help with dynamic arrays

Hi everyone,
I'm trying to create a dynamic array to display information in a structure....now keep in mind I'm fairly new to C++. Here's what I have:
struct CandyBar
{
char brand[20];
float weight;
int calories;
};

int main()
{
CandyBar* p_candy = new CandyBar[3];
p_candy[0] = {"Chunky", 3.5, 200};
p_candy[1] = {"Monkey", 3.4, 100};
p_candy[2] = {"Choco", 3.3., 50};
------------------------------------------------------------
Now, this doesn't work obviously, and I'm curious how i can create a dynamic array to access structure members. I'm under the impression that to create a dynamic array, you need the type, and the pointer set to a memory location of the type...so whatever I'm doing wrong I just don't understand.. Any help would be great. thanks -Chap PS tell me if you need any more info

well, you could do:

Code:

strcpy(p_candy[0].brand, "Chunky");
p_candy[0].weight = 3.5;
p_candy[0].calories = 200;

...or just use an std::string for the name like:

Code:

struct CandyBar
{
string brand;
float weight;
int calories;
};

p_candy[0].brand = "Chunky";
p_candy[0].weight = 3.5;
p_candy[0].calories = 200;

or give the structure a member function to set the values:

Code:

struct CandyBar
{
 CandyBar(const char * _brand = "", float _weight = 0, int calories = 0) 
 {
  make(_brand, _weight, _calories);
 }
 CandyBar & make(const char * _brand, float _weight, int calories)
 {
  brand = _brand;
  weight = _weight;
  calories = _calories;
  return *this;
 }
 CandyBar & operator = (const CandyBar & cb)
 {
  return make(cb.brand, cb.weight, cb.calories);
 }
string brand;
float weight;
int calories;
};


int main()
{
 CandyBar candy("Chunky", 6.0, 1200);
 CandyBar* p_candy = new CandyBar[3];
 p_candy[0] = candy;
 p_candy[1].make("Monkey", 3.4, 100);
 p_candy[2].make("Choco", 3.3., 50);
 delete [] p_candy;
 return 0;
}

One word: Constructors

Code:

#include <cstring>
#include <iostream>

struct CandyBar
{
  char brand[20];
  float weight;
  int calories;

  CandyBar()
  {
    brand[0] = '\0';
    weight = 0;
    calories = 0;
  }

  CandyBar ( char name[], float wt, int cal )
  {
    std::strcpy ( brand, name );
    weight = wt;
    calories = cal;
  }
};

int main()
{
  CandyBar* p_candy = new CandyBar[3];

  p_candy[0] = CandyBar ( "Chunky", 3.5f, 200 );
  p_candy[1] = CandyBar ( "Monkey", 3.4f, 100 );
  p_candy[2] = CandyBar ( "Choco", 3.3f, 50 );

  for ( int i = 0; i < 3; i++ )
    std::cout<< p_candy[i].brand <<": "<< p_candy[i].weight <<" -- "<< p_candy[i].calories <<std::endl;
}

Of course, that just automates the task of assigning values. You could do away with them (despite their convenience) with relative ease:

Code:

#include <cstring>
#include <iostream>

struct CandyBar
{
  char brand[20];
  float weight;
  int calories;
};

int main()
{
  CandyBar* p_candy = new CandyBar[3];

  std::strcpy ( p_candy[0].brand, "Chunky" );
  p_candy[0].weight = 3.5f;
  p_candy[0].calories = 200;

  std::strcpy ( p_candy[1].brand, "Monkey" );
  p_candy[1].weight = 3.4f;
  p_candy[1].calories = 100;

  std::strcpy ( p_candy[2].brand, "Choco" );
  p_candy[2].weight = 3.3f;
  p_candy[2].calories = 50;

  for ( int i = 0; i < 3; i++ )
    std::cout<< p_candy[i].brand <<": "<< p_candy[i].weight <<" -- "<< p_candy[i].calories <<std::endl;
}

>char brand[20];
The array for a brand name is a little disconcerting. C-style strings are error prone and difficult to work with. You would be much happier using C++ strings:

Code:

#include <iostream>
#include <string>

struct CandyBar
{
  std::string brand;
  float weight;
  int calories;
};

int main()
{
  CandyBar* p_candy = new CandyBar[3];

  p_candy[0].brand = "Chunky";
  p_candy[0].weight = 3.5f;
  p_candy[0].calories = 200;

  p_candy[1].brand ="Monkey";
  p_candy[1].weight = 3.4f;
  p_candy[1].calories = 100;

  p_candy[2].brand = "Choco";
  p_candy[2].weight = 3.3f;
  p_candy[2].calories = 50;

  for ( int i = 0; i < 3; i++ )
    std::cout<< p_candy[i].brand <<": "<< p_candy[i].weight <<" -- "<< p_candy[i].calories <<std::endl;
}

>float weight;
This is fine if you understand that C++ defaults to double just about everywhere, this includes literal values, so you'll need to postfix them with f as in this code:

Code:

p_candy[0].weight = 3.5f; // The f in 3.5f forces 3.5 to be float

>or give the structure a member function to set the values: <snip intermediate level code>

now keep in mind I'm fairly new to C++.

thanks for all the great info! It helped a lot and I really appreciate it. One quick question, what exactly does "strcpy" do in the line: strcpy(p_candy[0].brand, "Chunky");

Once again thanks for all the help! -Chap

>what exactly does "strcpy" do in the line: strcpy(p_candy[0].brand, "Chunky");
The biggest problem with C-style strings is that you cannot assign them. So the following will not work:

Code:

int main()
{
  char buffer[10];

  buffer = "!testing!";
}

It begins to get scary when you use dynamic arrays rather than static arrays because assignment will compile but give you odd results:

Code:

int main()
{
  char *buffer = new char[10];

  buffer = "!testing!";
}

The above code compiles and runs, but is a memory leak and a bug waiting to happen. The assignment does not copy the string into the dynamic array, and that confuses a lot of people. So a function was added to the standard library that allows us to pass two strings: the string to assign to and the string to assign. You can read strcpy as the assignment operator for C-style strings.

Code:

#include <cstring>

int main()
{
  char buffer[10];

  strcpy ( buffer, "!testing!" );
}

Now the code works like you want. A copy of the string "!testing!" is placed in buffer.

As explained earlier, you can avoid all of this by using std::string rather than arrays wherever possible.

awesome! Thank you so much! -Chap