Sort array of pointers

Question

I'm looking to create a program that creates and stores N random complex numbers. However, it's necessary to use another array (of pointers) which points to every element on the complex array. Then, I have to present it sorted by its norm to the user.

To "sort" it, I just change to where the pointer array is pointing to. Then I just show the "sorted" array of pointers.

But I'm not getting it right. How can I achieve this?

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

#define FORMATLOG "FORMATLOG: invalid parameters: ex3 <N>"
#define RANGE 18 - 6

enum { true, false };

typedef struct {
    double real,
           imag,
           norm;
} Complex;

void generateComplex(int N) {

    int i, test;
    Complex vector[N];

    for(i = 0; i < N; i++) {
        vector[i].real = rand() % RANGE;
        do { vector[i].imag = rand() % RANGE; } while(vector[i].imag == 0);
        vector[i].norm = sqrt(pow(vector[i].real, 2) + pow(vector[i].imag, 2));
    }

    Complex *p_vect = &vector;

    /*  makes array point in order */
    while(test == false) {
        test == true;
        for(i = 0; i < N - 1; i++)
            if(vector[i].norm > vector[i + 1].norm) {
                *p_vect[i] = &vector[i + 1];
                *p_vect[i + 1] = &vector[i];
                test = false;
            }
    }

    for(i = 0; i < N; i++)
        printf("\t%d -| %2g  + %2gi | = %g\n", i, vector[i].real,
                                               vector[i].imag,
                                               vector[i].norm);
    printf("********************************\n");
    for(i = 0; i < N; i++)
        printf("\t%d -| %2g  + %2gi | = %g\n", i, p_vect[i].real,
                                               p_vect[i].imag,
                                               p_vect[i].norm);

}

int main(int argc, char **argv) {

    if(argc != 2) {
        puts(FORMATLOG);
        return false;
    }

    srand(time(NULL));

    int i, N = atoi(argv[1]);
    generateComplex(N);

    return true;
}

enum { true, false } is very confusing. Why do you define true as 0 and false as 1? — Christian Hujer
– Christian Hujer, Commented Nov 23, 2014 at 16:24
Minor: replace sqrt(pow(vector[i].real, 2) + pow(vector[i].imag, 2)) with hypot(vector[i].real, vector[i].imag) — chux
– chux, Commented Nov 24, 2014 at 3:55

John Kugelman · Accepted Answer · 2014-11-23 16:23:33Z

2

Complex *p_vect = &vector;

This creates a pointer to the existing array. The requirement is to create an array of pointers, not a pointer to an array.

Complex *p_vect[N];

for (i = 0; i < N; i++) {
    p_vect[i] = &vector[i];
}

If you start with this, you can then sort p_vect without touching vector. Your sorting code, when written correctly, will make no mention of vector at all.

answered Nov 23, 2014 at 16:23

John Kugelman

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3 Comments

Persixty Over a year ago

Also, because (in your code) *p_vect is essentially an alias for vector, these lines: *p_vect[i] = &vector[i + 1]; *p_vect[i + 1] = &vector[i]; Are just overwriting element i with i+1. The second line is having no effect. You need to use a third variable to swap values.

user3050910 Over a year ago

Oh, I see. Thank you. But then I'm having problems with sorting the array. I'm doing: /* makes array pointer in order */ while(test == false) { test == true; for(i = 0; i < N - 1; i++) if(p_vect[i]->norm > p_vect[i + 1]->norm) { Complex *temp = p_vect[i]; p_vect[i] = p_vect[i + 1]; p_vect[i + 1] = temp; test = false; } } But the pointer array just doesn't sort.

John Kugelman Over a year ago

I'll give you a hint. There's a typo with the test variable.

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