Evaluating strings

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 ....

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]

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.

Quote:

---

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.

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.

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.

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

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.

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!)

what you want in this case is arrays.

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