返回指向数组中最大数字的指针的函数

Question

as you can tell by the title I need to write a function that returns a pointer to the largest number in an array, the functions gets a pointer to a double array and it's size. In addition I need to write a main function that will use this function. Here is the code that I wrote:

#include <stdio.h>
#include <stdlib.h>
void BigEl(double* arr, double arrSize);

void BigEl(double* arr, double arrSize)
{
    int i;
    double  maximum, *x;
    maximum = arr[0];
    for (i = 1; i < arrSize; i++)
    {
        if (arr[i]>maximum)
        {
            maximum = arr[i];
        }
    }
    *x = maximum;
}
void main()
{
    double myarr[10];
    int i;
    printf("Please insert 10 numbers to the array\n");
    for (i = 0; i < 10; i++)
    {
        scanf("%d", &myarr[i]);
    }
    BigEl(myarr, 10);


}

I get this error:

Error   1   error C4700: uninitialized local variable 'x' used

I don't understand what I did wrong because I did initialized x. Any kind of help is appreciated, in addition, tell me if the idea of my code was right because Im not sure if I understood the question correctly.

Answer 1

You did not initialize the variable x . You merely wrote to the the location pointed to by x , here:

    *x = maximum;

when x was uninitialized, which is what the compiler is complaining about.

You want something like:

double *
BigEl(double* arr, size_t arrSize)
{
    size_t i;
    double *max = arr;
    for (i = 1; i < arrSize; i++)
        if (arr[i] > *max)
            max = &arr[i];
    return max;
}

Things I've changed:

1. Use size_t for the array size and the counter, not a double and an int .
2. Retain a pointer to the maximum element, not the maximum element's value.
3. Return the pointer to the maximum element.
4. Remove superflous braces.

Answer 2

You're not returning anything. Also, it might be good to take into consideration the case when the array size is 0. Moreover, the size should be passed as a constant. No other answer has mentioned this.

double* BigEl(double* arr, const size_t iSize)
{
    if(iSize == 0)
        return 0;

    double max = arr[0], *x = &arr[0];
    for(unsigned int i=1;i<iSize;++i){
        if(arr[i] > max){
            max = arr[i];
            x = &arr[i];
        }
    }
    return x;
}

Answer 3

//function that returns a pointer to the largest number
double *BigEl(double* arr, int arrSize)
{
    int i;
    double  *maximum;
    maximum = &arr[0];
    for (i = 1; i < arrSize; i++)
    {
        if (arr[i] > *maximum)
        {
            maximum = &arr[i];
        }
    }
    return maximum;
}
int main(void)
{
    double myarr[10];
    int i;
    printf("Please insert 10 numbers to the array\n");
    for (i = 0; i < 10; i++)
    {
        scanf("%lf", &myarr[i]);
    }
    printf("%f\n", *BigEl(myarr, 10));

    return 0;    
}

Answer 4

Your assignment to *x is incorrect - you are saying "assign to the location pointed to by x , without first saying where that is. Aside from that, there are a couple of other issues:

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

// return pointer to location from function
double * BigEl(double* arr, double arrSize)
{
    int i;
    // initialise both variables
    double maximum = arr[0], *max_pos = arr;

    for (i = 1; i < arrSize; i++)
    {
        if (arr[i]>maximum)
    {
        maximum = arr[i];
        // assign address here
        max_pos = &arr[i];
        }
    }
    // return address
    return max_pos;
}

int main()
{
    double myarr[10];
    double * max_pos;
    int i;
    printf("Please insert 10 numbers to the array\n");
    for (i = 0; i < 10; i++)
    {
        scanf("%lf", &myarr[i]);
    }
    // use return value here
    max_pos = BigEl(myarr, 10);

    return 0;
}

Answer 5

I need to write a function that returns a pointer to the largest number in an array

I think you need the follwoing

#include <stdio.h>

double * largest_element( const double *a, int n )
{
    const double *largest = a;
    int i;

    for ( i = 1; i < n; i++ )
    {
        if ( *largest < a[i] ) largest = a + i;
    }

    return ( double *)largest;

}

#define N   10

int main(void) 
{
    double a[N];
    int i;

    printf("Please insert %d numbers to the array: ", N );

    for ( i = 0; i < N; i++ )
    {
        scanf( "%lf", &a[i] );
    }

    printf( "\nThe largest element of the array is %lf\n", *largest_element( a, N ) );

    return 0;
}

If to enter for example

2.2 1.5 5.2 1.8 3.9 5.9 7.7 6.8 2.9 0.8

then the program output will be

The largest element of the array is 7.700000