How to read a file into vector in C++? - c++

Question

I need to read from a .data or .txt file containing a new float number on each line into a vector.
I have searched far and wide and applied numerous different methods but every time I get the same result, of a Main.size() of 0 and an error saying "Vector Subscript out of Range", so evidently the vector is just not reading anything into the file.
Note: the file is both in the folder and also included in the VS project.
Anyway, here's my code:
#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
#include <string>
using namespace std;
int main() {
vector<double> Main;
int count;
string lineData;
double tmp;
ifstream myfile ("test.data", ios::in);
double number;
myfile >> count;
for(int i = 0; i < count; i++) {
myfile >> tmp;
Main.push_back(tmp);
cout << count;
}
cout << "Numbers:\n";
cout << Main.size();
for (int i=0; i=((Main.size())-1); i++) {
cout << Main[i] << '\n';
}
cin.get();
return 0;
}
The result I get is always simply:
Numbers:
0

Answer 1

Your loop is wrong:
for (int i=0; i=((Main.size())-1); i++) {
Try this:
for (int i=0; i < Main.size(); i++) {
Also, a more idiomatic way of reading numbers into a vector and writing them to stdout is something along these lines:
#include <iostream>
#include <iterator>
#include <fstream>
#include <vector>
#include <algorithm> // for std::copy
int main()
{
std::ifstream is("numbers.txt");
std::istream_iterator<double> start(is), end;
std::vector<double> numbers(start, end);
std::cout << "Read " << numbers.size() << " numbers" << std::endl;
// print the numbers to stdout
std::cout << "numbers read in:\n";
std::copy(numbers.begin(), numbers.end(),
std::ostream_iterator<double>(std::cout, " "));
std::cout << std::endl;
}
although you should check the status of the ifstream for read errors.

Answer 2

Just to expand on juanchopanza's answer a bit...
for (int i=0; i=((Main.size())-1); i++) {
cout << Main[i] << '\n';
}
does this:
Create i and set it to 0.
Set i to Main.size() - 1. Since Main is empty, Main.size() is 0, and i gets set to -1.
Main[-1] is an out-of-bounds access. Kaboom.

Answer 3

Just a piece of advice.
Instead of writing
for (int i=0; i=((Main.size())-1); i++) {
cout << Main[i] << '\n';
}
as suggested above, write a:
for (vector<double>::iterator it=Main.begin(); it!=Main.end(); it++) {
cout << *it << '\n';
}
to use iterators. If you have C++11 support, you can declare i as auto i=Main.begin() (just a handy shortcut though)
This avoids the nasty one-position-out-of-bound error caused by leaving out a -1 unintentionally.

Answer 4

1.
In the loop you are assigning value rather than comparing value so
i=((Main.size())-1) -> i=(-1) since Main.size()
Main[i] will yield "Vector Subscript out of Range" coz i = -1.
2.
You get Main.size() as 0 maybe becuase its not it can't find the file. Give the file path and check the output. Also it would be good to initialize the variables.

Answer 5

#include <iostream>
#include <fstream>
#include <vector>
using namespace std;
int main()
{
fstream dataFile;
string name , word , new_word;
vector<string> test;
char fileName[80];
cout<<"Please enter the file name : ";
cin >> fileName;
dataFile.open(fileName);
if(dataFile.fail())
{
cout<<"File can not open.\n";
return 0;
}
cout<<"File opened.\n";
cout<<"Please enter the word : ";
cin>>word;
cout<<"Please enter the new word : ";
cin >> new_word;
while (!dataFile.fail() && !dataFile.eof())
{
dataFile >> name;
test.push_back(name);
}
dataFile.close();
}

Answer 6

//file name must be of the form filename.yourfileExtension
std::vector<std::string> source;
bool getFileContent(std::string & fileName)
{
if (fileName.substr(fileName.find_last_of(".") + 1) =="yourfileExtension")
{
// Open the File
std::ifstream in(fileName.c_str());
// Check if object is valid
if (!in)
{
std::cerr << "Cannot open the File : " << fileName << std::endl;
return false;
}
std::string str;
// Read the next line from File untill it reaches the end.
while (std::getline(in, str))
{
// Line contains string of length > 0 then save it in vector
if (str.size() > 0)
source.push_back(str);
}
/*for (size_t i = 0; i < source.size(); i++)
{
lexer(source[i], i);
cout << source[i] << endl;
}
*/
//Close The File
in.close();
return true;
}
else
{
std::cerr << ":VIP doe\'s not support this file type" << std::endl;
std::cerr << "supported extensions is filename.yourfileExtension" << endl;
}
}