New @ Programming - Need Help w/ Arrays

[saketbash]

Hi guys. This is my first post and I hope I can get some help. Thanks so much in advance for this. I have what I'm sure is simple but I'm missing something here. I have a file being read from (input.txt) which has no more than 50 grades w/ -1 being a Sentinel value to end the file when looped. I am trying to find a way to create a frequency array that tells me how many times a grade is used so that I can find the mode for it.

This is what I have so far but anytime I run it , the program crashes. I'm using VS2007 and it doesnt give any errors when compiling but crashes when I run it. Please help me out. Thanks!

// Analyze and Output Mean, Median, Mode of Grades from File
//************************************************************************

#include <iostream>	// I/O functions
#include <iomanip>	// I/O formatting functions
#include <string>	// string class
#include <cstdlib>  // abs(), rand(), atof(), etc.
#include <cmath>	// exp(), pow(), trig functions, floor/ceiling, etc.
#include <fstream>  // read to and from files
using namespace std;
 
void BubbleSort(int qAns[], int n);
const int MAX_SIZE_GRADE = 50; // Max number of grades.
const int MAX_SIZE_FREQ = 101; //Max for freq array
const int SENTINEL = -1;  //Value to end loop when read

int main() 
{
	
	int i, freqArray[MAX_SIZE_FREQ], gradeArray[MAX_SIZE_GRADE], mode;
	float mean, median;
	ifstream inFile;
	ofstream outFile;
	
	
	inFile.open("input.txt");
	outFile.open("output.txt");

	//Initialize frequency array elements to 0
	for(i=0; i<MAX_SIZE_FREQ; i++)
	{
	freqArray[i]=0;
	}
	
	i=0;
	//Read input file and store into array
	while(gradeArray[i] != SENTINEL)
	{
		
		inFile>>gradeArray[i];
		freqArray[gradeArray[i]]++;  //Creating frequency array
		i++;
	}
	
	
	//Output what was read in file (for debugging)
	i=0; 
	while(gradeArray[i] != SENTINEL)
	{
		cout<<gradeArray[i]<<", ";
		i++;

	}

	return 0;
}

[Narue]

>inFile.open("input.txt");
>outFile.open("output.txt");
Let's start here. You're not checking to see if the files were opened successfully. All kinds of gremlins can pop up if you use a file stream that failed to open.

>//Read input file and store into array
>while(gradeArray != SENTINEL)
You never initialized gradeArray, so gradeArray is garbage. You access a garbage value and expect predictable behavior.

>inFile>>gradeArray;
>freqArray[gradeArray]++;
This is scary on three levels:

1. You don't check the input request for success
2. You don't put a bound on i, so if the file has more than 50 numbers, you're overflowing the gradeArray
3. You're using values from the file without checking their validity. This is also a potential overflow, but of freqArray this time.

That might help you pinpoint the problem more easily.

[saketbash]

Thanks for the reply.

Questions though:

1. How do I know what to initialize gradeArray to? Since I technically dont know how many grades are in the array, should I create a loop and just initialize all the elements (50 of them even if there are lets say 10 grades) to 0?

2. I know its not the file stream error since if I take out the freqArray[gradeArray]++ statement, the output for debugging statements show all the grades in the file.

3. I guess my question then is how do I set up the frequency array since it seems thats whats causing my error.

[Narue]

>1. How do I know what to initialize gradeArray to?
Well, SENTINEL wouldn't be a bad idea, and you can initialize it just like you do the frequency array. But you can get away with not initializing it at all, if you're careful:

i = 0; // Restart the gradeArray iteration

while ( i < MAX_SIZE_GRADE ) {
  int input;

  // Bomb out if we can't read anything
  if ( !( inFile>> input ) )
    break;

  // We'll assign the sentinel after the loop
  if ( input == SENTINEL )
    break;

  gradeArray[i] = input;

  if ( input >= 0 && input < MAX_SIZE_FREQ )
    ++freqArray[input];

  ++i;
}

// Make sure the grades are terminated
gradeArray[i] = SENTINEL;

I bet if you use that code, you'll find that the problem goes away.

>if I take out the freqArray[gradeArray]++ statement, the
>output for debugging statements show all the grades in the file
Think about it for a second. If you take that statement out, it doesn't crash. Could it be that you're hitting one of the problems I mentioned? Re-read scary #3 and think about what happens when you read the sentinel from your file.

>3. I guess my question then is how do I set up the frequency array
You're on the right track. But this experience should teach you that validating your assumptions is priority 1.

[saketbash]

a

[saketbash]

Wow that fixed everything. Thanks so much and because of that I was able to find out all the other information.