keskiverto]

Read: http://www.cplusplus.com/reference/istream/istream/operator%3E%3E/
It says: "Errors are signaled by modifying the internal state flags"
and: "Return value is reference to the istream object"

The latter you should already know, because it allows "chaining" operations:

cin >> val1 >> val2;
// is same as
(cin >> val1) >> val2;
// is same as
cin >> val1;
cin >> val2;

and:

cin >> foo;
if ( cin ) ...
// is same as
if ( cin >> foo ) ...

So what does the if ( cin ) do?
http://www.cplusplus.com/reference/ios/ios/operator_bool/
Seems to be same as if ( ! cin.fail() ), which
http://www.cplusplus.com/reference/ios/ios/fail/

Now we return to the "errors are signaled". If the user has written |, then the next cin >> val will think that the input cannot be interpreted as a valid value of int and sets the failbit.

The while needs a condition. The cin >> val1 >> val2 can be used as a condition expression.


There is only a minor flaw: every "invalid character" will abort the loop. Not just the '|'.
However, you should already be able (with above information) to write a program that reads pairs of ints and prints them, until you type a non-integer (non-whitespace) character. Do that.