Counting of objects in linked list - c

Question

I have a linked list and I need to count only some of records according some criteria. I have got this code.
int count_records() {
int number = 0;
RECORD *re;
char criteria[20];
for (re = first; re != NULL; re = re->next) {
criteria = (re->criteria);
if(criteria=="something")
number++;
}
return number; }
Structure of linked list is this:
typedef struct record {
char criteria[20];
struct record *next;
} RECORD;
Problem is that it gives me this error on line criteria = (re->criteria); Error: Invalid array assignment
Any ideas?
Thanks.

Answer 1

You have arrays of char´s. To copy or compare an array, you´ll need to copy/compare each element separately instead of using = and == on the whole array in one go.
And instead of making a loop, you can use strcpy and strcmp in this case:
strcpy(criteria, re->criteria);
if(!strcmp(criteria, "something"))
By the way, this is more C than C++. If you want a C++-like solution, see std::string (then you can work with = and == too)

Answer 2

Indeed, you can't copy arrays using =. Neither can you compare them using ==. They are quite strange, unfriendly things, which need a certain amount of care to use.
If you want to use arrays, you can use the C library to compare them:
if (std::strcmp(re->criteria, "something") == 0)
Since this is C++, you might prefer to use std::string to represent strings; then it's just
if (re->criteria == "something")
and you can copy it with = if you need to (although you don't need to here).

Answer 3

In C++ you can and should use std::string for handling strings. You can find the reference (here).
Just define criteria as std::string criteria; instead of char criteria[20];.
Don't forget to add #include <string> to the include section.

Answer 4

You need to use memcpy in order to copy arrays, but be careful using memcpy when exposed on a public interface. I think there was a recent security issue involving this...

Answer 5

I think that you may wish to pass in the criterion to your count_records() function. I'd render it like this:
int count_records(RECORD *first, const char *criterion) {
int count = 0;
RECORD *re;
for (re = first; re != NULL; re = re->next) {
/* add to count if the strings match */
if(!strcmp(criterion, re->criteria))
count++;
}
return count;
}
Note that it also doesn't rely on first being a global variable which makes the code more reliable and easier to understand.

Answer 6

If you want to copy a char array into another, you need to use strcpy. For example:
// will copy from array1 to array2
strcpy(array2, array1);
You can't directly do array1 = array2. Because in this case you would manipulate the adresses (char *) of the arrays and not their values.
In addition, in order to compare a char array to another, use strcmp instead:
if(strcmp(criteria, "something") == 0)
P.S.: Since you're using C++, std::string is your friend, which you can do assignment using = and comparison using ==.

Answer 7

Dealing with strings in C is a royal pain when you are starting out with the language. You can checkout an online tutorial to get you started.
In the mean time, you can change your code to:
for (re = first; re != NULL; re = re->next) {
if (strcmp(re->criteria, "something) == 0)
number++;
}
You don't need to copy re->criteria to a local variable.