File Input from structue???

This is a discussion on File Input from structue??? within the C++ Programming forums, part of the General Programming Boards category; I have a quick question about inputting data. More specifically data that is of the fallowing struct Code: enum sex ...

  1. #1
    Registered Abuser Loic's Avatar
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    File Input from structue???

    I have a quick question about inputting data. More specifically data that is of the fallowing struct
    Code:
    enum sex {male, female};
    
    struct data
    {
    	char name[30];
    	sex MF;
    	int age;
    	double weight;
    };
    Some one has asked me for some help with this but I cant seem to work out how to input the data from this file. if anyone wants to view the file. I have been told that its formatted as per the above structure.

    All of the input and output I have always uses was using the fallowing method.
    Code:
    FILE *input;  // declare a pointer of type FILE
    char buffer[400];  // a buufer array
    input=fopen("C:\\Sales.txt","r");  // opens file
    read(buffer,1,200,input);  // read 200 characters to buffer
    Is there someway to use the above method? Or is there an alternative?

  2. #2
    and the hat of wrongness Salem's Avatar
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    Perhaps
    struct data student;
    fread( &student, sizeof student, 1, input );


    Though you will need to open the file with "rb" mode.

    You also need to know that reading a binary file in unportable in a large number of ways.
    - structure padding and alignment varies between compilers.
    - endian-ess and data type sizes vary from machine to machine.
    To counteract all these issues, you would have to read the file one byte at a time.
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  3. #3
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    This will work. I don't know how to do it using c++ streams though. You have to open the file in binary mode and read a chunk of data the size of your structure. If you pass fread() a pointer to your structure it will populate your structure (assuming the file has the correct format).

    Code:
    int main(int argc, char *argv[])
    {
        FILE * sFile;
        struct data student[10];
        
        if (NULL == (sFile = fopen("student.data", "rb")))
        {
            cout << "Could not open file" << endl;
            system("PAUSE");
            return -1;
        }
        cout << "name\tsex\tage\tweight:" << endl;
        for (int i=0; i<10; i++)
        {
            fread(&student[i], sizeof(data), 1, sFile);
            cout << student[i].name << "\t";
            student[i].MF?(cout << "female\t"):(cout << "male\t");     
            cout << student[i].age << "\t" << student[i].weight << endl;
        }
        system("PAUSE");
        return EXIT_SUCCESS;
    }
    kyle

  4. #4
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    A structure can be read by writing your own operator>> and operator<< functions that takes that structure as an argument (and a istream/ostream as well, of course).

    You can use >> and << inside the structure reading function, if that's what you want to do.

    Note that this will read/write data in text form (assuming you use the >>/<< operators), not in binary form like the fread/fwrite functions do - there are good and bad things about that.


    If you want to use binary form, use the fstream::read and fstream::write
    http://www.cplusplus.com/reference/i...ream/read.html
    http://www.cplusplus.com/reference/i...eam/write.html

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  5. #5
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    Thanks for the info. So heres the revised example which contains both the c method and the c++ file stream method:
    Code:
    #define USE_STREAMS
    
    enum sex {male, female};
    struct data
    {
    	char name[30];
    	sex MF;
    	int age;
    	double weight;
    };
    
    using namespace std;
    int main(int argc, char *argv[])
    {
        int length;
        struct data student[10];
        
        #ifdef USE_STREAMS
        ifstream isFile;
        isFile.open ("student.data", ios::binary );
    
        isFile.seekg (0, ios::beg);
        // read data 
        for (int i=0; i<10; i++)
        {            
            isFile.read ((char *)(&student[i]),sizeof(data));
        }
        isFile.close();
        cout << "name\tsex\tage\tweight:" << endl;
        for (int i=0; i<10; i++)
        {
            cout << student[i].name << "\t";
            student[i].MF?(cout << "female\t"):(cout << "male\t");     
            cout << student[i].age << "\t" << student[i].weight << endl;
        }
        #else
        FILE * sFile;
         
        if (NULL == (sFile = fopen("student.data", "rb")))
        {
            cout << "Could not open file" << endl;
            system("PAUSE");
            return -1;
        }
        cout << "name\tsex\tage\tweight:" << endl;
        for (int i=0; i<10; i++)
        {
            fread(&student[i], sizeof(data), 1, sFile);
            cout << student[i].name << "\t";
            student[i].MF?(cout << "female\t"):(cout << "male\t");     
            cout << student[i].age << "\t" << student[i].weight << endl;
        }
        #endif //USE_STREAMS
        system("PAUSE");
        return EXIT_SUCCESS;
    }
    kyle

  6. #6
    Frequently Quite Prolix dwks's Avatar
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    Code:
    student[i].MF?(cout << "female\t"):(cout << "male\t");
    Seems to me that this would be simpler:
    Code:
    cout << (student[i].MF ? "female : "male") << '\t';
    Also note that you'd need to include <fstream> for the streams version and <cstdio> for the C version.
    dwk

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