2D arrays as arguments - c++

Question

I have read a book about C, and I did try, so please be soft with me.
I am trying to understand how memory really works.
I would like to have some words array like this ( in C) :
char builts[20][10]={"light","temp"}; //should it looks like this ?
Then, I would like to pass that array to a function (in another class)
//some class
char *builtinFunctions;
void Intepreter::setBuiltIns( char *builtins)
{
// here - should I save a copy to builtinFunctions ?? how ?
}
The other class need to have access to that array of words all the time.
Why this is gives error : intepreter.setBuiltIns(&builts); ?
How would one declare builtinFunctions ? as a pointer or array ? should it be copied to?
How exactly the whole thing should look like ??

Answer 1

There are multiple ways to pass a 2D array to a function:
The parameter is a 2D array
int array[10][10];
void passFunc(int a[][10])
{
// ...
}
passFunc(array);
The parameter is an array containing pointers
int *array[10];
for(int i = 0; i < 10; i++)
array[i] = (int*)malloc(40); //array[i] = new int[10];
void passFunc(int *a[10]) //Array containing pointers
{
// ...
}
passFunc(array);
The parameter is a pointer to a pointer
int **array;
array = (int*)malloc(40);//array = new int *[10];
for(int i = 0; i <10; i++)
array[i] = (int*)malloc(40); //array[i] = new int[10];
void passFunc(int **a)
{
// ...
}
passFunc(array);

Answer 2

When passing a simple array to a function, you pass it as a pointer (array name is decayed as a pointer to the first element of the array), like this :
int foo[3] = { 3, 2, 1 };
bar(foo);
So your function takes a pointer to an int as parameter :
void bar(int *data) { }
Here you have an array which contains NULL terminated string of length 10 which are arrays too. So, builts is a pointer to his first element so a pointer to an array of 10 char :
char builts[20][10] = {"light", "temp"};
char (*foo)[10] = builts; // This is valid, foo is a pointer to an array of 10 char
So, your function must take an argument of type char (*)[10], because you pass a pointer to an array of 10 char :
void bar(char (*data)[10]) { }

Answer 3

You can pass the 2 day array by just using its name while calling a function, like:
test_2d(builts);
You can either pass the total number of words as a separate argument, or alternatively, you can make a special word to indicate end of your array. For instance, I have used a special word "\0" to indicate that this marks the end of array. Overall the code looks like.
#include<stdio.h>
#include<string.h>
int main(void)
{
char builts[20][10]={"light","temp", "\0"};
test_2d(builts); /* without passing total number of args; depends on special word at the end */
test_2d_num(builts, 2); /* also pass total num of elements as argument */
return 0;
}
/* This needs a special word at the end to indicate the end */
void test_2d(char builts[][10])
{
int i;
char tmp[10];
/* just print the words from word array */
for (i=0; *builts[i] != '\0'; i++ )
printf("%s\n", builts[i]);
/* also try copy words to a tmp word and print */
for (i=0; *builts[i] != '\0'; i++ ) {
strcpy(tmp, builts[i]);
printf("%s\n", tmp);
}
/* Do something */
}
/* Also get total number of elements as a parameter */
void test_2d_num(char builts[][10], int tot_elem)
{
int i;
/* Process each word from the array */
for (i = 0; i < tot_elem; i++) {
printf("%s\n", builts[i]);
/* Do something */
}
}
Note that this function can only process arrays like, builts[][10], and not builts[][11], or builts[][9].
If you want want a generic function, then you need to store the addresses of individual words in an char *arr[] and pass this array to the function. Like
int main()
{
/* store the addresses of individual words in an `char *arr[] */
char *arr[] = {"hello", "this", "that", NULL};
test_2d_gen(arr);
return 0;
}
void test_2d_gen(char *arr[])
{
int i;
/* process each word */
for (i = 0; arr[i] != NULL; i++) {
printf("%s\n", arr[i]);
/* Do something */
}
}