Why not Eval()?

Post #107,396 by tablizer 6/26/03 12:55:37 PM Reply	Why not Eval()? I have done data conversions where I put a bunch of expressions (equations) in a table and then processed each record through the conversions using Eval(). The accountant I was working with caught on to the expressions pretty quickly and started writing in some corrections to my initial guesses to the fixed assets depreciation calc conversions. For those of you not familiar with such functions, you can do things like this: x = "23 + b" b = 5 r = eval(x) print(r) // result: 28 Closures give you a bit more scope control than Eval() or Execute(), but for smaller programs it is not much of an issue. Again, I am not saying that closures won't simplify anything, I am just saying that the difference is relatively minor, and the inclusion of closures in a language often dooms it. Eval() or Execute() are non-intrusive syntax-wise; they are just functions like any other. There are not that many places where Eval()-type things are needed, at least not all over the program, so a 5% reduction in syntax or a slight improvement in packaging for those few spots of the code is not a huge selling point. I see no reason to move them to the top of my wishlist as long as I have Eval-like functions. ________________ oop.ismad.com
Post #107,404 by ChrisR 6/26/03 2:02:54 PM 6/26/03 2:05:28 PM Reply	The problem is that the equations are part of the data Eval doesn't do any good unless you want to parse a string to represent the equations - and even then you need some way to represent the equations as data (i.e. be able to manipulate it). Your misunderstanding is to assume that I know the equation. I don't know the equation, I know one or more forms of the equation that I'd like to solve for (i.e. find the coefficients and r-square values). In your example, you are using of: x = "23 + b" - meaning that the a0 coefficient is 23 and the a1 coefficient is 1. The regression is designed to input some numbers [0,23], [1,24], [2,25], and figure out the coefficients (23,1). That just happens to be a simple linear equation that can be done without resorting to closures. However, what if wanted to solve the coefficients for: y = a0 + a1x + a2xx + a3xxx and just had a bunch of (x,y) data points. Or how about throwing in some additional exogenous variables: y = a0 + a1x + a2z + a3xz + a4xx + a5zz Now, how do you propose to solve for these coefficients given a series of data points? Having done this in a version of Pascal that does not have closures, my solution was to build a parser that allowed you to represent the form of the equation as a string. Problem is that a parser becomes a mini-language and occupies a good amount of space and requires an attention to detail. Whereas, by using closures in Python or code blocks in Smalltalk, I can sling together equation forms (i.e. lambda functions or anonymous functions). Edited by ChrisR June 26, 2003, 02:05:28 PM EDT Expand All History
Post #107,458 by tablizer 6/27/03 2:02:25 AM Reply	I am not following you My math is too rusty I guess. The equations can be stored anywhere. In tables, in files, etc. You can have something like: equation = prompt("Enter equation:") // assumes equation has "a" and "b" in it a = prompt("Enter A:") b = prompt("Enter B:") printLine("Result is " . eval(equation)) Example run: Enter equation: a2 + bb Enter A: 3 Enter B: 4 Result is 22 (Validation and security is another lesson) ________________ oop.ismad.com
Post #107,497 by ChrisR 6/27/03 10:08:47 AM Reply	Do you not understand Regression? The equations can be stored anywhere. It's easy enuf to store the equations - IF YOU HAVE THE EQUATIONS! That's your problem, you're assuming that all you are doing is storing coefficients. // assumes equation has "a" and "b" in it a = prompt("Enter A:") b = prompt("Enter B:") There's your problem. Your program is not smart enuf to figure out the coefficients A && B. What if I give you a table of: CREATE TABLE MyData(x INT, y INT) INSERT INTO MyData VALUES(1,0) INSERT INTO MyData VALUES(3,1) INSERT INTO MyData VALUES(5,2) and I told you the equation was of the form: y = mx + b Now give me a program that uses the above data to derive m and b. Don't give me a program asking the end user to give me m & b. When you get that done, then start changing the form of the equation around, adding more variables and various combinations of power orders. Something simple like: y = a + bx + cxx + dxz + ezz + fz And don't ask the operator to supply the a,b,...f coefficients.
