This code is leading me to an infinite cycle and I can’t figure out why

Question

This code is leading me to an infinite cycle and I can’t figure out why.

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

void
troca (double *n1, double *n2)
{
    double n;

    n    = *n1;
    *n1  = *n2;
    *n2  = n  ;
}

void
ordena (double *r, int size)
{
    int t;
    int i;

    while (1)
    {
        t = 0;
        {
            for (i=0;i<size-1;i++)
            {
                if(r[i] > r[i+1])
                {
                    troca (&r[i], &r[i+1]);
                    t      = 1          ;
                }
            }
        if(t == 0)
            break;
        }
    }
}


int
main(int argc, char **argv)
{
    FILE   *f1   ;
    int    i     ;
    int    j=0   ;
    int    N     ;               
    int    k     ;             
    double Min   ;
    double Max   ;
    double *reais;           
    double n     ;
    char    c    ;

    sscanf(argv[1],"%d",&N)   ;
    sscanf(argv[2],"%lf",&Min);
    sscanf(argv[3],"%lf",&Max);

    f1    = fopen("f1.txt", "wb")            ; 

    srand(time(NULL));

    while (1)
    {

    if (j == 0)
    {
        reais = (double*)malloc((N)*sizeof(double));
        j=1;
    }

    else
        reais = realloc(reais, (N)*sizeof(double));

    n = Max - Min;
    for (i=0;i<N;i++)
    {
        reais[i] = Max - ((double) rand()/(double) RAND_MAX )*n;
        //printf("%9lf",reais[i]);
    }

    ordena(reais, N);

    k = 0;
    for (i=0;i<N;i++)
    {
        printf("%9lf",reais[i]);
        k++;

        if (k == 5)
        {
            k = 0;
            printf("\n");
        }
    }

    printf("\nDeseja repetir as operacoes:\n1-Repetir\n2-Sair\nR:");
    c = getchar();

    if (c == '1')
    {
        printf("\nIntroduza novos valores de N, Min e Max:");
        scanf("%d %lf %lf", &N, &Min, &Max);
        continue;
    }

    if (c == '2')
    {
        free(reais);
        break;
    }

    }

    return 0;
}

The program takes three arguments from the command line, N, the number of reals the user wants to generate, Max and Min the limits of the generated values. With these values is generated a vector of doubles with the necessary size that is then ordered. So far everything works well. The problem is that when I ask for new arguments, I enter an infinite loop

Answer 1

You are ordering using the bubble strategy, with right to a flag to mark whether there have been exchanges or not.

Mathematically, you need to do the most o( (n^2 - n)/2 ) operations. By rotating once the inner side, there is a guarantee that the largest number will be at the end of the vector. Then the next sorting bid would not need to go to the last position, only to the penultimate, then the two largest elements will be in their definitive positions. In that reply, called this strategy of minimizing the amount of repetitions of "minimal bubblesort". You can put the flag to indicate that if there was no swap, then the vector is already ordered and no longer need to work on it.

This operation the way you’re doing is slow. Don’t be surprised by it. My suggestion for improvement at best doubles the speed, but also remains slow (more relative performance information on another linked answer).

In theory, his ordination would one day end up considering that the exchange operation is correct and that the comparison is correct. I couldn’t see any problems with these operations. If you want to do a debug, but do not have the most traditional tools for it, you can insert a printf to indicate when there is exchange and what are the values being exchanged. Obviously this would be a debugging code that should be removed in the final version.

Then, the loop failure must be at another point. The other loop there is at the main function. You are using the getchar(). What happens if this function returns a value you don’t expect? From what I understand of the code, it returns the loop weakly, without changing any of the parameters.

Or maybe he just isn’t reacting to you typing? According to function reference, it only returns the moment you press ENTER. It wasn’t clear to me what happens if you happened to read something earlier.

Answer 2

If I understand the code correctly the problem is in the method ordena; how do you have a for covering all the positions of the r, doesn’t need the while(1).

The loop was because the t == 0 never occurs, since whenever you enter the iteration for (i=0;i<size-1;i++) this guy spins and the t = 1; also.

voi ordena (double *r, int size)
{
    int t = 0;

    for (int i = 0; i < size - 1; i++)
    {
        if (r[i] > r[i+1])
        {
            troca (&r[i], &r[i+1]);
            t = 1;
        }
    }
}

OBS: I kept the structure of the variables and such, but tries to use more intuitive names, it is much easier to understand the code.

Besides, there are some things to improve, for example the FILE *f1;. You just create this variable and open a file, but use it for nothing.