Use variable to specify structure field

Hi, as a brief outline, I'm using a function to sort some data, held in an array of structures. Depending on what is passed to the function, it will need to sort based on a different field.

i.e. if "field1" is passed to the function, the function will sort based on structure.field1. If "field2" is passed, it will be sorted based on structure.field2.

The problem I'm having is using a variable to specify the structure field. How can I set it so that the code effectively reads

structure.variablevalue

depending on what is passed to the function as variablevalue.

Thanks in advance :)

Are all the fields the same type?

What are you trying to do?

im sorting an array of structures. there are 5 fields - one long and 4 ints. The structures will be ordered in the array by one of the 4 int fields in them, and the user selects which one to sort by. Basically, I want to be able to pass this

fieldvariable = "field1";


....

then refer later in code to the field denoted by the variable

e.g.

if fieldvariable is as above, I could have

Code:

---

structure.fieldvariable

---

would be seen by the program as

Code:

---

structure.field1

---

you will need to pass the address of the structure and then the offset of the member to be sorted.

Could you imagine for a second that I am the complete novice/idiot that I am, and give me the big print version with pictures (and preferably sample code)?Lol. Cheers for the rapid response!

edit: that was a failure, just use gcc (dunno if you can with visual studio/cl) and offsetof
edit: If you're using cl and it doesn't have offsetof or vs doesn't

Code:

---

#define offsetof(type, field)    ((int) (& (((type *) 0)->field)))

---

edit: edit: edit:
sorry, I dropped the code

Code:

---

int main(...)
{
    ...
    struct structType *aStruct;
    AccessArbitraryField(aStruct, offsetof(structType, fieldName);
    ...
}

AccessArbitraryField(void *aStruct, int fOff)
{
    access it as *(aStruct + fOff)
}

---

I think that should work

Maybe change your structure values to an array of ints and pass in the index to sort on.

So if this were say excel, you'd be looking for some kind of "sort by column" feature?

@Valis, I'm trying to get your code to work, but it complains at me for using the (aStruct + fOff) code, apparently "void" is not suitable for arithmetic.

@WaltP, I don't think I can do that, please re-read my first post, if I'm wrong, could you explain what you mean?

@Salem, yes exactly. I'm passing the array of structures to a bubble sort routine.

Code:

---

typedef enum {
  FT_INT,
  FT_LONG,
  FT_DOUBLE,
  FT_STRING,
  FT_MAX
} cellType;

typedef struct {
  cellType  type;
  union {
    int    iField;
    long    lField;
    double  dField;
    char  *sField;
  } data;
} cell;

---

The whole sheet would be

Code:

---

cell sheet[10][5];

---

Setting an item would be

Code:

---

sheet[0][0].type = FT_INT;
sheet[0][0].data.iField = 123;

---

When it comes to sorting, you look at the type field and choose the appropriate comparison.

Quote:

---

Originally Posted by Rick87

@WaltP, I don't think I can do that, please re-read my first post, if I'm wrong, could you explain what you mean?

---

You said

Quote:

---

i.e. if "field1" is passed to the function, the function will sort based on structure.field1. If "field2" is passed, it will be sorted based on structure.field2.

---

My idea:
i.e. if "1" is passed to the function, the function will sort based on structure.field[1]. If "2" is passed, it will be sorted based on structure.field[2].

Turn .fieldn into an array .field[n]