Using enums to access 2D array indices

Question

I haven't had a chance to try this and my understanding of C is amateur at best. The closest I found to this question was for c++ but dealt with fixed enum values.

Say you have a 2D integer array dynamically allocated like this:

4x4

would it be possible to use enum to assign dynamic names to each column and row and then access those rows and columns by their name?

Like:

        apples   bread   carrot   dinosaur  
apples    0        0       0         0  
bread     0        0       0         0  
carrot    0        0       0         0   
dinosaur  0        0       0         0

Allowing me to do something like this:

matrix[apples][bread] += 1;

EDIT When I say "dynamic" what I mean is when a value is compiled at run time with no fixed size. So on one run the matrix may be 2x2 or 82x82 and the enum values could be apple, bear, or apple, bear, teddy etc.

I don't know what "dynamic" means in this context, but `enum` values are numerical and suitable enough for indexing arrays. — Eugene Sh., May 08 '19 at 13:56
*the enum values could be apple, bear, or apple, bear, teddy etc.* – and where should those values come from at runtime? At compiletime: `enum { apple, bear } foo;` ... that's it. You cannot add any values to that `enum foo` at runtime. And the names don't exist at runtime either. — Swordfish, May 08 '19 at 14:03
Well. That ruins my plans. @Ayxan a flat array? I'm assuming that's a 1D array? No, I need a 2D array as this is meant to be a correlation matrix. — Aevitas, May 08 '19 at 14:05
A 1d array can be a projection of a 2d array: `int foo[16]; size_t x = 3; size_t y = 2; /* let width be 4 */ foo[y * 4 + x];` When you allocate an "2d array" with `malloc()` you have an array of pointers. These point to memory regions that represent the rows. That's not an array. Something like that is called "jagged array" at best. The main characteristic of an array is that all its elements are consecutive in memory. Such "jagged arrays" only hurt performance. — Swordfish, May 08 '19 at 14:07
@Swordfish I think I have to do something like that anyway as the array needs to be variable. — Aevitas, May 08 '19 at 14:11

Aykhan Hagverdili · Accepted Answer · 2019-05-08T14:05:13.300

3

C is a statically typed language and you cannot add variable names to your program at runtime. This includes enums too. A hash table like std::map may be used for your purposes, but C does not provide such a type, so you have to implement it yourself.

edited May 08 '19 at 14:05

answered May 08 '19 at 14:00

Aykhan Hagverdili

28,141
6
41
93

John Bode · Answer 2 · 2019-05-08T14:18:56.000

Yes, you can use enumerations to create symbolic constants for each index:

enum { apples, carrot, bread, dinosaur };

You can also use regular variables:

const int apples   = 0;
const int carrot   = 1;
const int bread    = 2;
const int dinosaur = 3;

You can also use the preprocessor:

#define APPLES   0
#define CARROT   1
#define BREAD    2
#define DINOSAUR 3

What you can't do is create any of these things at runtime. If you decide at runtime to create a 5x5 array, you cannot also create a new symbolic constant (via an enum or variable or macro). That can only be done at compile time.

EDIT

What you can do is create some kind of associative data structure (map or lookup table) that associates a string with an integer value, and then you can do something like...

Some_Map_Type map;
...
addIndex( map, "apples", 0 );
addIndex( map, "carrot", 1 );
addIndex( map, "bread", 2 );
addIndex( map, "dinosaur", 3 );
...
do_something_with( matrix[getIndex(map, "apples")][getIndex(map, "bread")] );

and then at runtime prompt for a new index name and value, like:

printf( "Gimme a new index: " );
scanf( "%s %d", name, &value ); // doing it this way *just* for brevity - don't actually do it like this
addIndex( map, name, value );
...
do_something_with( matrix[getIndex(map, name)][getIndex(map, "bread")] );

That may or may not be worth the effort for you.

Thanks for taking the time to write that. But sadly, that won't work for me. I need to find another way to solve the problem. — Aevitas, May 08 '19 at 14:28

score 1 · Answer 3 · answered May 08 '19 at 14:04

Enum in C are nothing more than a mapping between names and integers. So you could do something like

enum Fruit{apple, orange, banana};
array[apple][banana] = 1;

But like its name points, you must enumerate it, BEFORE you use it. So it's impossible to add one at runtime. Some kind of map structure might work though.

Using enums to access 2D array indices

3 Answers3