Not sorting of struct using arrays - c++

Question

It seems to be a simply exercise, but something doesn't work with the following qsort algorithmus. The struct abcSort correctly shows all assigned values. Unfortunately, the qsort is not sorting anything.
typedef int(*compfn)(const void*, const void*);
struct ABCSort
{
int svSort[10];
string itemSort[10];
};
struct ABCSort abcSort;
int compare(struct ABCSort*, struct ABCSort*);
void mainSort()
{
for (int i = 0; i < 10; i++)
{
abcSort.svSort[i] = 100 - i;
abcSort.itemSort[i] = arrayREAD[i][2];
}
qsort( (void*)&abcSort, 10, sizeof(struct ABCSort), (compfn)compare );
}
int compare(struct ABCSort *elem1, struct ABCSort *elem2)
{
if (elem1->svSort< elem2->svSort)
return -1;
else if (elem1->svSort > elem2->svSort)
return 1;
else
return 0;
}

Answer 1

You have build two arrays of integer and strings and you want to sort them by the numbers, keeping the initial pairing. That's the first problem, you should have created one array of structs, each struct containg a number and a string and a function that compares the integer member of that struct to obtain the sort order.
You also tagged this question as C++ but you are using qsort, arrays and function pointers like C, so I'll present two complete C programs that solve your problem.
Let's see, using an array of structs, what your code could be like:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 6
#define CMAX 15
typedef int(*cmp_fn)(const void*, const void*);
typedef struct {
int num;
char str[CMAX];
} numABC;
int comp_struct(const numABC *lhs, const numABC *rhs ) {
if ( lhs->num < rhs->num )
return -1;
if ( lhs->num > rhs->num )
return 1;
return 0;
}
int main(void) {
numABC myArray[MAX] = { {6, "cat"}, {4, "dog"}, {8, "panter"},
{2, "red fish"}, {1, "hawk"}, {6, "snake"} };
int i;
// sort the array of structs by int member
qsort(myArray, MAX, sizeof(numABC), (cmp_fn)comp_struct);
// print out the sorted array
printf("\nSort by numbers:\n");
for ( i = 0; i < MAX; ++i ) {
printf("%d %s\n",myArray[i].num,myArray[i].str);
}
return 0;
}
If you want to use this code, but you have a couple of arrays instead, one option is to convert those arrays:
int nums[MAX] = {6,4,8,2,1,3};
char str[MAX][CMAX] = {"cat","dog","panter","red fish","hawk","snake"};
int i;
// create the array of structs from the two arrays
numABC myArray[MAX];
for ( i = 0; i < MAX; ++i ) {
myArray[i].num = nums[i];
strcpy(myArray[i].str, str[i]);
}
Another option to sort two different arrays (mantaining the pairing or alignment between the two) is to use a more complicated method which consists in sorting an array of indeces instead. to keep the relationships between the two original arrays I'll have to use global variables which can be accessed inside the compare function(s). Once the indeces are sorted, the original arrays are changed accordingly.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 6
#define CMAX 15
const size_t I_DIM = MAX * sizeof(int);
const size_t SS_DIM = MAX * sizeof(char*);
const size_t S_DIM = MAX * CMAX;
// global variables needed to perform comparisons
int *pg_int;
char **pg_str;
typedef int(*cmp_fn)(const void*, const void*);
int comp_num(const int *lhs, const int *rhs ) {
if (pg_int[*lhs] < pg_int[*rhs])
return -1;
if (pg_int[*lhs] > pg_int[*rhs])
return 1;
return 0;
}
int comp_str(const int *lhs, const int *rhs ) {
return strcmp(pg_str[*lhs],pg_str[*rhs]);
}
int main(void) {
int nums[MAX] = {6,4,8,2,1,3};
char str[MAX][CMAX] = {"cat","dog","panter","red fish","hawk","snake"};
int i;
// create an array of indeces
int index[MAX];
for ( i = 0; i < MAX; ++i ) {
index[i] = i;
}
// set global copies
pg_int = malloc(I_DIM);
memcpy(pg_int,nums,I_DIM);
pg_str = malloc(SS_DIM);
pg_str[0] = malloc(S_DIM);
memcpy(pg_str[0],str[0],S_DIM);
for ( i = 1; i < MAX; i++ ) {
pg_str[i] = pg_str[0] + i * CMAX;
}
// sort the indeces ordering by ints
qsort(index, MAX, sizeof(int), (cmp_fn)comp_num);
//update the two arrays
for ( i = 0; i < MAX; ++i ) {
nums[i] = pg_int[index[i]];
strcpy(str[i],pg_str[index[i]]);
}
// print out sorted couples
printf("Sort by numbers:\n");
for ( i = 0; i < MAX; ++i ) {
printf("%d %s\n",nums[i],str[i]);
}
// sort the indeces ordering by strings
qsort(index, MAX, sizeof(int), (cmp_fn)comp_str);
//update the two arrays
for ( i = 0; i < MAX; ++i ) {
nums[i] = pg_int[index[i]];
strcpy(str[i],pg_str[index[i]]);
}
// print out sorted couples
printf("\nSort by strings:\n");
for ( i = 0; i < MAX; ++i ) {
printf("%d %s\n",nums[i],str[i]);
}
free(pg_int);
free(pg_str[0]);
free(pg_str);
return 0;
}
The output (sorry for the silly example) is:
Sort by numbers:
1 hawk
2 red fish
3 snake
4 dog
6 cat
8 panter
Sort by strings:
6 cat
4 dog
1 hawk
8 panter
2 red fish
3 snake
If you want to accomplish the same task in C++ you should take advantage of the Standard Library and use std::vector as a container and std::sort as algorithm:
#include <iostream>
#include <vector>
#include <algorithm>
using std::vector;
using std::string;
using std::cout;
struct numABC {
double num;
string str;
// instead of a compare function I can overload operator <
friend bool operator<( const numABC &lhs, const numABC &rhs ) {
return lhs.num < rhs.num;
}
};
// or use a lambda
auto cmp_str = []( const numABC &lhs, const numABC &rhs ) -> bool {
return lhs.str < rhs.str;
};
int main() {
vector<numABC> my_data = { {3.6, "cat"}, {5.7, "dog"}, {7.1, "panter"},
{0.2, "red fish"}, {1.8, "hawk"}, {1.1, "snake"}};
std::sort(my_data.begin(), my_data.end());
std::cout << "Sort by numbers:\n";
for ( auto & s : my_data ) {
std::cout << s.num << ' ' << s.str << '\n';
}
std::sort(my_data.begin(), my_data.end(), cmp_str);
// passing a lambda to specify how to compare ^^
std::cout << "Sort by strings:\n";
// if you don't like c++11 range for:
for ( int i = 0; i < my_data.size(); ++i ) {
std::cout << my_data[i].num << ' ' << my_data[i].str << '\n';
}
return 0;
}
Note that I have initialized my_data as a vector of objects of type numABC. If you have to start from two arrays, you can create the vector like this:
vector<double> nums = {3.6, 5.7, 7.1, 0.2, 1.8, 1.1};
vector<string> str = {"cat", "dog", "panter", "red fish", "hawk", "snake"};
vector<numABC> my_data;
for ( int i = 0; i < nums.size(); ++i ) {
my_data.push_back(numABC{nums[i],str[i]});
}
After sorting, if you have to extract the two vectors again (instead of simply looping through my_data) you can do something like this:
for ( int i = 0; i < my_data.size(); ++i ) {
nums[i] = my_data[i].num;
str[i] = my_data[i].str;
}
Alternatively you could implement an algorithm similar to the one I used before and sort the two vectors nums and srt using an auxiliary vector of indeces:
vector<double> nums = {3.6, 5.7, 7.1, 0.2, 1.8, 1.1};
vector<string> str = {"cat", "dog", "panter", "red fish", "hawk", "snake"};
// create the vector of indeces
vector<int> idx(nums.size());
std::iota(idx.begin(),idx.end(),0); // fill the vector, require #include <numeric>
// thanks to the lambda variable capture you don't need globals
auto cmp_ind = [ &nums ]
( const int &lhs, const int &rhs ) -> bool {
return nums[lhs] < nums[rhs];
};
// sort indeces
std::sort(idx.begin(), idx.end(),cmp_ind);
// create sorted arrays. It could be done in place but it's more difficult
vector<double> sorted_nums(nums.size());
vector<string> sorted_str(str.size());
for ( int i = 0; i < nums.size(); ++i ) {
sorted_nums[i] = nums[idx[i]];
sorted_str[i] = str[idx[i]];
}
std::cout << "Sort by numbers:\n";
for ( int i = 0; i < nums.size(); ++i ) {
std::cout << sorted_nums[i] << ' ' << sorted_str[i] << '\n';
}

