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; } 5 Answers
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.
- You can run previous code in a loop (or two loops) and throw away a defined number of values - for example, if you need the value at point (97, 60), you have to skip 5996 (= 60 * 100 + 96) values and use the last one. This will work if you're interested only in specified value.
- You can load the data into an array - as Jerry Coffin sugested. He already gave you quite nice class, which will solve the problem. Alternatively, you can use simple array to store the data.
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; 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>()}; The input operator for number skips leading whitespace, so you can just read the number in a loop:
while (myfile >> a) { // ... } 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; } } You can use a 2D vector for storing the numbers that you read from the text file as shown below:
#include <iostream> #include <vector> #include <string> #include <sstream> #include<fstream> int main() { std::string line; double word; std::ifstream inFile("data.txt"); //create/use a std::vector std::vector<std::vector<double>> vec; if(inFile) { while(getline(inFile, line, '\n')) { //create a temporary vector that will contain all the columns std::vector<double> tempVec; std::istringstream ss(line); //read word by word(or double by double) while(ss >> word) { //std::cout<<"word:"<<word<<std::endl; //add the word to the temporary vector tempVec.push_back(word); } //now all the words from the current line has been added to the temporary vector vec.emplace_back(tempVec); } } else { std::cout<<"file cannot be opened"<<std::endl; } inFile.close(); //lets check out the elements of the 2D vector so the we can confirm if it contains all the right elements(rows and columns) for(std::vector<double> &newvec: vec) { for(const double &elem: newvec) { std::cout<<elem<<" "; } std::cout<<std::endl; } return 0; } The output of the above program can be seen here.
