Sorting an array of structures using merge sort

Question

So I have a structure called product and I have an array of products.

I am trying to sort the array by product price using merge sort, the problem is that my code is not swapping anything and I don't understand why. If someone could help me I would appreciate it a lot.

Structure:

typedef struct product {
    int ident;      /* idp of a product */
    char desc[64];  /* string that describes a product eg. "bread" */
    int price;      /* price of the product */
    int weight;     /* weight of the product eg. 2kg */
    int quant;      /* quantity of the product in stock */
    int state_prod; /* state of a product, if 1 it is in the system else is 0 and is not in the sistem */
} product;

Merge sort algorithm

void mergesort_price(product a[], int left, int right) {
    int m = (right + left) / 2;
    if (right <= left)
      return;
    mergesort_price(a, left, m);
    mergesort_price(a, m + 1, right);
    merge_price(a, left, m, right);
}

void merge_price(product a[], int left, int m, int right) { 
    int i, j, k;
    for (i = m + 1; i > left; i--) 
        aux[i - 1] = a[i - 1];
    for (j = m; j < right; j++)
        aux[right + m - j] = a[j + 1];
    for (k = left; k <= right; k++)
        if (aux[j].price < aux[i].price || i > m)
            a[k] = aux[j--];
        else
        if ((aux[j].price == aux[i].price) || i > m) {
            if ((aux[j].ident < aux[i].ident) || i > m) {
                a[k] = aux[j--];
            }
        } else
            a[k] = aux[i++];
}

Example:

My input:

desc: pao, price:2, weight: 2, quant:200
desc: ovos, price:1, weight: 1, quant:100
desc: iscas, price:3, weight: 3, quant:300
desc: fiambre, price:2, weight: 2, quant:200
desc: queijo, price:2, weight: 2, quant:200

Correct output:

desc: ovos, price:1, weight: 1, quant:100
desc: pao, price:2, weight: 2, quant:200
desc: fiambre, price:2, weight: 2, quant:200
desc: queijo, price:2, weight: 2, quant:200
desc: iscas, price:3, weight: 3, quant:300

Program:

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

#define MAX_PROD 10000 /* max products in the system */

typedef struct product {
   int ident;      /* idp of a product */
   char desc[64];  /* string that describes a product eg. "bread" */
   int price;      /* price of the product */
   int weight;     /* weight of the product eg. 2kg */
   int quant;      /* quantity of the product in stock */
   int state_prod; /* state of a product, if 1 it is in the system else is 0 and is not in the system */
} product;

product p1 = { 0, "pao", 2, 2, 200, 1 };
product p2 = { 1, "ovos", 1, 1, 100, 1 };
product p3 = { 2, "iscas", 3, 3, 300, 1 };
product p4 = { 3, "bacon", 2, 5, 400, 1 };
product p5 = { 4, "abacate", 2, 6, 500, 1 };

product sistem[5] = {};  /* array that contains the products */
product sistem2[5] = {}; /* Will be used has a copy of sistem */

sistem[5] = { p1, p2, p3, p4, p5}
product aux[5]; /* Auxliar array used in mergesort */

void mergesort_price(product a[], int left, int right) {
    int m = (right + left) / 2;
    if (right <= left)
        return;
    mergesort_price(a, left, m);
    mergesort_price(a, m + 1, right);
    merge_price(a, left, m, right);
}

void merge_price(product a[], int left, int m, int right) { 
    int i, j, k;
    for (i = m + 1; i > left; i--) 
        aux[i - 1] = a[i - 1];
    for (j = m; j < right; j++)
        aux[right + m - j] = a[j + 1];
    for (k = left; k <= right; k++)
        if (aux[j].price < aux[i].price || i > m)
            a[k] = aux[j--];
        else if ((aux[j].price == aux[i].price) || i > m) {
            if ((aux[j].ident < aux[i].ident) || i > m) {
                a[k] = aux[j--];
            }
        } else
            a[k] = aux[i++];
}

void print_array(product arr[]) {
    int i;
    for (i = 0; i < 5; i++) {
         printf("* %s %d %d %d\n", arr[i].desc, arr[i].price, arr[i].quant, arr[i].ident);
    }
}

void copy_el(product source[], product dest[], int size) {
    int i;
    for (i = 0; i < size; i++) {
        dest[i] = source[i];
    }
}

int main() {   
    copy_el(sistem, sistem2, 5);
    printf("The products in the system are:\n");
    print_array(sistem);

    printf("\nSorted products by price in the system:\n");
    mergesort_price(sistem2, 5, 0);
    print_array(sistem2);

    return 0;
}

I know the input that I typed is not coded in the program I just gave the input in that way as an example to make it more comprehensible.

