An alliance with an insidious mistake

Having a problem understanding that when we want to go through the whole array and compare each value of the array with the number present in the array, say arr [0], then why is it recommended to initialize int with arr [0], just like int acomp = arr [0], and compare acomp with any integer present in the array, than compare any integer present in the array with arr [0]? For example, in the following union code, I was told that Code 2 is better than Code 1, but I'm not entirely sure why.

int unionarr(int p, int q){            //Code 1
    for(int i=0;i<size;i++)
        if(arr[i]==arr[p])
            arr[i]=arr[q];}

int unionarr(int p, int q){            //Code 2
    int pid=arr[p];
    int qid=arr[q];
        for(int i=0;i<size;i++)
        if(arr[i]==pid)
            arr[i]=qid;}