Comparing two instances of a struct

Question

I have my own data type since C89 doesn't allow bool type. I can't use C99 sadly.

I have tested the below code and my program is not even recognizing that the names are different. The names are working just fine. I have those loaded in an instance of the struct.

struct ROOM roomList [MAX_ROOMS_COUNT];

I used memset and strcpy to copy in a list of rooms from a room array. Testing this gives me the results I want like so...

printf("%s", roomList[1].name) printf("%s", roomList[2].name) ... printf("%s", roomList[7].name)

The rooms array has 7 values.

#define MAX_ROOMS_COUNT 7 typedef enum { true = 1, false = 0 } boolean; struct ROOM { char name[8]; char* type; int numConnections; char* connections [MAX_CONNECTIONS_COUNT]; }; boolean isSameRoom(struct ROOM x, struct ROOM y) { //printf("Is Same Room, 1:%s, 2:%s\n", x.name, y.name); if (x.name == y.name) { //printf("ROOM IS SAME!"); return true; } else { return false; } } struct ROOM getRandomRoom() { int random = rand() % MAX_ROOMS_COUNT; return roomList[random]; } void addRandomConnection() { struct ROOM A; struct ROOM B; A = getRandomRoom(); do { B = getRandomRoom(); }while(isSameRoom(A, B) == true); //printf("Add Random Connection, 1:%s, 2:%s\n", A.name, B.name); //for testing purposes }

Using the print statements I can see that the isSameRoom function isn't working. It's generating random rooms just fine, but there are instances when I run the program that I'm getting the same room for A and B. Any idea why? Thanks! :)

You cannot compare strings with ==, you have to use strcmp — Pablo
– Pablo, Commented Oct 27, 2019 at 19:04
Aside: you say you output the room names with roomList[1].name thru roomList[7].name for #define MAX_ROOMS_COUNT 7 rooms. In C arrays are indexed from 0 so that should be roomList[0].name thru roomList[6].name. — Weather Vane
– Weather Vane, Commented Oct 27, 2019 at 20:00

cnst · Accepted Answer · 2019-10-27 19:20:48Z

1

First of all, you're using pass-by-value for these structures, which is pretty inefficient. You should do pass by reference.

Second, as others mentioned, you cannot compare fixed-size strings with == in C; you have to use strncmp or some other structure.

Perhaps a better idea would be to do the whole comparison of the whole structure with memcmp?

return memcmp(&A, &B, sizeof(struct ROOM)) == 0;

answered Oct 27, 2019 at 19:20

cnst

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2 Comments

Weather Vane Over a year ago

If there is padding between struct members, is memcmp() uncertain? For an initialised struct I guess no because uninitialised parts are zeroed, but for a struct where members' values are individually assigned? For an unpadded struct, in the case where the only difference might be a string with unused space in the array, could they compare different when they are actually the same? Moreover, if any member is a pointer to a data value, the data might be the same, but the pointer different. I propose that the only safe way is to compare each member, according to its type.

ikegami Over a year ago

@WeatherVane, Obviously, this would only work for a shallow comparions (re: pointer), but good point about the string. And I see that you already mentioned that. woops! Comment deleted.

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