Evaluating strings

**whitel1977** · 11-23-2002

Hi!

Am very new to this board ... have checked out the FAQs etc by the way before posting this questions as i can't find what i'm after anywhere - so somone might be able to help

Having a Coldfusion background, there was a very handy function called Evaluate(), and i am looking for the C equivalent of it (if it exists). As an example of what it does (explanation after code sample):

<cfset variable1 = 10>
<cfset i = 1>
<cfset myvariable = Evaluate("variable" & i)>

Variables variable1 and i are defined,
myvariable is set to the value of variable1 (10) by evaluating the string in the Evaluate() function - the string being the text "variable" concatenated with the value of variable i.

I hope this is clear - i have done a little C programming before, but knowing if this could be achieved would be great ...

Thanks in advance ....

**Shiro** · 11-23-2002

You can use the function strcat(), you will need to include string.h for this.

The declaration of strcat() is:

char *strcat (char * restrict s1, const char * restrict s2);

It puts the string pointed to by s2 after s1. So if s1 is the string "Hello " and s2 is the string "World", then strcat (s1, s2) would result in s1 becoming "Hello World".

[edit]
More information on string-functions and other standard functions can be found here.

http://www.f.kth.se/~f95-pax/refs/C%20Library%20Ref/
[/edit]

**orbitz** · 11-23-2002

Shiro, that is not what evaluate does. What he wants is something not possible in C, as far as I know. All variable names becomes address at compile/link time. Be able to refer to a variable by it's name in a string is only possible in interpreted languages. C does not have this capability.

**moi** · 11-23-2002

Originally posted by whitel1977
Hi!

Am very new to this board ... have checked out the FAQs etc by the way before posting this questions as i can't find what i'm after anywhere - so somone might be able to help

Having a Coldfusion background, there was a very handy function called Evaluate(), and i am looking for the C equivalent of it (if it exists). As an example of what it does (explanation after code sample):

<cfset variable1 = 10>
<cfset i = 1>
<cfset myvariable = Evaluate("variable" & i)>

Variables variable1 and i are defined,
myvariable is set to the value of variable1 (10) by evaluating the string in the Evaluate() function - the string being the text "variable" concatenated with the value of variable i.

I hope this is clear - i have done a little C programming before, but knowing if this could be achieved would be great ...

Thanks in advance ....

C is a compiled, not interpreted language.

**Shiro** · 11-23-2002

After rereading the post and reading some info on the web I think whitel1977 means something different. The argument to Evaluate evaluates to an expression.

http://www.houseoffusion.com/cfdocs1...on/dwa07_2.htm

In the example of whitel1977, the name of a variabele is being build. Concatenating the string "variable" with the value 1 results in variable1. It makes me think of hash-tables, like Perl implements them:

$h{"a"} = 1;
$h{"b"} = 2;
print $h{"a"}, "\n"; # prints 1
print $h{"b"}, "\n"; # prints 2

Such hash-tables can be implemented in C, but not in the easy way like in ColdFusion or Perl. A very basic way is to implement a linked list where each node has a unique string which identifies the node.

**orbitz** · 11-23-2002

I think you are misunderstanding Shiro. What Evaluate does is, among other things, takes the string you give it, and then returns the value of the variable named that string.

myass = 10;

mypantsize = Evaluate("myass");

Would make mypantsize be equal to 10. If you can do this in C then be sure to tell us all how it's done.

**moi** · 11-23-2002

i've seen it done in C, sort of:

Code:

int foo, bar, bed, grok;

typedef struct
{
  int *pi;
  const char *name;
} fooentry_t;

const int numentries = 4;

fooentry_t fooentries[4] = {
{&foo, "foo"},
{&bar, "bar"},
{&bed, "bed"},
{&grok, "grok"}
};

int setfoo (int val, const char *name)
{ // attempts to set foo with name name to val val
  int i;
  for (i = 0; i < numentries; i++)
  {
    if (strcmp (name, fooentries[i].name) == 0)
    {
      *(fooentries[i].pi) = val;
      return 0;
    }
  }
  return 1; // fail
}

start with somethng like that

**orbitz** · 11-23-2002

Oh, ickie! hehhee

Why don't we all just edit C so it allows this, huh? I'm sure the ISO and ANSI groups will appreciate it. I'll start working on my gcc patch ASAP.

Your code is theortically sound, but I it lacks the runtime necesity. Also lacks the ability to deal with arrays, etc. But that would be too much work for a post and all. Anyways, This is just my personal opinion, but I think any code which requires eomthing like 'Evaluate' is probably rank anyways and needs a rewrite.

**whitel1977** · 11-23-2002

Phew .... didn't expect to get so many replies! - thanks ....
well, that clears one thing up at least, knowing that there isn't a predefined function to do that,..... but you do have point of what i was looking for ...

Basically what I am trying to do (which i can achieve with alot of repetitive code) is that i have a whole lot of variable pairs specifying width/height
eg

dk1_WIDTH = 10;
dk1_HEIGHT = 2;
dk2_WIDTH = 20;
dk2_HEIGHT = 5;

etc...

and the idea behind requiring Evaluate is that i have another variable (like an index), and if it is set to

index = 1;

then i want to set a generic width/height variable pair to that specified by the index

eg (partly in C and pseudo-ish code)

WIDTH = 0;
HEIGHT = 0;

WIDTH = Evaluate("dk" & index & "_WIDTH");
HEIGHT = Evaluate("dk" & index & "_HEIGHT");

therefore, index = 1 means that WIDTH = 10 and HEIGHT = 2 (based on the values set earlier) .....

is there another way to do this (i currently am using a whole bunch of case statements containing LOTS of repetitive code!)

**moi** · 11-23-2002

what you want in this case is arrays.

**whitel1977** · 11-23-2002

I think i see what you mean - store the value pairs in each index of the array and extract the values as need be ....

Thanks again ... been very helpful

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thanks

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