如何摆脱固定的哨兵值（-32767）？

Question

Below is the program I wrote to find sum of a subarray from given array, however somehow I am not getting how can I get rid of the sentinel value (-32767 in this case)? and how can I optimise it? and how can I keep track of range of max subarray?

#define EVALUE -32767

using namespace std;

int findMaxSubArray(vector<int>,int low,int high);
int findMaxSubArray_Mid(vector<int>,int low,int high);

int main()
{
    vector<int> v;
    int j=0;

    cout << "Enter array values(-32767 to end): ";
    while(1)
    {
        cin >> j;
        if (EVALUE==j)
            break;
        v.push_back(j);
    }

if(v.size()!=0)
    cout << "Max sum is: " << findMaxSubArray(v,0,v.size()-1) << "\n";
else
    cout << "No array elements entered, exiting...\n";

system("pause");
return 0;
}

int findMaxSubArray(vector<int> v, int low, int high)
{
    if(low==high) return v[low];

    int max_mid_sum=findMaxSubArray_Mid(v,low,high);
    int max_left_sum=findMaxSubArray(v,low,(low+high)/2);
    int max_right_sum=findMaxSubArray(v,(low+high)/2+1,high);

    if (max_mid_sum>max_left_sum) return (max_mid_sum>max_right_sum?max_mid_sum:max_right_sum);
    else return(max_left_sum>max_right_sum?max_left_sum:max_right_sum);
}

int findMaxSubArray_Mid(vector<int> v,int low,int high)
{
    int mid=high/2;
    int max_left_sum=0;
    int max_right_sum=0;
    int sum=0;

    for(int i=mid;i>=low;--i)
    {
        sum+=v[i];
        if(sum>max_left_sum)    
        {
            max_left_sum=sum;
        }
    }

    sum=0;
    for(int i=mid+1;i<=high;++i)
    {
        sum+=v[i];
        if(sum>max_right_sum)   
        {
            max_right_sum=sum;
        }
    }

    return (max_right_sum+max_left_sum);
}

Answer 1

When reading from a textfile, the last character that cin will get is the "EOF" character, or end of file character. You can send this character to your program in the command line with control+d. You're going to want to check for this rather than -32767.

This is a basic program that should identify a simple fix for your problem:

#include <vector>
#include <iostream>

using namespace std;

int main()
{
    vector<int> v;
    int j;

    cout << "Enter array values (Control+D (EOF) to end): ";
    cin >> j;
    while(cin.good())
    {
        v.push_back(j);
        cin >> j;
    }

    return 0;
}

If you want to get really smart you can use the below and it will directly insert the contents of the memory at cin (from the beginning until EOF) into your vector. As far as running time goes, this will probably be faster than your solution and the above solution.

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>

using namespace std;

int main()
{
    vector<int> v;
    cout << "Enter array values (Control+D (EOF) to end): ";

    istream_iterator<int> in(cin);
    istream_iterator<int> eof;
    copy(in, eof, back_inserter(v));

    ostream_iterator<int> out(cout, "\n");
    copy(v.begin(), v.end(), out);

    return 0;
}

Answer 2

About the sentinel, IIRC with Control+D you close standard input(may depend of OS). That will cause the << to fail (I am not sure how, probably you'll have to catch an exception).

Anyway, the rest of the code is just a recursive (binomial) adding of the vector. You can sustitute all of it with a simple for

for(int i = 0; i < v.size(); i++) {
  total += v[i]
}

The question about range of max subarray is already managed by the class Vector