help with dynamically allocated arrays

[joelw]

hello,
how would i write a function that will dynamically allocate an array of integers. The function should accept an integer argument indicating the number of elements to allocate. Then return a pointer to the array?
Thanks in advance.

[redhead]

int* my_function(int* array, int size)
{
  if(size <= 0 || !(array = (int*) malloc(size*sizeof(int))))
    return NULL;
  return array;
}

Use it like:

...
int* my_array;
...
my_array = my_function(my_array, 10);
...

[joelw]

ok now i need to fugure out how to write a program that dynamically allocates an array large enough to hold any number of test scores the user wishes to enter using the function i wrote in the earlier submission. after the score are enter, the array should be passed to a function that sorts theem in ascending order and another fucntion should then be called to calculate the average score. can anyone help?
Thanks in advance.

[redhead]

Look at realloc() instead of malloc()

[joelw]

what do you mean?

[toe_cutter]

man realloc

[Valmont]

Do you want the allocation to be done in C or C++ by the way?

[joelw]

ok i got my program written however im having two problems im trying to get the lowest score to drop off when doing the average score. secondly if you compile my program you can clearly see i have it in acsending order it looks something like this
score 1: 50
score 2: 25
score 3: 75

which is fine but i want it to look like this though
score 2: 25
score 1: 50
score 3: 75

here is my code if anyone can help much appreciated.

.

#include <iostream>


void sortArray( double arr[], int sz );
void sortArray( double ( &arr[] ), int sz );
void displayArray( double arr[], int sz );


int main()
{
    double *scores, total = 0;  // average;
    int count;
    int numAmount;
    
	cout << "How many test scores are you going to record? ";
	cin >> numAmount;
    scores = new double[numAmount];
    
	cout << "Please enter in your test scores below.\n";
    for ( count = 0; count < numAmount; count++ )
    {
        cout << "Test Score " << (count + 1) << ": ";
        cin >> scores[count];
    }
	
	
	sortArray( scores, numAmount );
	displayArray( scores, numAmount );
	
	delete [] scores;

	return 0;
}

void sortArray( double arr[], int sz )
{
	
}

void displayArray( double arr[], int sz )
{
	cout << std::endl;  // for spacing
	for ( int i = 0; i < sz; i++ )
	{
		std::cout << "Score " << ( i + 1 ) << " : ";
		
		std::cout << arr[i] << std::endl;
	}
}

[toe_cutter]

I'd bet good money it doesn't sort in any order.

I think your missing a little code.

[joelw]

hello,
ok this code works fine but it still dosnt compute the avearage with the lowest score dropped.
here is my code.

#include <iostream>
#include <iomanip>

using namespace std;

void sortArray( double arr[], int sz, int scoresPos[], int size );
void displayArray( double arr[], int sz, int scoresPos[], int size );


int main()
{
    double *scores, total = 0;  // average;
	
    int count;
    int numAmount;
    
    cout << "How many test scores are you going to record? ";
    cin >> numAmount;
    scores = new double[numAmount];
    int *scorePos = new int[numAmount];

    cout << "Please enter in your test scores below.\n";
    for ( count = 0; count < numAmount; count++ )
    {
        cout << "Test Score " << (count + 1) << ": ";
        cin >> scores[count];
	scorePos[count] = count + 1;
    }
	

	sortArray( scores, numAmount, scorePos, numAmount );
	displayArray( scores, numAmount, scorePos, numAmount );
	
	delete [] scores;
	delete [] scorePos;


    return 0;
}

//**************************************************  *********************
// Definition of a sortArray                                            *
// This function excepts a double array and its size as an              *             
// argument. The results will be diplayed in ascending order            *
//**************************************************  *********************


void sortArray( double arr[], int sz, int scoresPos[], int size )
{
    double temp = 0;
    int posTemp = 0;

    for ( int i = 0; i < sz - 1; i++ )
    {
        for ( int j = ( i + 1 ); j < sz; j++ )
	{
            if ( arr[i] > arr[j] )
           {
                temp = arr[i];
		posTemp = scoresPos[i];
                arr[i] = arr[j];
		scoresPos[i] = scoresPos[j];
                arr[j] = temp;
		scoresPos[j] = posTemp;
           }
        }
     }
}

