Function doesn't seem to modify array

Question

I'm writing a program that is supposed to take in an input of zeroes and ones and assign them into an array. Then it passes the array to edge function and formats it in the following way.

Assign a 1 to the output bit pattern whenever two consecutive bits (one bit and it’s previous bit) are different
Assign a 0 to the output bit pattern whenever two consecutive bit (one bit and it’s previous bit) are the same
Assign 0 to the first output bit since there is no previous bit for the first bit

The problem is that when the array is passed to the function and printf is called, it simply prints out the original input. I've looked at it from different angles and can't seem to see whats wrong.

#include <stdio.h>

void edge(int n, int a1[], int a2[])
{
    int i = 1;
    a2[0] = 0;
    int last = a1[i-1];
    printf("%d", a2[0]);
    for(i = 1; i < n; i++)
    {
        if(last == a1[i])
        {
                a2[i] = 0;
        }
        else
        {
                a2[i] = 1;
        }
    printf("%1d", a2[i]);
}
}

int main(void)
{
    int i = 0;
    int num;

    int array1[8];
    int array2[8]={0};

    printf("Enter an 8-digit barcode: \n");
    for(i = 0; i < 8; i++)
    {
        scanf("%1d", &num);
        if(num == 1)
        {
                array1[i] = 1;
        }
    }
    printf("Output: ");

    edge(8, array1, array2);

    return 0;
}

You don't update the value you give to last in your loop ; could your problem originate there ? — m.raynal - bye toxic community
– m.raynal - bye toxic community, Commented Feb 5, 2017 at 19:08
Since you talk in your explanation about "2 consecutive bits", probably you'd want to add last = a1[i] just after your call to printf in the for loop. — m.raynal - bye toxic community
– m.raynal - bye toxic community, Commented Feb 5, 2017 at 19:10
@Dustynana: Don't delete necessary parts of your question just because you have an answer. — ShadowRanger
– ShadowRanger, Commented Feb 6, 2017 at 14:17

m.raynal - bye toxic community · Accepted Answer · 2017-02-05 19:15:56Z

1

If you don't update last within the loop, you'll always compare the values stored in a2[] with the same value, which does not seem to be your aim. Thus updating last in the loop.

void edge(int n, int a1[], int a2[])
{
    int i;
    a2[0] = 0;
    int last;
    printf("%d", a2[0]);
    for(i = 1; i < n; i++) {
        last = a1[i-1];
        if(last == a1[i])
            a2[i] = 0;
        else 
            a2[i] = 1;
        printf("%1d", a2[i]);
    }
}

answered Feb 5, 2017 at 19:15

m.raynal - bye toxic community

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2 Comments

Dustynana Over a year ago

You're completely right, i did this just now and it seems to be working fine.

Dustynana Over a year ago

got rid of last completely and switched it out in the loop for a1[i-1], where the if statement is.

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