getting c++ array length

[Cosizzle]

Hey guys,

 

Sorry for the simplistic question, I come from a PHP/Java background.

 

What im trying to figure out is how I can find out the array size or length of an array being passed into a class function.

 

so:

int cmds[] = {5,5,4,5,9,2}
...
turtleOne.processTurtleMoves(cmds);

=====[in the class now]=====

void TurtleGraphics:: processTurtleMoves(const int commands[])
{
// how would I get the size here (6)
// my understanding is that I should be able to go:
cout << sizeof commands / sizeof(int);
// however I get 1 as a result. Is this because of the constant?
}

 

Thank you kindly for your help!

-Cody

[Daniel0]

You can't.

 

Arrays are just short hand notation for pointers. When you pass an array, you are passing the memory address of the first element.

 

Say you have an array of 10 chars for instance. One char takes up exactly 1 byte, so an array of 10 chars take up 10 bytes. These are all stored sequentially in memory, so when you initialize a char array of size 10, you are allocating 10 byte somewhere in memory.

 

This means the following things are equivalent for your char array:

arr[0]   arr*     or: (arr + 0)*
arr[1]   (arr+1)*
arr[2]   (arr+2)*
arr[n]   (arr+n)*
etc.

 

So even though the highest index in this array is only 9, doing arr[1000] is valid because (arr+1000)* is valid even though it's out of bounds and you might really be reading anything.

 

This is also why array indices start from 0 in case you ever wondered.

 

The solution is to pass the size of your array as an extra argument to the function.

 

 

So, something like char *argv[] and char **argv (which is often seen as argument in the main function) are entirely equivalent. This is also the reason why you have the argc argument: you don't know how long argv is otherwise!

[Andy-H]

If the array is null terminated you can use this:

 

#include <iostream>

using namespace std;

int arrayLen(char *arr);

int main()
{
   char characterArray[6] = { 'a', 'b', 'c', 'd', 'e', '\0' };

   cout << endl << "Array Length: " << arrayLen(&characterArray) << endl;

   return 0;
}

int arrayLen(char *arr)
{

   register int total = 0;

   for (; *arr[total]; total++);

   return i;
}

 

Don't take my word on that tho, I have only really been learning C++ for about a week. Haven't even finished the book I'm using lol

[greatstar00]

Andy-H, in his example, he is passing a int to the function

so, your point definitely dont work

[Andy-H]

You could use Vectors ?