Thread: Array Sorting with pointers in C

Dear all,

Hi.

I am new to this coding field. In this code, I am trying to do array sorting using pointers. However, I am fairly sure that I am missing something while incrementing the pointers. Any guidance would be appreciated.

Please find the attached file for the code.

Code:

/*
Write a program using pointers to sort an array in ascending and/or descending order.
*/

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define rng 1000

int sort_ascending(int a[],int size,int *loc_ptr);
int sort_descending(int b[],int size,int *loc_ptr);

int main (void)
{
    int input_selection, sort_selection, n,i;

    //Program initialisation and input method selection
    printf("\n\tThis program sorts and displays the array in ascending/descending order as per user selection.");
    printf("\n\tThe array elements can either be through an RNG or be user fed.");
    printf("\n\n\tPress 1 for RNG fed array.\n\tPress 2 for user fed values.");
    printf("\n\tSelection : ");
    scanf("%d",&input_selection);
    if (((input_selection==1)||(input_selection==2))==0)
    {
        printf("\n\tPlease make a valid selection next time. Ending program execution.\n");
        exit(0);
    }

    //Sort Selection
    printf("\n\tPress 1 for sorting array in ascending order.");
    printf("\n\tPress 2 for sorting array in descending order.");
    printf("\n\tPress 3 for sorting array in both ascending and descending order.");
    printf("\n\tEnter the sort selection : ");
    scanf("%d",&sort_selection);
    if (((sort_selection==1)||(sort_selection==2)||(sort_selection==3))==0)
    {
        printf("\n\tPlease make a valid selection next time. Ending program execution.\n");
        exit(0);
    }

    //Array specs
    printf("\n\tEnter the size of the array : ");
    scanf("%d",&n);

    int array_1[n],array_2[n];

    if(input_selection==1)
    {
        printf("\n\tRandom numbers will be generated between 1 and %d.",rng);
        srand(time(NULL));
        for(i=0;i<n;i++)
        {
            array_1[i]=1+rand()%rng;
            array_2[i]=array_1[i];
        }
    }
    else
    {
        printf("\n\tPlease enter the array elements.\n");
        for(i=0;i<n;i++)
        {
            printf("\tArray[%d] : ",i);
            scanf("%d",&array_1[i]);
            array_2[i]=array_1[i];
        }
    }

    //Print the array
    printf("\n\tThe array is : \n");
    for(i=0;i<n;i++)
    {
        printf("\t%d",array_1[i]);
        if((i+1)%10==0)
            printf("\n");
    }

    //Array Sorting and printing
    printf("\n\n\tThe array after sorting is : ");
    if(sort_selection==1)
    {
        sort_ascending(array_1,n,&array_1[0]);
        printf("\n\n\tAscending order : ");
        for(i=0;i<n;i++)
        {
            printf("\t%d",array_1[i]);
            if((i+1)%10==0)
                printf("\n");
        }
    }
    else if (sort_selection==2)
    {
        sort_descending(array_2,n,&array_2[0]);
        printf("\n\n\tDescending order : ");
        for(i=0;i<n;i++)
        {
            printf("\t%d",array_2[i]);
            if((i+1)%10==0)
                printf("\n");
        }
    }
    else
    {
        sort_ascending(array_1,n,&array_1[0]);
        sort_descending(array_2,n,&array_2[0]);

        printf("\n\n\tAscending order : ");
        for(i=0;i<n;i++)
        {
            printf("\t%d",array_1[i]);
            if((i+1)%10==0)
                printf("\n");
        }
        printf("\n\n\tDescending order : ");
        for(i=0;i<n;i++)
        {
            printf("\t%d",array_2[i]);
            if((i+1)%10==0)
                printf("\n");

        }
    }

    printf("\n\n");
}


int sort_ascending(int a[],int size,int *loc_ptr)
{
    int x,temp,pass;

    for(pass=0;x<size-1;pass++)
    {
        for(x=0;x<size-1;x++)
        {
            if(*(loc_ptr+pass)>*(loc_ptr+x))
                {
                    temp=*(loc_ptr+pass);
                    *(loc_ptr+pass)=*(loc_ptr+x);
                    *(loc_ptr+x)=temp;
                }
        }

    }
    return 0;
}


int sort_descending(int b[],int size,int *loc_ptr)
{
    int y,temp=0,pass;

    for(pass=0;y<size;pass++)
    {
        for(y=0;y<size;y++)
        {
            if(*(loc_ptr+pass)<*(loc_ptr+y))
                {
                    temp=*(loc_ptr+pass);
                    *(loc_ptr+pass)=*(loc_ptr+y);
                    *(loc_ptr+y)=temp;
                }
        }

    }
    return 0;
}

First step, add some debug print (or use a debugger) to get some visual indication of what's going on.

Code:

int sort_ascending(int a[],int size,int *loc_ptr)
{
    int x,temp,pass;
 
    for(pass=0;x<size-1;pass++)
    {
        printf("DEBUG: pass %d\n",pass);
        for(x=0;x<size-1;x++)
        {
            printf("DEBUG: x %d\n",x);
            if(*(loc_ptr+pass)>*(loc_ptr+x))
                {
                    temp=*(loc_ptr+pass);
                    *(loc_ptr+pass)=*(loc_ptr+x);
                    *(loc_ptr+x)=temp;
                    printf("DEBUG: swapped between %d and %d\n", pass, x);
                }
        }
 
    }
    return 0;
}

Produces for example.
Enter the size of the array : 5

Random numbers will be generated between 1 and 1000.
The array is :
719 314 518 935 231

The array after sorting is : DEBUG: pass 0
DEBUG: x 0
DEBUG: x 1
DEBUG: swapped between 0 and 1
DEBUG: x 2
DEBUG: x 3


Ascending order : 314 719 518 935 231

Your outer loop only runs once, so that's a good place to start.

You'll kick yourself when you see it