Answer 2

You seem to intend to sort an array of integers (your compare function looks like that). But what you are actually handing over to qsort for sorting is a pointer to a structure that holds, among other stuff, an array.
So what you are actually trying to sort is one single struct ABCSort which is initialized and 9 other, uninitialized structures. This must fail.
Your qsort line should look like so:
qsort ((void*)&(abcsort.svsort), 10, sizeof (int), (compfn)compare);
Also, you should change the compare function so that it takes and works on two pointers to integers:
int compare (int * e1, int * e2) {
return *e1 - *e2;
}
EDIT:
After you have explained a bit better what you want, have a look at the following:
typedef int(compfn)(const void, const void*);
#define MAXCARS 5
struct car {
int sortKey;
double displacement;
char name[15]; /* Note I have decided this should be C */
};
/* This time we have an array of structs */
struct car cars [MAXCARS] = {
{ 0, 1.9, "BMW" },
{ 0, 6.3, "Audi" },
{ 0, 0.5, "Fiat" },
{ 0, 25.0, "Humvee" },
{ 0, 0.05, "Minibike" }
};
int compare(struct car*, struct car*);
void main(int argc, char *argv []) {
int i;
for (i = 0; i < MAXCARS; i++)
cars[i].sortKey = 100 - i;
qsort((void *)&cars, MAXCARS, sizeof(struct car), (compfn)compare);
}
/* Note we are comparing struct car-s in here, based on their displacement */
int compare(struct car *elem1, struct car *elem2) {
return elem1->sortKey - elem2->sortKey;
}