How to subtract string arrays?

I was wondering if someone could tell me how to subtract two string arrays. I use getline() that is why the array is a string. It is in my main, but the below code is my function.

Also the data inside the array is just numbers such as: 88 91 100 etc, Just want to subtract those numbers

I Get the errors from these two lines of code.

Code:

`HUFFMAN[i] = dataCASE1[i] - dataCASE1[i-16];`

Code:

`HUFFMAN[i] = dataCASE1[i] - dataCASE1[i-1];`

Code:

`void algor1(int sizeCASE1, int columnCASE1, string dataCASE1[]){ `

string HUFFMAN[256];

ofstream outStream;

outStream.open("outfile.txt");

if(outStream.fail())

{

cout << "Output file opening failed.\n";

exit(1);

}

HUFFMAN[0] = dataCASE1[0];

for(int i = 1; i < sizeCASE1; i++)

{

if(i == columnCASE1)

{

HUFFMAN[i] = dataCASE1[i] - dataCASE1[i-16];

}

else

HUFFMAN[i] = dataCASE1[i] - dataCASE1[i-1];

}

for(int j = 0; j < sizeCASE1; j++)

{

outStream << HUFFMAN[j];

}

outStream.close();

}

Thanks