Understanding/Implementing Enumeration

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;
}

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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?

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:

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

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

Code:

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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);

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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

Code:

---

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

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*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.

Code:

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

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Btw, tabstop... tsk, tsk, you should know better :)
String literals should be const char* (and not const char * either :D).

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:

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

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

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

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?

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.)

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:

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

---

... prints "Sun".

Quote:

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Originally Posted by Elysia

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

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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 ;)

Code:

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

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

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:

Quote:

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Originally Posted by C_ntua

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

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

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.

> Why on Earth would anyone want the week to start on Sunday!?!
Because it's the standard in many, many countries :p. Is America different? :confused:

Quote:

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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 :p. Is America different? :confused:

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

Quote:

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Originally Posted by Iconate

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

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It's a double pointer the same way dayn is.

In my code snippet days_names is an array just like dayn at least in usage.

days_names is just the same array shifted over one.

dayn
[0] contains pointer to "Sun"
[1] contains pointer to "Mon"
[2] contains pointer to "Tue"

days_names
[0] contains pointer to ??? (equivalent to dayn[-1], so don't try to access it)
[1] contains pointer to "Sun" (equivalent to dayn[0])
[2] contains pointer to "Mon" (equivalent to dayn[1])