Use binary search to find interleaving sequence in C

Question

I have two arrays, A and X, where A >= X. I want to find the max interleaving factor i for X^i such that X^i is a subsequence of A. For example, if A = [4,3,2,1,4,3,2,1,4,3,2,1,4,3,2,1], and X = [1,2,3], then i = 1, because X^1 = [1,2,3] and that sequence is in A. My program should use a binary search to find this max interleaving factor i and trace whether or not each iteration is a sequence of A. So using binary search for the above example, i would start = 3 (as max possible for A/X = 6), and X^3 = [1,1,1,2,2,2,3,3,3] and that is not a sequence in A.

Here is my code so far:

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

void create_initial_arrays(int size_a, int *A, int size_x, int *X);
void binary_search(int size_a, int * A, int size_x, int *X, int max_i, int min_i);

int main(){
    int size_a, size_x;
    scanf("%d", &size_a);
    scanf("%d", &size_x);

    int max_i = size_a / size_x;
    int min_i = 0;

    printf("Max: %d\n", max_i);

    int *A = (int*) malloc(size_a *sizeof(int));
    int *X = (int*) malloc(size_x *sizeof(int));

    create_initial_arrays(size_a, A, size_x, X);

    printf("Old X: ");
    for(int i = 0; i < size_x; i++){
        printf("%d ", X[i]);
    }
    printf("\n");

    binary_search(size_a, A, size_x, X, max_i, min_i); //practice reallocating size of array


    //for(int i = 0; i < size_x; i++){
      //  printf("%d ", A[i]);
    //}



}

void create_initial_arrays(int size_a, int *A, int size_x, int *X){
    int i, throwaway;

    for(i = 0; i < size_a; i++){
        scanf("%d", &A[i]);
    }

    scanf("%d", &throwaway);

    for(i = 0; i < size_x; i++){
        scanf("%d", &X[i]);
    }

    scanf("%d", &throwaway);
}



void binary_search(int size_a, int * A, int size_x, int *X, int max_i, int min_i){

    int i, j, k, count = 0, max_repeat = 0;

    while(min_i <= max_i){

    int repeats = (max_i + min_i)/2;

    printf("\n");

    int * temp = realloc(X, size_x * sizeof(int) * repeats);
    X = temp;

    for(k = 0; k < size_x; ++k){
        int idx = size_x - k -1;
        temp = &X[idx];
        for(j = 0; j < repeats; ++j){
            X[idx * repeats + j] = *temp;
        }
    }

    printf("New X: ");
        for(i = 0; i < size_x * repeats; i++){
            printf("%d ", X[i]);
        }

    for(i = 0; i < size_x * repeats; i++){
        for(j = 0; j < size_a; j++){
            if(A[j] == X[i]){
                count++;
                i++;
            }
        }
    }

    if (count == size_x * repeats){
        printf("Low: %d Mid %d High % d Passes\n", min_i, repeats, max_i);
        min_i = repeats + 1;
        max_repeat++;
    }
    else
        printf("Low: %d Mid %d High % d Fails\n", min_i, repeats, max_i);
        max_i = repeats - 1;
    }

    printf("Max repeat: %d", max_repeat);
}

Here is my current output:

New X: 1 1 1 2 2 2 3 3 3 Low: 0 Mid 3 High  6 Fails

New X: 1 1 1 Low: 0 Mid 1 High  2 Fails

New X: Low: 0 Mid 0 High  0 Fails

I am expecting this:

New X: 1 1 1 2 2 2 3 3 3 Low: 0 Mid 3 High  6 Fails

New X: 1 2 3 Low: 0 Mid 1 High  2 Passes

New X: Low: 2 Mid 2 High  2 Fails

Max i = 1.

Meaning, that my code is not creating the correct array on the second iteration. X^1 should equal [1,2,3] not [1,1,1]. Why is it not creating the array properly on the second iteration but it does on the first?

Answer 1

Why is it not creating the array properly on the second iteration but it does on the first?

In the first loop you take X which is {1, 2, 3} and change it into {1, 1, 1, 2, 2, 2, 3, 3, 3} by repeating the first number 3 times, repeating the second number 3 times and repeating the third number 3 times.

In the second loop you start with X being {1, 1, 1, 2, 2, 2, 3, 3, 3}. Now you construct a new X by repeating the first number 1 time, repeating the second number 1 time and repeating the third number 1 time.

As the first, the second and the third numbers are all 1 you end up with {1, 1, 1}

In other words: Your first loop changed X and therefore you second loop use another value for X than the first loop. Consequently, the second loop produce an unexpected value for X