c++ - How to find the low & high in Merge Sort -

i have basic understanding of merge sort algorithm. reason can't wrap head around values low , high coming from. code working merge.

void practicemerge(int a[], int low, int mid, int high) {     int b[10000];     int = low, j = mid + 1, k = 0;      while (i <= mid && j <= high) {         if (a[i] <= a[j])             b[k++] = a[i++];         else             b[k++] = a[j++];     }     while (i <= mid)     b[k++] = a[i++];      while (j <= high)         b[k++] = a[j++];      k--;     while (k >= 0) {         a[low + k] = b[k];         k--;     } } 

let me explain commenting code:

//sort array index low index high merging 2 //subarrays of go low mid , mid high. void practicemerge(int a[], int low, int mid, int high) {     int b[10000]; //buffer     int = low; //starting index in first sub array     int j = mid + 1; //starting index in second sub array     int k = 0; //starting index in buffer      //while not go beyond limit of first subarray     //nor second     while (i <= mid && j <= high) {         if (a[i] <= a[j])             //put value of first subarray, , move              b[k++] = a[i++];         else             //put value of first subarray, , move             b[k++] = a[j++];     }     //finish copying first subarray if not done yet     while (i <= mid)     b[k++] = a[i++];     //finish copying second subarray if not done yet     while (j <= high)         b[k++] = a[j++];      //copy buffer array     k--;     while (k >= 0) {         a[low + k] = b[k];         k--;     } } 

basically, low, mid , high, borders of array "a" looking at. "a" bigger. ex:

a =  3 2 1 5 6 7 0 1  low = 0 mid = 2 high = 4 

here sorting first half of a.

edit: function merges arrays. main merge sort function, splits array. be:

void merge_sort(int a[], int lo, int hi) {    if (low >= hi)       return; //nothing sort    int mid = (lo+hi)/2; //the guy between lo , hi.    merge_sort(a,lo, mid); //sort left    merge_sort(a, mid, hi); //sort right    practicemerge(a, lo, mid, hi); //this merges array } 

to understand (don't copy paste!) think of way: merge_sort sorts chunk of array, not entire array, bit between lo , hi. in order so, sorts 1 half, other. merges result array. therefore, inside merge_sort, "hi" , "lo" computed parameter. now, you, user, might want sort array 0 end, or 10th 99th index. choice. , parameters pass call.

void main() {    //bla bla bla    merge_sort(songs, 89, 250); //only sort songs } 

think of this, black box. call parameters, box thing. because box uses itself, knows how call (i.e. know how compute low, high , mid) initial call in recursion responsability user.

p.s.: feel im not being clear...