Question

enum in C

Asked by: margarit

Hello,

I define typedef enum. Then define parameter from this type.
What would happen If i am trying to set an unillegal value to this parameter? What value would it get?
Is there any way to check if temp contains legal parameters from enum?

THANKS
Margarit

typedef enum Numbers
{
     one = 1,
     two =2,
    three = 3
} myParam;
 
void main ()
{
   temp =4;
myParam = temp;
}

                                  
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Asked On
2009-09-01 at 23:49:20ID24700373
Tags

C

Topic

C Programming Language

Participating Experts
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Answers

 

by: evilrixPosted on 2009-09-01 at 23:52:49ID: 25238469

The behavior is undefined.

BTW: I think your code was meant to be something like below...

typedef enum Numbers
{
     one = 1,
     two =2,
    three = 3
} myParam;
 
void main ()
{
   int temp = 4;
   myParam p = temp;
}
                                              
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by: margaritPosted on 2009-09-02 at 00:05:13ID: 25238524

Thanks,

Numbers - is a name of enum?

myParam - defines a type ?

Can I define it like this?

typedef enum {
    one = 1,
    two =2,
   three = 3
} myParam;

 

 

by: Infinity08Posted on 2009-09-02 at 00:06:08ID: 25238531

>> I define typedef enum. Then define parameter from this type.

That's not what you do. Currently, you typedef the enum as myParam. ie. myParam is a type name (alias for the enum), and not an instance of the enum.

See below for corrected code ...


>> What would happen If i am trying to set an unillegal value to this parameter? What value would it get?

Assigning an int value to an enum value is only legal if the int value corresponds to one of the enum's possible values (in this case 1, 2 or 3). Otherwise, the behavior is undefined ... so you shouldn't use it :)


>> Is there any way to check if temp contains legal parameters from enum?

You could check whether it falls in the range of the enum values (ie. 1 <= temp <= 3)

typedef enum
{
    one = 1,
    two = 2,
    three = 3
} Numbers;                /* the typedef */
 
Numbers myParam;          /* the instance */
 
int main ()               /* main HAS to return int */
{
    int temp = 4;         /* forgot the type ;) */
    myParam = temp;
    return 0;             /* main HAS to return an int */
}

                                              
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by: Infinity08Posted on 2009-09-02 at 00:06:50ID: 25238533

Oops ... I have been really slow in typing today lol. Sorry, evil one :)

 

by: evilrixPosted on 2009-09-02 at 00:11:54ID: 25238561

>> Can I define it like this?
BTW: You don't need to specifiy the values specifically, they are always +1 of the previous item. If you want to start from a specific value you only need to specify that.

typedef enum
{
    one = 1,
    two,
    three
} Numbers;

>> main HAS to return an int
Heh. Very true -- missed that one (too busy looking at the enum).

 

by: masheikPosted on 2009-09-02 at 00:40:42ID: 25238710

Can I define it like this?
yes.you can.

Debug your code to see the behaviour,.

#include <stdio.h>
#include <stdlib.h>
#define ENABLE_DEBUG //Comment out this line to see the output
#define TRUE 1
/*
Enum usually used for type check of constant inputs.It is good coding practice to use CAPITAL letters for
enum members.usually typedef not used with enum
*/
 
typedef enum Numbers
{
    ONE = 1,
    TWO =2,
    THREE = 3
} myParam;
 
int main ()
{
 
    int input;
    int temp=4;
    myParam p;
    while (TRUE)
    {
#ifdef ENABLE_DEBUG
        p=4;
#endif
 
        printf("\nvalue of p is now %d \n",p);
        printf("Enter any value:");
        scanf("%d",&input);
 
        if (input==ONE)
        {
            printf("one");
        }
        else if (input==TWO)
        {
            printf("two");
        }
        else if (input==THREE)
        {
            printf("three");
        }
        else
        {
            printf("Not valid\n");
        }
    }
    return 0;
}
                                              
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by: cupPosted on 2009-09-03 at 12:17:55ID: 25253962

A lot depends on your enum definitions.  If they are consecutive, it is not a problem checking whether something is out of range or not.  You can easily have something like

typedef enum {
    start = 1,
    one = 1,
    two,
    three,
    max
} Number;

...

if (temp < start || temp >= max)  printf ("Illegal value\n");

The problem arises when there are gaps in the enums.  For instance

typedf enum
{
    aa  = 1,
    bb, cc, dd, ee,
    ii = 9,
   oo = 15,
   uu = 21
} Vowels;

A value like 6 is not a valid enum in that case and there is no simple way to test for it.  Sometimes, enums are used for bit patterns

typedef enum
{
    DSR = 1 << 0,
    DTR = 1 << 1,
    RTS = 1 << 2,
   CTS = 1 << 4
} Bitpat;

Again, there are gaps so testing is difficult.  What's even more difficult is when these are ORed together.  For instance, if the value is CTS | RTS, we get 12.

If you want to print enums, put all of them in one file as follows

NUMBER(one)
NUMBER(two)
NUMBER(three)

Then to declare them

#define NUMBER(x) x,
typedef enum
{
#include "number.h"
   max
};
#undef NUMBER

To get them as strings

#define NUMBER(x) #x,
const char* numstr[] =
{
   #include "number.h"
   ""
};
#undef NUMBER

This way, the enums and the strings never go out of sync.   To print the enum of x (provided it is in range), it is simply

printf("%s\n", numstr[x]);

Of course, if you don't like macros, this technique isn't for you.

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