You should edit out the second copy of the code. Have you run through this in a debugger at all to see what's happening? I find that using more descriptive variable names makes the code easier to follow and can point out problems better than having to wonder if you should be using i or j. — Retired Ninja
– Retired Ninja, Commented Apr 6, 2020 at 20:04
In merge_price, i, j, and k are uninitialized in their respective for loops (e.g.) for (i; does not initialize i — Craig Estey
– Craig Estey, Commented Apr 6, 2020 at 20:17
sistem[5] = {p1,p2,p3,p4,p5} doesn't do what it seems you're hoping for. sistem[5] is out of bounds for that array, and you can't stuff 5 items into 1 slot. — Retired Ninja
– Retired Ninja, Commented Apr 6, 2020 at 22:18

Craig Estey · Accepted Answer · 2020-04-06 22:59:06Z

Remember as I mentioned in the top comments, your for loops are not initializing i, j, or k

Your sort looks a bit complicated.

There are two ways to do the merge:

(1) copy to temp array and merge back to original (this is what you're doing)

(2) merge from original left/right arrays to temp array and the copy the result back to the original (seems simpler to me)

Anyway, here's a refactored version:

#include <stdio.h>

typedef struct product {
    int ident;                          /* idp of a product */
    char desc[64];                      /* string that describes a product eg.
                                            "bread" */
    int price;                          /* price of the product */
    int weight;                         /* weight of the product eg. 2kg */
    int quant;                          /* quantity of the product in stock */
    int state_prod;                     /* state of a product, if its 1 its in
                                            the system else is 0 and is not in
                                            the system */
} product;

#define AMAX    28
product a[AMAX];
product aux[AMAX];

void merge_price(product a[], int low, int mid, int high);

void
mergesort_price(product a[], int low, int high)
{
    int mid = (high + low) / 2;

    if (high <= low)
        return;
    mergesort_price(a, low, mid);
    mergesort_price(a, mid + 1, high);
    merge_price(a, low, mid, high);
}

void
merge_price(product a[], int low, int mid, int high)
{
    int right;
    int left;
    int k;
    int i;

    k = 0;
    left = low;
    right = mid + 1;

    for (;  (left <= mid) && (right <= high);  ++k) {
        if (a[left].price <= a[right].price)
            aux[k] = a[left++];
        else
            aux[k] = a[right++];
    }

    for (;  left <= mid;  ++left, ++k)
        aux[k] = a[left];

    for (;  right <= high;  ++right, ++k)
        aux[k] = a[right];

    k = low;
    i = 0;
    for (;  k <= high;  ++k, ++i)
        a[k] = aux[i];
}

void
show(void)
{
    for (int idx = 0;  idx < AMAX;  ++idx)
        printf(" %d",a[idx].price);
    printf("\n");
}

int
main(void)
{

    for (int idx = 0;  idx < AMAX;  ++idx)
        a[idx].price = AMAX - idx;
    show();

    mergesort_price(a,0,AMAX - 1);
    show();
}

UPDATE:

ok it worked thanks but when i tried to do it but for strings it didnt work. I only changed the if statement really, like can u explain me why it didnt pls.

Okay, I've created a version that generates random product descriptions and sorts by price. It then resorts based on desc [a string sort].

I did this with a pointer to a comparison function that is passed as an extra argument to the existing functions. This is similar to the last argument of libc's qsort function.

This could be extended to a "multikey" sort (e.g. sort by price and then by desc [or whatever struct members you wish]). See: cmpbyboth and cmpbyall.

Note that, at first glance, cmpbyall should be faster than cmpbyboth but with (e.g.) -O2 the compiler may choose to inline the [simple] functions so that both functions may generate exactly the same executable code.

