calling a function saved in a varible

#1 @Joseph - Is that line of code stored on a GUI control or another object?

#2 If you have a field defined as you have shown...

%object.ability[0] = "grenade();";

then this

eval(%object.ability[0]);

should call that function.
If you have a string that does not evaluate to valid code you will see the message "parse error" appear in the console with no context at all, so it helps to debug these carefully.

Um, unless you are working engine-side. Then you would probably want to use

evaluate(ability[0]);

but I haven't really played with that much and it probably requires even more careful debugging.

#3 In TGB you can assign a GUI element a Command inside it's GuiCtrl, either through the Gui Builder, or through direct script. And then call upon that function, just like any other:

//Example 1
new GuiBitmapButtonCtrl() {
  Command = "doThis();";};

function doThis()
{ echo("I did it!");}
//Example 2
new GuiBitmapButtonCtrl(myGui) {
  Command = "myGui.doIt();";};

function myGui::doIt()
{ echo("I did it again!");}

#4 My apologies, after re-reading your question, I believe Richard was on the right track with eval, so you'd have to set up something like this:

myClass.ability[0] = "grenade();"
myClass.setAbility();

function myClass::setAbility(%this)
{
  %count = myClass.getAbiliyCount();
  for(%i=0;%i<%count;%i++)
  {
    eval(%this.ability[%i]);
  }

}

function myClass.getAbilityCount(%this)
{
  %count = 0;
  while(true)
  {
    if(%this.ability[%count] $= "")
      return false;
    else
      %count++;
  }
  return %count;
}

function grenade()
{
  echo("Boom!");
}

#5 thank you that works