Post #107,537 by Arkadiy 6/27/03 11:39:49 AM Reply	Eval can be used to compute arbitrary function with arbitrary number of arguments. It's really inconvenient, but possible, because most evals have access to global variables (and some even to local scope variables). So you do something like this: getFunctionValue(string f, real x[], real a[]) { idx = 0; foreach anX in x { eval("x" . idx . " = " . x); } idx = 0; foreach anA in a { eval("a" . idx . " = " . a); } return eval(f); } And then you build your "f"s so that they use x0, x1 and so on for argument names, and a0, a1 and so forth for parameter names. And then you can look for minimum sum of squares by calling getFunctionValue in the known data points x[] and varying a[]. And then you throw up in disgust at this slow unvieldy buggy contraption and choose a language that supports real anonymous functions. You do. But not Bryce. He is happy. He has just achieved another triumph of Eval. -- Less Is More. In my book, About Face, I introduce over 50 powerful design axioms. This is one of them. --Alan Cooper. The Inmates Are Running the Asylum
Post #107,556 by tablizer 6/27/03 1:02:48 PM Reply	Eval is "good enuf" for occassional use. It's really inconvenient, but possible, because most evals have access to global variables (and some even to local scope variables). In the languages I am familiar with, they had the same scope as if the expression itself was there in the code where the Eval function was. It is just like a code replacment. I suppose I should have stated that assumption, but I have yet to see it otherwise. And then you throw up in disgust at this slow unvieldy buggy contraption and choose a language that supports real anonymous functions. Buggy? You have not identified any specific bugs. True, you get no compile-time checking, but if we are generating stuff during run-time, we don't have that anyhow. Slow? Perhaps. But I don't use it for finite element simulations or the like. You seem to be agreeing with me that Eval is "good enough" for occassional usage, which is my point. If code was littered with eval's or dynamic execute's, then one might find closures would clean up the scoping issues more. And no, I don't remember multiple regression. That was many many moons ago. My brain does a pretty good job of house-cleaning, except that it never asks permission before tossing. Kinda like my wife. Maybe if I studied your code long enuf I might figure it out, but I have other more practical things I would rather study if I go into study mode. ________________ oop.ismad.com
Post #107,563 by ben_tilly 6/27/03 1:51:44 PM Reply	You are not familiar with Ruby then In the languages I am familiar with, they had the same scope as if the expression itself was there in the code where the Eval function was. It is just like a code replacment. I suppose I should have stated that assumption, but I have yet to see it otherwise. Ruby has several different eval functions running around. They vary in terms of whose scope they evaluate in. After all your code running in my package may do something very different from your code running in your package. Ruby gives you choice. If you want to do something more sophisticated than the basic built-in options, there is even the notion of a binding, which can be taken anywhere, and then from anywhere else you can eval with that binding. This is explained at [link\|http://www.pragmaticprogrammer.com/ruby/articles/dynacall.html\|http://www.pragmatic...les/dynacall.html] Cheers, Ben "good ideas and bad code build communities, the other three combinations do not" - [link\|http://archives.real-time.com/pipermail/cocoon-devel/2000-October/003023.html\|Stefano Mazzocchi]
Post #107,714 by JimWeirich 6/28/03 3:50:32 PM Reply	Of course Eval()? Tablizer: I have done data conversions where I put a bunch of expressions (equations) in a table and then processed each record through the conversions using Eval(). Actually, this is a reasonable example of using Eval (provided you understand the dangers of passing external data to eval, but that's a different topic). This is an example where a closure would NOT be a good choice. Closures and Eval are NOT interchangable, except in trivial examples. -- -- Jim Weirich jweirich@one.net [link\|http://onestepback.org\|http://onestepback.org] --------------------------------------------------------------------- "Beware of bugs in the above code; I have only proved it correct, not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)

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