C ++指针动态数组和函数

Question

This is from a beginning C++ class, no grade involved since I'm simply following along trying to remember stuff after too many years. The class has reached the point where we're using pointers and dynamic arrays. I'm trying to pass the array (or the pointer) to different functions having to do with various calculations on a list of integers. Some of the functions are working and others aren't. But I'm passing them all the same way and I'm calling them all the same way as well. Since some of the functions are returning garbage values, it feels like I'm not actually pointing where I think I am AND the functions that are working are only working by accident.

Before someone helpfully suggests that I should be using vectors and not arrays, the point of the exercise is to use dynamically allocated arrays.

Here's my code so far. It's the findavg() and findmedian() that are returning garbage.

#include <iostream>
#include <iomanip>

using namespace std;

void getarray(int *a, int &l)
{
    int input = 0;

    //-1 is not important here, just to make the while loop run
    while(input != '-1') 
    {
        cout << "Enter length of the list of numbers: ";

        if(!(cin >> input))
        {
            cout << "Please enter numeric characters only." << "\n";    
            //show error if they don't enter a number
            cin.clear();                                                
            //get rid of invalid characters
            cin.ignore(10000,'\n');
        }

        else if(input <= 0)
        {
            cout << "Please enter a number greater than 0." << "\n";    
            //show error if they enter a non-positive number
            cin.clear();                                                
            //get rid of invalid characters
            cin.ignore(10000,'\n');
        }
        else
        {
            l = input;  //input is a positive number
            break; //got good input, break out of while loop
        }

    }


    int i;
    int x; 

    cout << "Enter " << l << " integers on one line seperated by spaces.\n";


    for( i = 0; i < l  &&  cin >> x; ++i)
    {
        a[i] = x;
    }

}

void printarray(int *a, int &l)
{
    int i;

    cout << "The array of integers is:\n";
    for( i = 0; i < l; ++i)
        cout << setw(4) << a[i];
    cout << "\n";

}

int findmin(int *a, int &l)
{
    int min = 0;
    min = a[0]; 

    for(int i = 1; i<l; i++)
    {
        if(a[i] < min)
            min = a[i];
    }
    return min; 
}

int findmax(int *a, int &l)
{
    int max = 0;
    max = a[0];

    for(int i = 1; i<l; i++)
    {
        if(a[i] > max)
            max = a[i];
    }
    return max; 
}

float findavg(int *a, int &l)
{
    int total = 0;
    total = a[0];

    for(int i = 1; i<l; i++)
    {
        total = total + a[i];
    }

    return static_cast<float>(total/static_cast<float>(l));

}

float findmedian(int *a, int &l)
{
    int max = 0;
    int min = 0;

    max = findmax(a, l);
    min = findmin(a, l);

    return static_cast<float>((max + min)/2.0);

}


int main()
{

    int length = 0;
    int *an_array;

    an_array = new int[length];

    getarray(an_array, length);

    printarray(an_array, length);

    cout << "Maximum value in list is: "<< findmax(an_array, length) << "\n";
    cout << "Minimum value in list is: "<< findmin(an_array, length) << "\n";
    printf("Average is: %10.5f", findavg(an_array, length));
    cout << "\n";
    printf("Median is: %10.5f", findmedian(an_array, length));
    cout << "\n";

    delete [] an_array;

    return 0;
}

Answer 1

int length = 0;
int *an_array;

an_array = new int[length]; //What is this ?

You're allocating zero bytes for an_array , because length is 0 . That is one problem I can see.

You should allocate memory for an_array in getarray function, after reading the length of array:

void getarray(int * &a, int &l) //see the change here!
{
    //code which reads length is omitted for brevity

    a = new int[l]; //alllocate memory after reading length

    //now read array elements here
}

---

Alternatively,you can write a struct as:

struct array
{
      int *data;
      size_t size;
};

And then use this everywhere. It is better because you tied the size and data together, and instead of using two independent objects, you can use just one object of type array , as described below.

array getarray() 
{
    array arr;

    //read length into arr.size

    arr.data= new int[arr.size]; //alllocate memory after reading length

    //now read array elements into arr.data

    return arr; //returning arr means you're returning both: data and size!
}

And then call this as:

 array arr = getarray(); //no need to pass any argument!

Also, if you're going to use this, then other functions signature would become:

void printarray(array arr);
int findmax(array arr);
float findavg(array arr);
float findmedian(array arr);

Now you need to modify these functions as well, but fortunately, there will not be major change : wherever you're using a and l , use arr.data and arr.size respectively instead. And you're done!

With all these improvements, your main() would become this:

int main()
{
    array arr = getarray();

    printarray(arr);

    cout << "Maximum value in list is: "<< findmax(arr) << "\n";
    cout << "Minimum value in list is: "<< findmin(arr) << "\n";

    printf("Average is: %10.5f\n", findavg(arr));
    printf("Median is: %10.5f\n", findmedian(arr));

    delete [] arr.data;
    return 0;
}

This looks better.

Answer 2

You are not allocating any memory for the array. Undefined behaviour ensues.

Change your getarray function to do the allocate and return the newly created and populated array:

int* getarray(int &length)

or even

void AllocateAndPopulateArray(int* &array, int &length)

for a more symmetrical interface.

If you want a non-terminating loop use

while(true)

There is no need to bring input into your while loop.

There's no point passing the length by reference in any of the other routines since you are not modifying it. In fact you are asking for trouble passing by reference since a coding error may inadvertently change the value. You should declare it with const to make the compiler catch any silly mistakes you might make.

I most heartily agree with Nawaz's suggestion to tie together the pointer and length variables.

In your average calculation I personally would declare total to be a float and thus avoid the awkward looking casts.

Your implementation of median is plain wrong, but that seems to be a side issue.