Flushing the I/O Buffer and 2D Array Practice

Topics to Cover

cout << flush
2D array practice

Question 1


#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 the code above, I expect the following:

Got here ... will be printed out.
Then, the will be a Seg Fault error.

However, here's what I get instead:

~/$ ./prog 
Segmentation fault (core dumped)

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

Question 2


#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 the following:

* will be printed out
Then one second later see another *
Then one second later see another *,
And so on until 10 *'s are printed.

However, I see nothing for 10 whole seconds, then all at once I see 10 *'s.

Q: What's happening?

I/O Buffering

The answer to both question is that it's because 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.

When is the buffer flushed?

Such events include:

writing a "endl", or
asking for input with cin, or
writing a "flush" to cout.

We've seen these events but except 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.

Flushing the buffer

Therefore, 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;  // NOTE HERE
    sleep(1);
  }
  cout << endl;
  return 0;
}

Mandatory Practice Problem

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!


~/$ ./prob1
Problem p4 is hardest (ave = 48.5294%)

Here's the main function given for you.


int main()
{
  int ns, np;
  double** P = readGrades(&ns, &np);
  double* A = getAveragePercentage(P,ns,np);
  int imin = indexOfMin(A,np);
  cout << "Problem p" << imin+1 << " is hardest (ave = " << A[imin] << "%)" << endl;

  for(int i=0; i < ns; i++)
    delete[] P[i];
  delete [] P;
  delete [] A;

  return 0;
}

Define the functions.

Note: 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. Check out this solution.