Define a function

This is a discussion on Define a function within the C++ Programming forums, part of the General Programming Boards category; I do not know where to start with this requirement. I know I have to int isNull() and write it ...

  1. #1
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    Define a function

    I do not know where to start with this requirement. I know I have to int isNull() and write it to a file, but not really know how to const it.

    Just need an example on how to start this, thanks

    Need to define function isNull() in personal class and define function writeNullToFile to overwrite a record to be deleted by a null record. The null record is defined as having a nonnumeric character (a tombstone) in the first position of the SSN member.

    Code:
    #include <iostream>
    #include <fstream>
    #include <cstring>
    
    using namespace std;
    
    #ifndef PERSONAL
    #define PERSONAL
    
    class Personal 
    {
    public:
        Personal();
        Personal(char*,char*,char*,int,long);
        void writeToFile(fstream&) const;
        void readFromFile(fstream&);
        void readKey();
        int size() const 
    	{
            return 9 + nameLen + cityLen + sizeof(year) + sizeof(salary);
        }
        bool operator==(const Personal& pr) const 
    	{
            return strncmp(pr.SSN,SSN,9) == 0;
        }
    protected:
        const int nameLen, cityLen;
        char SSN[10], *name, *city;
        int year;
        long salary;
        ostream& writeLegibly(ostream&);
        friend ostream& operator<<(ostream& out, Personal& pr) 
    	{
            return pr.writeLegibly(out);
        }
        istream& readFromConsole(istream&);
        friend istream& operator>>(istream& in, Personal& pr) 
    	{
            return pr.readFromConsole(in);
        }
    };
    
    #endif
    
    
    
    
    
    
    
    #include "personal.h"
    
    Personal::Personal() : nameLen(10), cityLen(10) 
    {
        name = new char[nameLen+1];
        city = new char[cityLen+1];
    }
    
    Personal::Personal(char ........n, char *n, char *c, int y, long s) :
            nameLen(10), cityLen(10) 
    {
        name = new char[nameLen+1];
        city = new char[cityLen+1];
        strcpy(SSN,ssn);
        strcpy(name,n);
        strcpy(city,c);
        year = y;
        salary = s;
    }
    
    void Personal::writeToFile(fstream& out) const 
    {
        out.write(SSN,9);
        out.write(name,nameLen);
        out.write(city,cityLen);
        out.write(reinterpret_cast<const char*>(&year),sizeof(int));
        out.write(reinterpret_cast<const char*>(&salary),sizeof(int));
    }
    
    void Personal::readFromFile(fstream& in) 
    {
        in.read(SSN,9);
        in.read(name,nameLen);
        in.read(city,cityLen);
        in.read(reinterpret_cast<char*>(&year),sizeof(int));
        in.read(reinterpret_cast<char*>(&salary),sizeof(int));
    }
    
    void Personal::readKey() 
    {
        char s[80];
        cout << "Enter SSN: ";
        cin.getline(s,80);
        strncpy(SSN,s,9);
    }
    
    ostream& Personal::writeLegibly(ostream& out) 
    {
        SSN[9] = name[nameLen] = city[cityLen] = '\0';
        out << "SSN = " << SSN << ", name = " << name
            << ", city = " << city << ", year = " << year
            << ", salary = " << salary;
        return out;
    }
    
    istream& Personal::readFromConsole(istream& in) 
    {
        char s[80];
        cout << "SSN: ";
        in.getline(s,80);
        strncpy(SSN,s,9);
        cout << "Name: ";
        in.getline(s,80);
        strncpy(name,s,nameLen);
        cout << "City: ";
        in.getline(s,80);
        strncpy(city,s,cityLen);
        cout << "Birthyear: ";
        in >> year;
        cout << "Salary: ";
        in >> salary;
        in.getline(s,80); // get '\n'
        return in;
    }

  2. #2
    Registered User rynoon's Avatar
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    What exactly are you having trouble with? Defining the method, manipulating the file?

    May I ask if this is homework?
    Last edited by rynoon; 01-04-2007 at 03:57 AM.

  3. #3
    The larch
    Join Date
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    What you could do, is set the Personal's data to null-data and use the existing method to write to the file.

    Aside from that, I don't see how Personal could be allowed to do what you are required. Why should Mary Smith's personal record be able to delete John Doe's record???

    Also, to overwrite a record, the class would need to know its position in the file. I don't see how this information fits logically into the Personal class (well, you could pass file position as a parameter). Anyway, managing the data-base should not be the task of database data.

    (Edit) Reread the post, it appears that deleting the record is done by an external function. Well, I can think of two simple approaches: either add a bool to the Personal class telling if it is empty or not, or set Personal data to empty data (as described in the requirements) and check this to decide later if the object is empty.

    By the way, there is also a memory leak: you allocate memory in the constructor (which you don't need to do, since string lengths are constant), but there is no destructor to delete this memory. Also, because of dynamic memory allocation this class needs a copy constructor or it should be made non-copyable (which probably isn't an option).
    Last edited by anon; 01-04-2007 at 08:04 AM.

  4. #4
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    I am having a problem defining the method within the class.

  5. #5
    The larch
    Join Date
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    Which method? This one?
    define function writeNullToFile to overwrite a record to be deleted by a null record
    It doesn't make sense for Personal to overwrite any records in a file.

    If you insist so, the signature might be:
    Code:
    void Personal::writeNullToFile(fstream&, int where, int howManyBytes);
    Write any data to this place, but change the first character of SSN to a non-number before writing it to file.

    The isNull method would simply return, if the first character of SSN is non-numeric.

  6. #6
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    The isNull method, how would I start the int.

    int isNull()

    int a

    if (isNull() = a)

    wruteNullTo File

  7. #7
    CSharpener vart's Avatar
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    Quote Originally Posted by CRC05
    The isNull method, how would I start the int.

    int isNull()

    int a

    if (isNull() = a)

    wruteNullTo File
    I don't understand the question, but for comparison you should use ==
    if(isNull() == a)
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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