exit(1)

Q: If you want to exit the program, and you're not in main(), you can't just do "return 0;". Why?

A: Because it would only return from the current function.

So there is a function "exit" that you can call to exit from anywhere. In particular, consider the following statement:


exit(0);    // requires #include <cstdlib>
It will exit the program, no matter where you are.

Note

flush and i/o buffering

Here are two examples of strange behavior:

#include <iostream>
using namespace std;

int main()
{
	cout << "Got here ...";
	int* A = 0;
	A[0] = 25;
	cout << " and now I'm here!" << endl;
	return 0;
}
When I run this, I expect to see Got here ..., and then see a Seg Fault message. Here's what I get instead:
~/$ ./prog 
Segmentation fault (core dumped)

Q: What happened to the "Got here" message?


#include <iostream>
#include <unistd.h>
using namespace std;

int main()
{
  for(int i = 0; i < 10; i++)
  {
    cout << '*';
    sleep(1);
  }
  cout << endl;
  return 0;
}
When I run this, I expect to see a *, then one second later see another *, then one second later see another *, and so on until 10 *'s are printed.

Q: Instead, I see nothing for 10 whole seconds, then all at once I see 10 *'s. What's happening?

The answer in both cases is I/O bufferring is happening. When you write things with cout, the characters you write are "bufferred", that means stored temporarily until either enough have been collected to make writing to the screen worthwhile, or some event has occurred to cause the less-than-full buffer to be written to the screen despite having unused capacity. Such events include:

We've seen these events but the last one. The last event "flushes the buffer", i.e. forces the contents of the buffer to be written to the screen, without writing a newline. Using this, we can get the second example working as we'd like:

#include <iostream>
#include <unistd.h>
using namespace std;

int main()
{
  for(int i = 0; i < 10; i++)
  {
    cout << '*' << flush;
    sleep(1);
  }
  cout << endl;
  return 0;
}

Formatting numbers with cout (or any ofstream object)

Oftentimes the way cout decides to format things might not agree with the way we'd like to see them formatted. For example, if x is a double with value 4.0, cout will write "4". Or, for example, the number of decimal places cout shows might not be sufficient.

Formatting can be affected by "i/o manipulators" (you have to include the library iomanip for some of these).

fixed

If you want to force cout to always show a fixed length of fractional part, you "send" the manipulator fixed to cout. This manipulator fixed does not come from iomanip; rather it is included in the iostream library.


cout << 3.01 << endl;
cout << fixed << 3.01 << endl;
cout << fixed << 3.010 << endl;
YIELDS
3.01
3.010000
3.010000

setprecision()

If you want to set the precision to be displayed in printing a double object use the setprecision(k) manipulator, where k is the number of digits you want shown (which does require the iomanip library). For example:

double x = 3.1234567890123456789;
cout << x << endl;
cout << setprecision(15) << x << endl;
YIELDS
3.123456
3.12345678901235

Remember! Include the library iomanip when using setprecision!

Problems

  1. Here's a good problem to work on, as it takes into account a number of the things we've talked about: The file scores.txt contains the scores of students on various problems on an exam. Each row corresponds to a student, and the scores along that row are that student's scores on problems 1, 2, 3 etc.
    Your job: figure out which problem was the hardest! You may assume that for every problem, at least one student got full credit for that problem. If the average score for problem X as a percentage of the full credit score for X is less than the average score for problem Y as a percentage of the full credit score for Y, then problem X is "harder" than problem Y.
    
    ~/$ ./prob1
    Problem p4 is hardest (ave = 48.5294%)
    Check out this solution.
  2. Use the file letters.txt and write a program that prints out the "NAVY" horizonatally. I.e. we want it to run like this:
    $ ./prog
    #.    #      #      #     #   #\   /#
    # #   #     # #      #   #     #\_/# 
    #  `# #    #===#      # #        #   
    #    `#   #     #      #         #   
    Here is the solution.

    A further challenge is to print out "NAVY" twice on the same line, like this:

    $ ./prog
    #.    #      #      #     #   #\   /#   #.    #      #      #     #   #\   /#
    # #   #     # #      #   #     #\_/#    # #   #     # #      #   #     #\_/# 
    #  `# #    #===#      # #        #      #  `# #    #===#      # #        #   
    #    `#   #     #      #         #      #    `#   #     #      #         #   
    Why is this interesting? Because the challenge here is to design the right "print" function. We can't print a whole letter at once now, because printing, for example, the first "N" requires newlines, and that would mess up printing the subsequent letters. Part of the Art of Programming is to make good choices about what functions to make, and exactly what they should do.

    If you're feeling really adventurous, I have a file alphabet.txt in the same format, but with all the letters of the alphabet. You could write a program that takes a message from the user and prints it out in the big letters. Something like this:

    ~/$ ./prog
    Enter message (all lower-case, no spaces): beatarmy
    
    
    ######\   #######      #      #######      #      ######    #\   /#   #\   /#
    #_____/   #____       # #        #        # #     #     #   # # # #    #\_/# 
    #     \   #          #===#       #       #===#    ######    #  *  #      #   
    ######/   #######   #     #      #      #     #   #     #   #     #      #