Reading data from file into array of structs C++

Question

I have a sample txt file and want to read the contents of the file into an array of structs. My persons.txt file contains 5 arbitrary nos one on each line.

My program looks like this:

#include <iostream>
#include <fstream>

using namespace std;

struct PersonId
{
    typedef PersonId* ptr;
    PersonId();
    int fId;
}; 

istream& operator >> (istream& is, PersonId &p)
{
    is >> p;
    // return is;
    // return p.read(is);
}

// istream& PersonData::read(std::istream& is) 
// {
//  is >> fId;
//  return is;
// }


int main ()
{
    ifstream indata;
    int i, is;
    indata.open("persons.txt", ios::in); // opens the file

    if(!indata) 
    { // file couldn't be opened
          cout << "Error: file could not be opened" << endl;
          exit(1);
    }

    int n = 5;
    PersonId* p;
    p = (PersonId*) malloc (n * sizeof(PersonId));

    while ( !indata.eof() )
    { // keep reading until end-of-file
        // p[i].read();
        indata >> is;
        i++;
        cout << "The next number is "<< is << endl;
            cout << "PersonId [" << i << "] is " << p[i].fId << endl;
        // indata >> is; // sets EOF flag if no value found
    }
    return 0;
}

My output looks like this:

test6.cpp: In function ‘std::istream& operator>>(std::istream&, PersonId&)’:
test6.cpp:27: warning: control reaches end of non-void function
The next number is 7
PersonId [1] is 0
The next number is 6
PersonId [2] is 0
The next number is 4
PersonId [3] is 0
The next number is 3
PersonId [4] is 0
The next number is 2
PersonId [5] is 0

And your question is? BTW your code is very buggy.

Tadeusz Kopec for Ukraine
– Tadeusz Kopec for Ukraine

2009-08-05 12:39:52 +00:00
Commented Aug 5, 2009 at 12:39 — Tadeusz Kopec for Ukraine
– Tadeusz Kopec for Ukraine, Commented Aug 5, 2009 at 12:39
moving from C to C++ after a longgg time :)

vivekv80
– vivekv80

2009-08-05 13:04:33 +00:00
Commented Aug 5, 2009 at 13:04 — vivekv80
– vivekv80, Commented Aug 5, 2009 at 13:04

Cătălin Pitiș · Accepted Answer · 2009-08-05 12:35:37Z

2

istream& operator >> (istream& is, PersonId &p)
{
    is >> p.fId;
    return is;
}

(Reading the member fId of p, not the entire structure)

And the the while in main, read the structure, not a value:

instead of

indata >> is;

put

indata >> p[i];

answered Aug 5, 2009 at 12:35

Cătălin Pitiș

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2 Comments

ScottTx Over a year ago

Also, initialize i to zero before the while loop and move i++ AFTER your cout.

vivekv80 Over a year ago

struct PersonId { int fId; }; struct PersonData { public: typedef PersonData* Ptr; PersonData(); PersonId fId; istream& read(std::istream&); }; istream& PersonData::read(std::istream& is) { is >> fId; return is; } istream& operator >> (istream& is, PersonData &p) { // is >> p.fId; return p.read(is); } How will indata >>p[i] work in this scenario??

Community · Accepted Answer · 2017-05-23 11:48:38Z

0

See Neil Butterworth's reply to ifstream object.eof() not working for how to correctly read from an istream

edited May 23, 2017 at 11:48

CommunityBot

11 silver badge

answered Aug 5, 2009 at 12:42

tragomaskhalos

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2 Comments

vivekv80 Over a year ago

I avoided using getline() bcos in the future I will have lot of numbers in one line, which will be the items of the same structure.

tragomaskhalos Over a year ago

Getcha, but I was more worried about your use of eof().

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