shannon fano coding program in C

This is program for shanno fano coding .
It is a technique for constructing a prefix code based on a set of symbols and their probabilities (estimated or measured). It is suboptimal in the sense that it does not achieve the lowest possible expected code word length like Huffman coding; however unlike Huffman coding, it does guarantee that all code word lengths are within one bit of their theoretical ideal − logP(x).

// shannon.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include<stdio.h>

int g_level = 0;
void shannon(int start,int end,int arr[20], char code[20][20],int level) {
    int i=start;
    int j=end;
    int isum = arr[i],jsum = arr[j];

    if(level>g_level) {
        g_level = level;
    }
    while(i<(j-1)) {
        while(isum>jsum && i<(j-1)) {
            j--;
            jsum += arr[j];
        }
        while(isum<jsum && i<(j-1)) {
            i++;
            isum += arr[i];
        }
    }

    if(i==start) {
        code[start][level]='0';
    } else if((i-start)>=1) {
        for(int k=start;k<=i;++k) 
            code[k][level] = '0';

        shannon(start,i,arr,code,level+1);
    }
    if(j==end) {
        code[end][level]='1';
    } else if((end-j)>=1) {
        for(int k=j;k<=end;++k) 
            code[k][level] = '1';

        shannon(j,end,arr,code,level+1);
    }
}

int _tmain(int argc, _TCHAR* argv[])
{
    int arr[20];
    int n,i,j;
    printf("Enter the number of symbols :");
    scanf("%d",&n);

    // Take the symbols and sort them as user enters them using insertion sort 
    for(i=0; i<n; ++i) {
        int s;
        printf("Enter symbol frquency : ");
        scanf("%d",&s);
        j=i;
        while(j && arr[j-1]<s) {
            arr[j] = arr[j-1];
            j--;
        }
        arr[j] = s;
    }

    char code[20][20];
    
    for(i=0; i<n; ++i) {
        // Mark row as invalid
        for(j=0;j<n;j++) {
            code[i][j] = 'X';
        }
    }

    shannon(0,n-1,arr,code,0);

    printf("\n\n DATA  CODE\n");
    // Print the data and code 
    for(i=0; i<n; ++i) {
        printf("\n %4d  : ",arr[i]);
        for(j=0; j<=g_level; j++) {
            if(code[i][j]!='X') 
                printf("%c",code[i][j]);
        }
    }

    printf("\n\n");
    return 0;
}