typedef problem

Hi all,

I've got a question about typdef.
I want to save highscores for my schoolproject and I want to do it with a typedef.

Code:

typedef struct
     {
          char name[20];
          int hs;
     } HS;

HS highscore[4];

My program writes the highscores to a binairy file, but I don't know how to continue with the typedef...

I tried the following already:

Code:

if((f = fopen("hs.bin","rb")) == NULL)
     {
          strcpy(highscore[0].name, plyr_nm);
          highscore[0].hs = score;
          f = fopen("hs.bin", "wb");
          length = strlen(highscore);
          fwrite(&length, 1,sizeof(int),f);
          fwrite(&highscore, length+1, sizeof(HS), f);
          fclose(f);
     }
     else
     {
           fopen("hs.bin", "rb");
           fread(&length, sizeof(int),1,f);
           fread(&highscore, sizeof(char),length+1,f);
           fclose(f);
           strcpy(highscore[4].name, plyr_nm);
           highscore[4].hs = score;

           //than sort with qsort and write it back into the file
     }

I also don't understand qsort, I don't know how to write a comparison that works for typedef.

Can someone help me out please?

Greetings!

You should remove fopen("hs.bin", "rb"); from else
Why don't you write/read the whole structure?

Your Array has indexes 0,1,2,3 so highscore[4] does not exists
if you want to write the whole array use
fwrite(highscore,1,sizeof(highscore),f);
if you want to write the first element
fwrite(&highscore[0],1,sizeof(highscore[0]),f);

etc

I also don't understand qsort, I don't know how to write a comparison that works for typedef.

Use this code below as compare fucntion which u need to send as a parameter to your qsort function.

NOTE: This function coomapre on your struct hs member value, but u can still change the code so that it compares on the name rather than to be in hs. I leave it to you for that.

Code:

int compare (const void *a, const void *b)
{
    return ((*(HS *)a).hs - (*(HS *)b).hs);
}

And this is how u will call

Code:

qsort(highscore,5,sizeof(HS), compare);

Hope that help u

ssharish2005

Originally Posted by anarchypower

I also don't understand qsort, I don't know how to write a comparison that works for typedef.

Code:

int compare_highscores( const void* a, const void* b ) {
   HS* score1 = (HS*) a;
   HS* score2 = (HS*) b;
   if( score1->hs  < score2->hs ) return -1;
   else if( score1->hs == score2->hs ) return 0;
   else return 1;
 }

Kurt

Code:

int compare (const void *a, const void *b)
{
    return ((*(HS *)a).hs - (*(HS *)b).hs);
}

Although this is a very, very common compare function to see, it also has a very well known flaw: it has UB for integer overflow.

And there is no need to cast away constness.

Originally Posted by Dave_Sinkula

Code:

int compare (const void *a, const void *b)
{
    return ((*(HS *)a).hs - (*(HS *)b).hs);
}

Although this is a very, very common compare function to see, it also has a very well known flaw: it has UB for integer overflow.

And there is no need to cast away constness.

Dave true. I never through abut that. Well Zuks solution looks fine for the comparison.

But what i don't understand is why cast is not required. The reason why i casted was because the value which we get from the compare in of type void * which need casting to get member data types.

ssharish2005

Originally Posted by ssharish2005

The reason why i casted was because the value which we get from the compare in of type void * which need casting to get member data types.

You don't need casting because it's a void*.

I don't understand the following line of code

Originally Posted by Dave_Sinkula

You don't need casting because it's a void*.

I don't understand the following line of code

one thing which i dont understand is, when i was tried as suggested without casting a the const void *. I get these error. Do you why i cant do as u said.

Code:

testcom.c: In function âcompareâ:
testcom.c:12: warning: dereferencing âvoid *â pointer
testcom.c:12: error: request for member âhsâ in something not a structure or union
testcom.c:12: warning: dereferencing âvoid *â pointer
testcom.c:12: error: request for member âhsâ in something not a structure or union

i am using gcc 4.1.1 version

ssharish2005

> when i was tried as suggested without casting a the const void *. I get these error.
You still have to assign the void pointers to pointers of the correct type.
But you can do this without the need for explicit casts.