Pointers, Dynamic Arrays & Vectors

[tee360]

I get so close, and then it seems my brain shuts down ... I need to write a program that outputs a histogram of student grades for an assignment. The program should input each student's grade as an integer and store the grade in a vector. Grades should be entered until the user enters -1 for a grade. The program should then scan through the vector and compute the histogram. In computing the histogram, the minimum value for a grade is 0, but your program should determine the maximum value entered by the user. Use a dynamic array to store the histogram. Output the histogram to the console. For example, if the input is:

20

30

4

20

30

30

-1

Then the output should be:

Number of 4’s: 1

Number of 20’s: 2

Number of 30’s: 3

I can't quite get my output to look like that:

/*

***************************************
This program will display the histogram
of student grades for an assignment
***************************************/

#include <iostream>
#include <vector>
#include <stdlib.h>
#include <conio.h>

using namespace std;

const int SIZE = 100;

int main()
{
	//Create pointer
	int* p;

	//Variable declaration
	int max = 0;
	int inputVal;
	p = new int[SIZE];

	for(int i = 0; i < SIZE; i++)
	{
		p[i] = 0;
	}

	//Prompt the user to enter an input value
	cout << "Enter grades of the student (-1 to QUIT) \n";

	do
	{
		//Read the input value from the keyboard
		cin >> inputVal;

		//Compare input value with Max value
		if(inputVal > max)
			//assign input value to maximum value
			max = inputVal;

		if(inputVal >= 0)
			//Determines number of times grade is entered by the user
			p[inputVal]++;
	}while (inputVal != -1);

	//Display output
	for(int j = 0; j <= max; j++)
		cout << "Number of " << j << "'s: " << p[j] << endl;

	getch();
	return 0;
}//End Main 

Enter grades of the student (-1 to QUIT)
20
30
4
20
30
30
-1
Number of 0's: 0
Number of 1's: 0
Number of 2's: 0
Number of 3's: 0
Number of 4's: 1
Number of 5's: 0
Number of 6's: 0
Number of 7's: 0
Number of 8's: 0
Number of 9's: 0
Number of 10's: 0
Number of 11's: 0
Number of 12's: 0
Number of 13's: 0
Number of 14's: 0
Number of 15's: 0
Number of 16's: 0
Number of 17's: 0
Number of 18's: 0
Number of 19's: 0
Number of 20's: 2
Number of 21's: 0
Number of 22's: 0
Number of 23's: 0
Number of 24's: 0
Number of 25's: 0
Number of 26's: 0
Number of 27's: 0
Number of 28's: 0
Number of 29's: 0
Number of 30's: 3

I realize vectors are MIA, but I do have a failed attempt of using them which seemed at least just as far off from desired result, if not more off:

/*

***************************************
This program will display the histogram
of student grades for an assignment
***************************************/

#include <iostream>
#include <vector>
#include <stdlib.h>
#include <conio.h>

using namespace std;

const int SIZE = 100;

int main()
{
	//Create pointer
	int *p;

	//Variable declaration
	int max = 0;
	int inputVal;
	p = new int[SIZE];

	vector<int> values;

	for(int i = 0; i < SIZE; i++)
	{
		p[i] = 0;
	}

	//Prompt the user to enter an input value
	cout << "Enter grades of the student (-1 to QUIT) \n";

	do
	{
		//Read the input value from the keyboard
		cin >> inputVal;

		values.push_back(inputVal);

		//Compare input value with Max value
		if(inputVal > max)
			//assign input value to maximum value
			max = inputVal;

		if(inputVal >= 0)
			//Determines number of times grade is entered by the user
			p[inputVal]++;
	}while (inputVal != -1);

	//Display output
	for(int j = 0; j < values.size(); j++)
		cout << "Number of " << values[j] << "'s: " << p[j] << endl;

	delete p;

	getch();
	return 0;
}//End Main 

I feel I'm very close ... I probably just need to get something to eat, and get back to this, but if you can polish this code up for me, I'd surely appreciate it?

[vlad from moscow]

Without testing, using new features of C++ (except the range-based for statement) and without standard algorithm

***************************************
This program will display the histogram
of student grades for an assignment
***************************************/

#include <iostream>
#include <vector>
#include <conio.h>

using namespace std;

int main()
{
	vector<int> values;
	int inputVal;

	//Prompt the user to enter an input value
	cout << "Enter grades of the student (-1 to QUIT) \n";
	while ( cin >> inputVal && inputVal != -1 )
	{
		values.push_back(inputVal);
	}

	int max = 0;

	for ( int x : values )
	{
		if ( max < x ) max = x;
	}

	//Create pointer
	int *p = new int[max];


	for ( int i = 0; i < max; i++ )
	{
		p[i] = 0;
	}

	for ( int x : values )
	{
		if (  x >= 0 ) ++p[x];
	}


	//Display output
	for( int j = 0; j < max; j++)
	{
		if (  p[j] != 0 )  cout << "Number of " << j << "'s: " << p[j] << endl;
	}

	delete []p;

	getch();

	return 0;
} 

[tee360]

@vlad from moscow

That looks good, but I'm sure I'm overlooking something with the array in this for loop:

	//Display output
	for( int j = 0; j < max; j++)
	{
		if (  p[j] != 0 )  cout << "Number of " << j << "'s: " << p[j] << endl;
	}

[vlad from moscow]

My original code had a typo. Instead of

	//Create pointer
	int p = new int[max];

there shall be

	//Create pointer
	int *p = new int[max];

With the update it should be compiled and executed.

As for this loop

	//Display output
	for( int j = 0; j < max; j++)
	{
		if (  p[j] != 0 )  cout << "Number of " << j << "'s: " << p[j] << endl;
	}

when we simply traverse the dynamic array to see what values of the vector were registered in the array ( p[j] != 0 ) and display them. If for example for some j p[j] == 0 then it means that the vector had no such value among its elements.