Read from file
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Thread: Read from file

  1. #1
    Join Date
    Apr 2013

    Read from file

    I want my program to read variables from file "input.txt":
    Please choose the crystal lattice. Type 1 for BCC, 2 for FCC, and 3 for HCP: 2
    Please enter lattice parameter (a): 1
    For HCP please enter second lattice parameter (c): 1
    Please enter the number of translated cells along X axis: 2
    Please enter the number of translated cells along Y axis: 2
    Please enter the number of translated cells along Z axis: 2
    Moreover, I want program to read only the numbers after colon. How can I do it?
    #include <iostream>#include <fstream> // to read from file
    using namespace std;
    int main ()
        int latticeType;
        int dimX, dimY, dimZ; // number of translated cells along each axis
        float d; // lattice parameter
        float h; // second lattice parameter for HCP
        ifstream inputFile; // object of class 'ifstream' to read from it
    "input.txt", ios::in); // open 'input.txt' only for reading
        if (!inputFile){
            cerr << "Unable to open input file";
            return -1;
        while (!inputFile.eof()){
            inputFile >> latticeType;
            cout << latticeType << endl;
        if ( (latticeType!=1) && (latticeType!=2) && (latticeType!=3) ){
            cerr << "Invalid value of lattice type" << endl;
            return -1;
            inputFile >> d;
            cout << d << endl;
            if (latticeType==3){
                inputFile >> h;
                cout << d << endl;
            inputFile >> dimX;
            cout << dimX;
            inputFile >> dimY;
            cout << dimY;
            inputFile >> dimZ;
            cout << dimZ;
        return 0;
    Last edited by ted_kingdom; May 12th, 2013 at 02:26 PM.

  2. #2
    2kaud's Avatar
    2kaud is online now Super Moderator Power Poster
    Join Date
    Dec 2012

    Re: Read from file

    Possibly the easiest way is to read a complete line at a time from the file then parse for the string to the right of the :

    string fline;
    size_t coln;
    int  val;
    	getline(inputFile, fline);
    	if ((coln = fline.find_last_of(":")) != string::npos) {
    		val = atoi((fline.substr(coln + 1, fline.length() - coln)).c_str());
                    cout << "val: " << val << endl;
    This sample will read a line from the file and display the value of the integer to the right of the last : in the line. As you need to read multiple lines, this could be made into a function.

    Hope this helps.
    All advice is offered in good faith only. You are ultimately responsible for effects of your programs and the integrity of the machines they run on.
    C, C++ Compiler: Microsoft VS2015

  3. #3
    Join Date
    Apr 2013

    Re: Read from file

    Thank you for your answer, but I went a little bit different way:
    #include <iostream>
    #include <fstream>
    using namespace std;
    int main()
        float inputData [6];
        int i=0;
        ifstream file("input.txt");
        char c;
        while (file.good()) {
            c = file.get();
            if (c == ':') {
                file >> inputData[i];
        latticeType = inputData[0];
        d = inputData[1];
        h = inputData[2];
        dimX = inputData[3];
        dimY = inputData[4];
        dimZ = inputData[5];
        return 0;
    Last edited by ted_kingdom; May 12th, 2013 at 03:37 PM.

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