Referring to data both by name and by number in C

Question

I have twenty or so integers which I want to be able to refer to by name when they're being set, but I would like to also be able refer to them by number like they were in an array, so I can print them out one by one using a for loop. Any ideas how to code this in C? Here's what I'm talking about in pseudo code:

/* a data structure to keep a count of each make of car I own */
my_cars;

/* set the counts */
my_cars.saabs = 2;
my_cars.hondas = 3;
my_cars.porsches = 0;

/* print the counts */
for(all i in my_cars) {
    print my_cars[i];
}

Is this asking too much of a low level language like C?

Answer 1

You need two data structures. An array to hold the numbers, and a map from the name to the index in the array. In C++ you'd use one of the map classes in the standard library. I don't know what's available in C but I'm sure there are map implementations available.

Answer 2

struct car {
    const char *name;
    int count;
} my_cars[] = {{"Saab", 2}, {"Honda", 3}, {"Porsche", 0}};
int i;
for (i = 0; i < sizeof(my_cars) / sizeof(my_cars[0]); i++)
    printf("%s: %d\n", my_cars[i].name, my_cars[i].count);

Answer 3

To do that you should use an array instead of standalone data fields

#include <stdio.h>

typedef enum CarType {
  CART_SAAB,
  CART_HONDA,
  CART_PORSHE,

  CART_COUNT_
} CarType;

typedef struct MyCars {
  unsigned ncars[CART_COUNT_];
} MyCars;

int main(void)
{
  MyCars my_cars = { 0 } ;
  unsigned i;

  my_cars.ncars[CART_SAAB] = 2;
  my_cars.ncars[CART_HONDA] = 3;

  for (i = 0; i < CART_COUNT_; ++i)
    printf("%u\n", my_cars.ncars[i]);

  return 0;
}

Answer 4

enum Makes { SAAB, HONDA, PORSCHE, INVALID };
int my_cars[INVALID];
my_cars[SAAB] = 2;
my_cars[HONDAS] = 3;
my_cars[PORSCHE] = 0;

Answer 5

C can do anything any other language can do. This does look like homework and I bet you are expected to make something with a key. Remember, your instructor wants you to use the data structures he or she is trying to teach you. He doesn't really want the problem solved in any random way, he wants it solved applying the topics you have been discussing.

So think about a data structure containing both strings and counts, one that can be searched, and provide functions to do that. What you are likely to get here are nice, professional, simple solutions to the problem. And that's not really what your instructor wants...

Answer 6

There are maybe a couple of options.

It is possible to have the same space in memory defined (and used) in two different ways. In other words, you could have a struct with the named members and reference it either as the struct or as an array depending on how you intended to address it.

Alternatively, you could do an enumerated typedef that names the locations in the array, e.g.

typedef enum {
  SABS = 0,
  HONDAS,
  PORSCHES
} cars;

This would then allow you to refer to offsets in the array by name, e.g.

mycars[SABS] = 5;

Answer 7

Umm...based on what you've pseudo coded up there you could probably use a union. The answers others are giving seem oriented around allowing a mapping between names and numbers. If thats what you're looking for (as in, being able to print the names) then their answers will be better. However it sounds like to me you're simply looking for clarity in the code to allow you to reference things by name or number, in this case a union would be ideal I think. This is exactly the type of thing a low level language like C lets you do.

union my_cars {
    struct names {
        int saab;
        int ford;
        ...
    }

    int[NUM_MAKES] nums;
}

You will have to be careful to ensure NUM_MAKES is in sync with the number of makes you define. Then you can do something like:

my_cars.names.saab = 20;
my_cars.nums[0] = 30;

And modify the same element.

FYI, my C is a little rusty so there may be syntax errors there, feel free to correct.

EDIT:

Ahh, I read some of the other answers using ENUMs or DEFINEs and those might actually be simpler/easier than this one...but unions are still worth knowing.

Answer 8

The low-level C way to do this would be to wrap the cars structure into a union:

// define a structure for the cars.
typedef struct 
{
    int saabs;
    int hondas;
    int porsches;
} cars;

// wrap it into a union:
typedef union 
{
  cars byname;
  int  byid[3]; // Note: Number and type must match with the cars structure. 
} cars2;

int main (int argc, char **arg)
{
  cars2 my_cars;
  int i;

  // fill out by name:
  my_cars.byname.saabs = 1;
  my_cars.byname.hondas = 5;
  my_cars.byname.porsches = 3;

  // print by index:
  for (i=0; i<3; i++)
    printf ("%d\n", my_cars.byid[i]);
}