Writing sorted numbers from function out to text file? - c++

Question

I have two text files, each with an unknown number of integers sorted from lowest to highest... for example:
input file 1: 1 3 5 7 9 11...
input file 2: 2 4 6 8 10 ....
I want to take these numbers from both files, sort from low to high, and then output the full list of sorted numbers from both input files to a single output file. What I have so far...
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include "iosort.h"
int main()
{
const char* filename1 = "numberlist1.txt";
const char* filename2 = "numberlist2.txt";
std::ofstream ofs("output.txt");
std::ifstream ifs1, ifs2;
std::string input1, input2;
ifs1.open(filename1);
std::getline(ifs1, input1);
std::cout << "Contents of file 1: " << input1 << std::endl;
ifs2.open(filename2);
std::getline(ifs2, input2);
std::cout << "Contents of file 2: " << input2 << std::endl;
ioSort(ifs1, ifs2, ofs);
return 0;
}
and my function...
#include <fstream>
#include <sstream>
#include <vector>
#include "iosort.h"
void ioSort(std::ifstream& in1, std::ifstream& in2, std::ofstream& out)
{
int a, b;
std::vector<int> f1, f2, f3; //create one vector for each input stream
while (in1 >> a)
{
f1.push_back(a);
}
while (in2 >> b)
{
f2.push_back(b);
}
//now f1 and f2 are vectors that have the numbers from the input files
//we know that in these input files numbers are sorted from low to high
if (f1.size() > f2.size()) //input stream 1 was larger
{
for (int i = 0; i < f2.size(); i++)
{
if (f1[i] > f2[i]) //number at input vector 2 less that respective pos
{ //in input vector 1
f3.push_back(f2[i]);
}
else if(f1[i] == f2[i]) //numbers are equal
{
f3.push_back(f1[i]);
f3.push_back(f2[i]);
}
else //number in 1 is less than that in vector 2
{
f3.push_back(f1[i]);
}
}
for (int i = f2.size(); i < f1.size(); i++)
{
f3.push_back(f1[i]); //push remaining numbers from stream 1 into vector
}
}
else //input stream 2 was larger
{
for (int i = 0; i < f1.size(); i++)
{
if (f1[i] > f2[i]) //number at input vector 2 less that respective pos
{ //in input vector 1
f3.push_back(f2[i]);
}
else if(f1[i] == f2[i]) //numbers are equal
{
f3.push_back(f1[i]);
f3.push_back(f2[i]);
}
else //number in 1 is less than that in vector 2
{
f3.push_back(f1[i]);
}
}
for (int i = f1.size(); i < f2.size(); i++)
{
f3.push_back(f1[i]); //push remaining numbers from stream 2 into vector
}
}
//send vector contents to output file
for (int i = 0; i < f3.size(); i++)
{
out << f3[i] << " ";
}
}
Everytime I compile and run, the file output.txt is being created, but it is empty. Can anybody point me to what I am doing wrong. If, in main, I do something like:
out << 8 << " " << 9 << std::endl;
then it will show up in the output file.

Answer 1

AHA! Found your error. You're opening the file, then reading it directly to stdout (where you list the contents of your file), and then passing the same stream into your function. You cannot do this. Whenever you read from a file, the stream moves further through the file. By the time you're in your sorting function, you're at the end of the file, and so no numbers are read!
You need to remove the lines
std::getline(ifs1, input1);
std::cout << "Contents of file 1: " << input1 << std::endl;
and
std::getline(ifs2, input2);
std::cout << "Contents of file 2: " << input2 << std::endl;
Instead, print them out after you've stored them in the vector.
I'll leave the rest of my reply down below, since you, or posterity, might need it.
I'm not sure what's going on with your output file problem. Go through the whole chain and see where it's failing:
After you've read your file in, print out f1 and f2 with cout. Are they there and what you expect? If they are, we can move on.
After your algorithm has run, is your f3 there, and what you expect? If so, keep going!
This lets you diagnose the exact line where your code is failing (i.e. not doing it what you expect it do), and you know you can rule everything you've checked out.
Of course, instead of using cout you can launch this under a debugging environment and see what happens step by step, but if you don't know how, it'll take longer to do that to diagnose your problem the first time.
You do have other problems though, your merge function has errors. You end up skipping certain elements because you're only using one index for both arrays. Think about it: you only push one number into your output array in the f1[i] > f2[i] or the f1[i] < f2[i], but you discard both by incrementing i.
You can take your merge loop and simplify it by a lot, while also fixing your mistake :).
auto it = f1.cbegin();
auto jt = f2.cbegin();
while (it != f1.cend() && jt != f2.cend()) {
if (*it < *jt) f3.push_back(*jt++); //f2 was bigger, push f2>f3 and increment f2 index
else if (*it > *jt) f3.push_back(*it++); //f1 was bigger, push f1>f3 and increment f1 index
else { //implicit equals, only option left
f3.push_back(*jt++);
f3.push_back(*it++);
}
}
while (it != f1.cend()) f3.push_back(*it++);
while (jt != f2.cend()) f3.push_back(*jt++);
So now f3 contains your sorted array, sorted in O(m+n) time. If you're doing this for the sake of learning, I'd try to remedy your error using your way first before switching over to this.
If you want to write less code and speed isn't a problem, you can use <algorithm> to do this, too, but it's a terrible O((n+m)lg(n+m)).
auto it = f1.cbegin();
auto jt = f2.cbegin();
while (it != f1.cend()) f3.push_back(*it++);
while (jt != f2.cend()) f3.push_back(*jt++);
std::sort(f3.begin(), f3.end());

Answer 2

Since you're reading the file with std::getline() before calling ioSort(), there's nothing for the sorting function to read.
You can rewind back to the beginning of the file with seekg().
ifs1.clear();
ifs1.seekg(0, ifs1.beg);
ifs2.clear();
ifs2.seekg(0, ifs1.beg);
ioSort(ifs1, ifs2, ofs);
See How to read same file twice in a row

Answer 3

In order to be short:
#include <fstream>
#include <algorithm>
#include <iterator>
int main()
{
std::ifstream infile1("infile1.txt");
std::ifstream infile2("infile2.txt");
std::ofstream outfile("outfile.txt");
std::merge(
std::istream_iterator<int>{infile1}, std::istream_iterator<int>{},
std::istream_iterator<int>{infile2}, std::istream_iterator<int>{},
std::ostream_iterator<int>{outfile, " "}
);
}
std::merge is an STL algorithm that merge two sorted ranges into one sorted range. And the ranges are the files for this case. The files are viewed as ranges using std::istream_iterator<int>. The output file is accessed as a range using std::ostream_iterator<int>.