Comparing numbers in strings to numbers in int? - c++

Question

I'm trying to make a program that will open a txt file containing a list of names in this format (ignore the bullets):
3 Mark
4 Ralph
1 Ed
2 Kevin
and will create a file w/ organized names based on the number in front of them:
1 Ed
2 Kevin
3 Mark
4 Ralph
I think I'm experiencing trouble in line 40, where I try to compare the numbers stored in strings with a number stored in an int.
I can't think of any other way to tackle this, any advice would be wonderful!
#include <iostream>
#include <fstream>
#include <vector>
#include <cstdlib>
using namespace std;
int main()
{
ifstream in;
ofstream out;
string line;
string collection[5];
vector <string> lines;
vector <string> newLines;
in.open("infile.txt");
if (in.fail())
{
cout << "Input file opening failed. \n";
exit(1);
}
out.open("outfile.txt");
if (out.fail())
{
cout << "Output file opening failed. \n";
exit(1);
}
while (!in.eof())
{
getline(in, line);
lines.push_back(line);
}
for (int i = 0; i < lines.size(); i++)
{
collection[i] = lines[i];
}
for (int j = 0; j < lines.size(); j++)
{
for (int x = 0; x < lines.size(); x--)
{
if (collection[x][0] == j)
newLines.push_back(collection[x]);
}
}
for (int k = 0; k < newLines.size(); k++)
{
out << newLines[k] << endl;
}
in.close( );
out.close( );
return 0;
}

Answer 1

Using a debugger would tell you where you went wrong, but let me highlight the mistake:
if (collection[x][0] == j)
You're expecting a string like 3 Mark. The first character of this string is '3', but that has the ASCII value of 51, and that is the numerical value you'll get when trying work with it is this way! This will never equal j, unless you've got a lot of lines in your file, and then your search system will not work at all like you wanted. YOu need to convert that character into an integer, and then do your comparison.
C++ offers many way to process data via streams, including parsing simple datafiles and converting text to numbers and vice versa. Here's a simple standalone function that will read a datafile like you have (only with arbitrary text including spaces after the number on each line).
#include <algorithm>
// snip
struct file_entry { int i; std::string text; };
std::vector<file_entry> parse_file(std::istream& in)
{
std::vector<file_entry> data;
while (!in.eof())
{
file_entry e;
in >> e.i; // read the first number on the line
e.ignore(); // skip the space between the number and the text
std::getline(in, e.text); // read the whole of the rest of the line
data.push_back(e);
}
return data;
}
Because the standard way that >> works involves reading until the next space (or end of line), if you want to read a chunk of text which contains whitespace, it will be much easier to use std::getline to just slurp up the whole of the rest of the current line.
Note: I've made no attempt to handle malformed lines in the textfile, or any number of other possible error conditions. Writing a proper file parser is outside of the scope of this question, but there are plenty of tutorials out there on using C++'s stream functionality appropriately.
Now you have the file in a convenient structure, you can use other standard c++ features to sort it, rather than reinventing the wheel and trying to do it yourself:
int sort_file_entry(file_entry a, file_entry b)
{
return a.i < b.i;
}
int main()
{
// set up all your streams, etc.
std::vector<file_entry> lines = parse_file(in);
std::sort(lines.begin(), lines.end(), sort_file_entry);
// now you can write the sorted vector back out to disk.
}
Again, a full introduction to how iterators and containers work is well outside the scope of this answer, but the internet has no shortage of introductory C++ guides out there. Good luck!