help with c project using qsort

[VegasSte]

How do you use qsort on a text file of different length records(when you have to sort two things(record type(first character) and customer_code(chars 2-6))) when the data is in structures inside a union?
e.g.:

  struct i_r_record{
                            char rec_type,
                            cust_code[5],
                             part_num[6],
                              i_r_qty[4];
                            };
  struct c_record{
                           char rec_type,
                           cust_code[5],
                           cust_name[20],
                           cust_address[60],
                           cust_bal[9],
                           cust_limit[7];
                         };
  struct d_record{
                            char rec_type,
                               cust_code[5];
                        };
  union records {
                          struct i_r_records i;
                          struct c_records c;
                          struct d_records d;
                         };

[code][/code]tagged by Salem

[VegasSte]

I have tried to sort simple arrays, and even an array of structures, but these were all with fields that were of the same length!! I understand the compare function, but not how to implement it in the case of the file of different length records!!

[Salem]

But you've already read the text file (with its variable length records) into an array like this right?

union records database[100];

The union is as long as the longest member, so in memory at least, they're all the same length (which is good enough for qsort)

In addition, you'll note that the first two members of the union
    char rec_type, cust_code[5],
are common to all members of the union (which makes things a bit simpler)


So all you need do is

int compare ( const void *a, const void *b ) {
    const union records *pa = a;
    const union records *pb = b;
    if ( pa->rec_type < pb->rec_type ) return -1;
    if ( pa->rec_type > pb->rec_type ) return +1;
    // else equal
    return strncmp( pa->cust_code, pb->cust_code,5);
}

[bigtamscot]

As salem says, your union is the length of your biggest structure. The thing is that the fields you require to sort - the record type and the customer code - are common to each structure and no matter what the record, they hold the same position when your file is read in to memory. To acheive this, you will need to allocate memory for the number of records on file.

Fellow CIL student here...mail me, I sent you an e-mail.

to calculate the number of records on file...if your file is binary use fseek() and ftell() - remember to rewind the file pointerafterwards.

to read the file into memory, use 2 memory pointers and malloc. One pointer remains at the start of the allocated memory, the other is used to access the records.

call your qsort function.

once sorted read the records from memory to the sorted file.

[VegasSte]

Thanks fellas, I will try out your recommendations and get back to you!

[VegasSte]

I have just thought......I have to write a function/loop to count the number of records BEFORE i can read them into an array of unions, so how can I then set the size of an array of unions before I know the numbwer of records?

[VegasSte]

I know that it has more than 100 records, so would I be able to resize the array of unions to(say)200?