For example, if data in an external text file is like this:
45.78 67.90 87
34.89 346 0.98
How can I read this text file and assign each number to a variable in c++? Using ifstream, I am able to open the text file and assign first number to a variable, but I don't know how to read the next number after the spaces.
#include <iostream>
#include <fstream>
using namespace std;
int main()
{
float a;
ifstream myfile;
myfile.open("data.txt");
myfile >> a;
cout << a;
myfile.close();
system("pause");
return 0;
}
#include <iostream>
#include <fstream>
using namespace std;
int main()
{
int data[6], a, b, c, d, e, f;
ifstream myfile;
myfile.open("a.txt");
for(int i = 0; i << 6; i++)
myfile >> data[i];
myfile.close();
a = data[0];
b = data[1];
c = data[2];
d = data[3];
e = data[4];
f = data[5];
cout << a << "\t" << b << "\t" << c << "\t" << d << "\t" << e << "\t" << f << "\n";
system("pause");
return 0;
}
This question is related to
c++
Repeat >> reads in loop.
#include <iostream>
#include <fstream>
int main(int argc, char * argv[])
{
std::fstream myfile("D:\\data.txt", std::ios_base::in);
float a;
while (myfile >> a)
{
printf("%f ", a);
}
getchar();
return 0;
}
Result:
45.779999 67.900002 87.000000 34.889999 346.000000 0.980000
If you know exactly, how many elements there are in a file, you can chain >> operator:
int main(int argc, char * argv[])
{
std::fstream myfile("D:\\data.txt", std::ios_base::in);
float a, b, c, d, e, f;
myfile >> a >> b >> c >> d >> e >> f;
printf("%f\t%f\t%f\t%f\t%f\t%f\n", a, b, c, d, e, f);
getchar();
return 0;
}
Edit: In response to your comments in main question.
You have two options.
Edit: How to skip values in file
To choose the 1234th value, use the following code:
int skipped = 1233;
for (int i = 0; i < skipped; i++)
{
float tmp;
myfile >> tmp;
}
myfile >> value;
you could read and write to a seperately like others. But if you want to write into the same one, you could try with this:
#include <iostream>
#include <fstream>
using namespace std;
int main() {
double data[size of your data];
std::ifstream input("file.txt");
for (int i = 0; i < size of your data; i++) {
input >> data[i];
std::cout<< data[i]<<std::endl;
}
}
The input operator for number skips leading whitespace, so you can just read the number in a loop:
while (myfile >> a)
{
// ...
}
It can depend, especially on whether your file will have the same number of items on each row or not. If it will, then you probably want a 2D matrix class of some sort, usually something like this:
class array2D {
std::vector<double> data;
size_t columns;
public:
array2D(size_t x, size_t y) : columns(x), data(x*y) {}
double &operator(size_t x, size_t y) {
return data[y*columns+x];
}
};
Note that as it's written, this assumes you know the size you'll need up-front. That can be avoided, but the code gets a little larger and more complex.
In any case, to read the numbers and maintain the original structure, you'd typically read a line at a time into a string, then use a stringstream to read numbers from the line. This lets you store the data from each line into a separate row in your array.
If you don't know the size ahead of time or (especially) if different rows might not all contain the same number of numbers:
11 12 13
23 34 56 78
You might want to use a std::vector<std::vector<double> > instead. This does impose some overhead, but if different rows may have different sizes, it's an easy way to do the job.
std::vector<std::vector<double> > numbers;
std::string temp;
while (std::getline(infile, temp)) {
std::istringstream buffer(temp);
std::vector<double> line((std::istream_iterator<double>(buffer)),
std::istream_iterator<double>());
numbers.push_back(line);
}
...or, with a modern (C++11) compiler, you can use brackets for line's initialization:
std::vector<double> line{std::istream_iterator<double>(buffer),
std::istream_iterator<double>()};
Source: Stackoverflow.com