Re: [Gretl-users] Drastic discrepancy in performance

On Sat, 26 Apr 2014, Andreï | Андрей Викторович wrote: [I'm pushing the example down to insert my comments here.] The issue raised here is the difference in execution speed between variants of a hansl script in which the value of a loop index (say, j) is accessed (variant 1) as simply "j" or (variant 2) in the dollar-form, "$j". Andrei remarks that it "seems more correct" to use "$j", yet it turns out that this slows things down substantially. Short answer: Never use the dollar-form for a loop index unless you really need string substitution. It is always "more correct" to use the plain variable name if possible. Suppose the current value of the index j is 5. Then if you put plain "j" into a command or function call you get the numerical value 5 when the line is evaluated, while if you use "$j" the string "5" is substituted for "$j" in the line in question before it is evaluated (a "macro", so to speak). Consider one of your calls: y = cum(mnormal(T, 1))+5*j What you want here is just the numerical value of j; it would be a roundabout procedure to substitute the string representation of j's value into the line of hansl first, and then evaluate the line. And here's why it slows things down substantially. In the context of loops we attempt to "compile" (in a limited sense) assignments so they'll run faster. But if we find that a given line contains string substitution we abandon the attempt, since in principle the final form of the line could change in an arbitrary, unknown way from one loop iteration to another. Briefly, here's a case where you might want to use $j: series foo_$j = <some expression that depends on j> This gives you a way of defining an arbitrary number of series foo_1, foo_2, and so on, in a loop, something which could not be done conveniently in any other way at present (although we're working on a better mechanism). Allin Cottrell