Using stringstream to indent/center output - c++

Question

I'm learning c++ and got the project to send a pascal's triangle to output (after n-rows of calculation)., getting output like this, stored in a stringstream "buffer"
1
1 1
1 2 1
1 3 3 1
But what I want is rather
1
1 1
1 2 1
1 3 3 1
My idea was: calculate the difference of the last line and current line length (I know that the last one is the longest). Then pad each row using spaces (half of the line-length-difference).
My Problem now is:
I didn't get how getLine works, neither how I might extract a specific (-> last) line
I don't know and could not find how to edit one specific line in a stringstream
Somehow I got the feeling that I'm not on the best way using stringstream.
So this is rather a common question: How'd you solve this problem and if possible with stringstreams - how?

Answer 1

To know the indentation of the first line, you would need to know the number of lines in the input. Therefore you must first read in all of the input. I chose to use a vector to store the values for the convenience of the .size() member function which will give the total number of lines after reading in all input.
#include<iostream>
#include<sstream>
#include<vector>
#include<iomanip> // For setw
using namespace std;
int main()
{
stringstream ss;
vector<string> lines;
string s;
//Read all of the lines into a vector
while(getline(cin,s))
lines.push_back(s);
// setw() - sets the width of the line being output
// right - specifies that the output should be right justified
for(int i=0,sz=lines.size();i<sz;++i)
ss << setw((sz - i) + lines[i].length()) << right << lines[i] << endl;
cout << ss.str();
return 0;
}
In this example, I am using setw to set the width of the line to be right justified. The padding on the left side of the string is given by (sz - i) where sz is the total number of lines and i is the current line. Therefore every subsequent line has 1 less space on the left hand side.
Next I need to add in the original size of the line (lines[i].length()), otherwise the line will not contain a large enough space for the resulting string to have the correct padding on the left hand side.
setw((sz - i) + lines[i].length())
Hope this helps!

Answer 2

If you have access to the code that writes the initial output, and if you know the number of lines N you are writing, you could simply do:
for(int i = 0; i < N; ++i) {
for(int j = 0; j < N - 1 - i; ++j)
sstr << " "; // write N - 1 - i spaces, no spaces for i == N.
// now write your numbers the way you currently do
}