//**************************************************  ***********************
// Definition of a display array                                          *
// This function excepts array and its size as an argument. The           *
// average will be displayed dropping the lowest score.                   *
//**************************************************  ***********************


void displayArray( double arr[], int sz, int scoresPos[], int size )
{
	double average = 0;
	double sum = 0;

	std::cout << std::endl;  // for spacing
	for ( int i = 0; i < sz; i++ )
	{
		cout << "Score " << scoresPos[i] << " : ";
		cout << arr[i] << std::endl;
	}
	
	int j;
	for ( j = 1; j < sz; j++ )
	{
		sum += arr[j];
	}

	average = sum / j;

	cout << "\nThe average score disregarding the lowest score is: " << average << endl;
    cout << fixed << showpoint << setprecision(2);
    
}

[moodsey211]

Originally Posted by joelw

int j;
for ( j = 1; j < sz; j++ )
{
sum += arr[j];
}

average = sum / j;

[/code]

i guess your having problem because of the denominator. for example you have 3 elements in your array, with the scores 1,2,3 respectively. the above code would produce 1.66667 instead of 2.5. its because the code above would add 2 and three which would result to 5 and divide it with 3 which is the total number of elements. which should not be cause you only added two elements of the array and not the whole array.

tip: Life is simple, You just complicate things.

[DaveRErickson]

Here is a code snippet that can allocate a dynamic (flexible) array of pointers to functions inside a struct

(also known as struct hacking)

I racked my brains looking and then nothing specific, so I just did it

http://aiss.suffield.drdc-rddc.gc.ca...s/c_tips1.html

This works on GCC as is, with no command line switches

[ $ gcc -o test_example example.c ]
[ ./test_example ]

# DRE 2008

#include <stdlib.h>
#include <stdio.h>

typedef struct 
{
	char name[200];   	//! name of thing
	int stuff;		//! data stuff
	// last member struct hack  - see  All Questions
	void (**a)( int  );	//! ptr to array of  pointer to functions
}struct_array_t;

void fcn1(int a);

int main ()
{
	// struct ptr on heap
	 struct_array_t *  test;
	
	test = malloc (sizeof( struct_array_t ));
	// dynamic malloc on stack
	test->a = malloc ( sizeof(void*) *3 );
	
	test->a[0] = fcn1;
	test->a[1] = fcn1;
	test->a[2] = fcn1;

	printf("these are the addresses contained by array a[0]=%p a[1]=%p a[2]=%p \n", test->a[0],test->a[1],test->a[2]);

	// calling the struct member to execute function (address to execution)
	test->a[0](1);
	test->a[1](2);
	test->a[2](3);	
	return 0;
}


void fcn1(int a)
{
	printf(" I work! %d  ", a);
}

[landonmkelsey]

Long ago in an interview, I was asked to write a program to list the words(tokens) in a file and beside the word print the number of occurrences.

Well I wrote a long C program since I didn't know the STL.

I'll bet this is what they were looking for!

Did I get the job! NO!

// put this file in histo.cpp
#include <iostream>
#include <iterator>
#include <fstream>
#include <algorithm>
#include <map>
#include <string>
using namespace std;

map<string,int> histo;
void record(const string& s)
{
        histo[s]++;
}
void print(const pair<const string, int>& r)
{
        cout<<r.first<<" , "<<r.second<<endl;
}
int main()
{
        ifstream fin("histo.cpp");
        istream_iterator<string> is(fin);
        istream_iterator<string> eos;
        for_each(is,eos,record);
        for_each(histo.begin(), histo.end(), print);
        return 0;
}

great book on the STL by Schildt (available used)