Understanding/Implementing Enumeration

**Iconate** · 10-09-2008

Hello, I am currently teaching myself C, and am at a point where I am learning about enumeration. Id really like some help understanding its capabilities. Here in this example I was given.

Code:

#include <stdio.h>

enum colours {RED, ORANGE, YELLOW, GREEN, BLUE, INDIGO, VIOLET,
              NUMBER_OF_COLOURS};

typedef enum colours colour_t;

int main(void)
{
   colour_t sky, forest;

   printf("There are %d colours in the enum\n", NUMBER_OF_COLOURS);

   sky = BLUE;
   forest = GREEN;

   printf("sky = %d\n", (int)sky);
   printf("forest = %d\n", (int)forest);

   return 0;
}

Is it possible to re work this code such that, the user could be prompted to select a number (corresponding to the colours in the enumeration), so that the output is the colour instead of the coefficient it relates to.

Is this even possible with enumeration or are these lists just place holders for the value pertaining to it?

**tabstop** · 10-09-2008

There's no "native" way to get the enumeration name back from a value. (In fact, I don't believe the program itself is required to have the names anywhere in it -- all the names would be replaced by their values when compiling.)

There's a couple different workarounds; you can do switches, or you could do something like

Code:

char *colour_names[] = {"RED", "ORANGE", "YELLOW", "GREEN", "BLUE", "INDIGO", "VIOLET"};
.
.
.
printf("sky = &#37;s\n", colour_names[sky]);

**C_ntua** · 10-09-2008

It is just a holders in C. I believe in other languages it is available. Though you could do something like:

Code:

typedef struct colourTag {
    colour_t col,
    char name[10];
} color;

color sky, forest;
sky.col = BLUE;
strcpy(sky.name,"blue");

etc etc

OR

void colorName (colour_t col, color* colStruct) {
  colStruct.col = col;
   switch (col) {
   case BLUE: strcpy(colStruct.name, "blue");
                     break;                   
   etc etc
   }
}

color sky;
colName(BLUE, *sky);

Note that other languages just have enum as an object with built in functions. You can do the same thing in C, though you have to do everything by yourself

**Iconate** · 10-09-2008

Code:

char *colour_names[] = {"RED", "ORANGE", "YELLOW", "GREEN", "BLUE", "INDIGO", "VIOLET"};
.
.
.
printf("sky = &#37;s\n", colour_names[sky]);

*EDIT* just working on this now.....and What I basically want to do is this, and Just tell me what the easiest way would be...

Have a list, say...Work days, 1-sunday 2-monday...etc...

The user inputs the coefficient: ie. 7

The output would be : You have selected Saturday.

**Elysia** · 10-09-2008

Code:

const char* days_names[] = { "Monday",  "Tuesday", ..., "Sunday" };
printf("&#37;s", days_names[input]);

Btw, tabstop... tsk, tsk, you should know better

String literals should be const char* (and not const char * either

).

**C_ntua** · 10-09-2008

There is one thing you should pay attention though. That the first value MUST start from 0. Or you need a different array. Thus, if you have sunday=1, monday=2, ...., satarday=7 then you would do:

Code:

const char* days_name[] = {"", "Monday", ..., "Sunday"}
printf("%s", days_names[input]);
OR
const char* days_name[] = {"Monday", ..., "Sunday"}
printf("%s", days_names[input-1]);

if you want sunday = 0, ... then you use as Elysia wrote it.

**Elysia** · 10-09-2008

Why on Earth would anyone want the week to start on Sunday!?!

**Iconate** · 10-09-2008

Hey! My job is open 11pm - 12am every day!

But thanks guys, that makes sense. Follow up question though, why am I using a pointer for the array? Is it because if I called the array say WorkDay[1] and it wasn't a pointer than it would give me the address instead?

**tabstop** · 10-09-2008

You need an array of const char *, so that's what we've been making. (We don't want an array of just chars, since "Monday" is not a char but a const char *, i.e., a string.)

**nonoob** · 10-09-2008

If you don't want to do any math on-the-fly such as days_names[input-1], or waste an element on an empty string, how about

Code:

const char *dayn[] = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"}, **days_names = dayn - 1;
...
printf("day: &#37;s\n", days_names[1]);

... prints "Sun".

**C_ntua** · 10-09-2008

Originally Posted by Elysia

Why on Earth would anyone want the week to start on Sunday!?!

Well, dunno what Monday, Tuesday actually mean, but in greek it is: Sunday(Lordsday actually), Seconday, Thirday, ..., Fifthday, something, something. So it kind of makes sense for me

Besides, Mondays suck. Badly. They don't deserve to be first. At least with the sunday leading you can say that the week starts good and ends good and convince yourself that the rest of the week also has to be good...

Also, having an extra 4byte pointer wastes more memory than a 1byte empty string

**Iconate** · 10-09-2008

Code:

const char *dayn[] = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"}, **days_names = dayn - 1;

Wow I LOVE that bit of code at the end there, thats really helpful, do you mind explaining why its a double pointer

Here is my understanding tell me if I am correct

Since....dayn is already pointing to the const

days_names is then pointing to dayn? so its like...

days_names >>>>>>>> dayn >>>>>>>>>> --const--

Yes no?

EDIT:

Originally Posted by C_ntua

Also, having an extra 4byte pointer wastes more memory than a 1byte empty string

Do you mean just having a place holder in the beginning ie. { "EmptyString" "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"}

**@nthony** · 10-09-2008

Not quite, days_names is pointing to the memory location one before dayn, which may or may not be valid memory or memory owned by you. In other words, it's a bad idea and pretty useless if just to avoid using proper indexing. On the contrary, get comfortable with "Math-on-the-fly" when it comes to array indexing, because just about every well-known programming language uses a zero-based array indexing scheme.

**zacs7** · 10-10-2008

> Why on Earth would anyone want the week to start on Sunday!?!
Because it's the standard in many, many countries

. Is America different?

**Elysia** · 10-10-2008

Originally Posted by zacs7

> Why on Earth would anyone want the week to start on Sunday!?!
Because it's the standard in many, many countries

. Is America different?

I don't live in America. It's Monday first here, and Sunday last...

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