Thread: Dynamic array problem

Can anybody tell me why this code gives me errors?

Code:

	ifstream infile("text.txt");

	infile >> numrecords_str;

	numrecords = atoi(numrecords_str);

	Record records[numrecords];

Errors:

Code:

error C2057: expected constant expression
error C2466: cannot allocate an array of constant size 0
error C2133: 'records' : unknown size

Thanks

Hi,

Dynamic arrays have to be allocated with the 'new' operator or the 'malloc' function.

Code:

ifstream infile("text.txt");

infile >> numrecords_str;

numrecords = atoi(numrecords_str);

Record records = new Record[numrecords];

// After your done using the memory you allocated release it
delete [] records;

Or you can consider using "std::vector" and don't worry about these details!

Code:

error C2440: 'initializing' : cannot convert from 'class Record *' to 'class Record'

Change augur code to

Code:

ifstream infile("text.txt");

infile >> numrecords_str;

numrecords = atoi(numrecords_str);

Record *records = new Record[numrecords];

// After your done using the memory you allocated release it
delete [] records;

But actually u dont need to allocate dynamic memory

Code:

#include <iostream>
#include <fstream>
#include <conio.h>
#include <string>

using namespace std;

int main(void)
{
string numrecords_str;

ifstream infile("c:\\text.txt");

infile >> numrecords_str;

int numrecords = atoi(numrecords_str.c_str());

int array[numrecords];

for (int j = 0; j < numrecords; j++)
    array[j] = j*2+3;
    
for (int j = 0; j < numrecords; j++)
    cout << array[j] << endl;

getch();
return 0;
}

But still I would recommend dynamic memory allocation or i.e std::vector class that augur suggested, insteed of my code. If the array is to large you might encounter a stack overrun(overflow).

Originally posted by nag

It would return error again,can you declare an array of numrecords size because it is variable!!

Of course u can!

Perfect legal

Code:

#include <iostream>

int main(void)
{
const int avariable = 5;
int array[avariable];
return 0;
}

I´ve compiled my code(original) and tested the program and it runs well. Are u experiating some problem with it???

Maybe you are confused by the similair variable names numrecords_str and numrecords.

Code:

int numrecords = atoi(numrecords_str.c_str());

int array[numrecords];

Yeeh I wasn´t sure if this would work (my first reaction was that it wouln´t, beacuse it isn´t a constant expression) but when a compiled it with Dev c++ (ming) and it worked, I thoght I was wrong but apperntly I wasn´t. Hmmm, why does it work under Dev c++???

Record *records = new Record[numrecords];

if the number wasn't constant would you use pointer arithmetic on *records to access differnt values in the dynamic array? or is there an easier way?