Anyway, here's the code:

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

#define STRMAX      64
#define GENSTR      7

typedef struct product {
    int ident;                          /* idp of a product */
    char desc[STRMAX];                  /* string that describes a product eg.
                                            "bread" */
    int price;                          /* price of the product */
    int weight;                         /* weight of the product eg. 2kg */
    int quant;                          /* quantity of the product in stock */
    int state_prod;                     /* state of a product, if its 1 its in
                                            the system else is 0 and is not in
                                            the system */
} product;

typedef int (*cmpfnc_p)(const product *,const product *);

#define AMAX    28
product a[AMAX];
product aux[AMAX];

void merge_price(product a[], int low, int mid, int high,cmpfnc_p cmpfnc);

void
mergesort_price(product a[], int low, int high, cmpfnc_p cmpfnc)
{
    int mid = (high + low) / 2;

    if (high <= low)
        return;
    mergesort_price(a, low, mid, cmpfnc);
    mergesort_price(a, mid + 1, high, cmpfnc);
    merge_price(a, low, mid, high, cmpfnc);
}

void
merge_price(product a[], int low, int mid, int high,cmpfnc_p cmpfnc)
{
    int cmpflg;
    int right;
    int left;
    int k;
    int i;

    k = 0;
    left = low;
    right = mid + 1;

    for (;  (left <= mid) && (right <= high);  ++k) {
        cmpflg = cmpfnc(&a[left],&a[right]);
        if (cmpflg <= 0)
            aux[k] = a[left++];
        else
            aux[k] = a[right++];
    }

    for (;  left <= mid;  ++left, ++k)
        aux[k] = a[left];

    for (;  right <= high;  ++right, ++k)
        aux[k] = a[right];

    k = low;
    i = 0;
    for (;  k <= high;  ++k, ++i)
        a[k] = aux[i];
}

void
show(const char *tag)
{

    printf("\n");
    printf("%s",tag);

    for (int idx = 0;  idx < AMAX;  ++idx)
        printf(" %s=%d",a[idx].desc,a[idx].price);

    printf("\n");
}

// genstr -- get random string
void
genstr(char *desc)
{
    int len;
    int idx;
    int chr;
    int carry;

    // get random string length
    len = (rand() % GENSTR) + 1;

    // fill in random string
    for (idx = 0;  idx < len;  ++idx) {
        chr = rand() % 26;
        chr += 'a';
        *desc++ = chr;
    }

    *desc = 0;
}

// cmpbyprice -- compare by price
int
cmpbyprice(const product *left,const product *right)
{
    int cmpflg;

    cmpflg = left->price - right->price;

    return cmpflg;
}

// cmpbydesc -- compare by description
int
cmpbydesc(const product *left,const product *right)
{
    int cmpflg;

    cmpflg = strcmp(left->desc,right->desc);

    return cmpflg;
}

// cmpbyboth -- compare by price description
int
cmpbyboth(const product *left,const product *right)
{
    int cmpflg;

    do {
        cmpflg = cmpbyprice(left,right);
        if (cmpflg)
            break;

        cmpflg = cmpbydesc(left,right);
        if (cmpflg)
            break;

        // add more if you like ...
    } while (0);

    return cmpflg;
}

// cmpbyall -- compare by price description [faster version]
int
cmpbyall(const product *left,const product *right)
{
    int cmpflg;

    do {
        cmpflg = left->price - right->price;
        if (cmpflg)
            break;

        cmpflg = strcmp(left->desc,right->desc);
        if (cmpflg)
            break;

        // add more if you like ...
    } while (0);

    return cmpflg;
}

int
main(int argc,char **argv)
{
    unsigned int seed;
    product *ptr;

    --argc;
    ++argv;

    if (argc > 0)
        seed = atoi(*argv);
    else
        seed = time(NULL);
    printf("SEED: %u\n",seed);
    srand(seed);

    for (int idx = 0;  idx < AMAX;  ++idx) {
        ptr = &a[idx];
        ptr->price = AMAX - idx;
        genstr(ptr->desc);
    }
    show("INITED");

    mergesort_price(a,0,AMAX - 1,cmpbyprice);
    show("BYPRICE");

    mergesort_price(a,0,AMAX - 1,cmpbydesc);
    show("BYDESC");

    mergesort_price(a,0,AMAX - 1,cmpbyboth);
    show("BYBOTH");

    mergesort_price(a,0,AMAX - 1,cmpbyall);
    show("BYALL");
}

ok it worked thanks but when i tried to do it but for strings it didnt work. I only changed the if statement really, like can u explain me why it didnt pls.

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