The audacity to do such a thing…
The answer is a map or dictionary.
Or sometimes even just an array. The first time I thought I wanted to do this was 2003 and I was writing a perl script, and I was trying to loop through some sort of array, and write the outputs of some calculations to $val0 $val1 and so on, and I was neck deep into some horrible dark constructs like
${"val" . $i}
before I actually realized that I really just wanted an array, you know, like the one I was already using.It took me forever to understand map (the metafunction).
It depends. If the variable names are arbitrary, then a map is best. If the variable names are just x_1, x_2, x_3, …, x_n, then a list or dynamic array would be more natural. If n is constant, then a vector or static array is even better.
I’ve just figured out linked lists and I want to call that the solution ¯_(ツ)_/¯
When you get a new hammer…
I mean, I wouldn’t recommend it, but seems like a data structure that could work.
In python, ‘eval()’ is your friend.
/maliciouscompliance
locals()[“x”] = 1
Nah, the locals() or globals() object is much better for this.
In c, nothing and nobody is your friend.
Source: me
++ is your friend
Objectively…
# is your worst enemy despite being a copy of a good language
In Perl,
eval
can do similar things, but symbolic references are “better” (I’m fairly sure it’s where PHP got the idea, and the syntax, from.) e.g.$foo = "bar"; $$foo = "potatoes"; # $$foo = access the variable named in $foo, i.e. $bar print $bar; # prints potatoes
Reading other responses, it seems like Python’s
globals
object is not entirely dissimilar, especially if you know how Perl deals with symbolic references under the hood.But just because you can doesn’t mean you should. If you
use strict;
in Perl, it will fail to compile most of this nonsense. Use a hash / associative array / dictionary / whatever your language (natural and/or programming) calls them instead.And I’m pretty sure that even without
strict
, local variables can’t be accessed at all the symbolic way, which is probably for the best. (NB:local
is a subtle thing in Perl. By “local” here, I mean the so-calledmy
variables that aren’t accessible outside their scope.local
variables are actually localised globals. Enjoy that thought.)
I distinctly remember asking this question during a 100 level programming class but I just can not remember why I’d ever want to do this?
What problem could I have possibly have been trying to solve where this would seem like the answer.
A common problem (before learning it is impossible/fraught with danger) is categorisation, like sorting of strings.
Say you have a text, and need to count words of different lengths.
One intuitive approach is to pass through it once and add each word to a list for the corresponding length, as well as making lists as needed. No 7 letter words, no 7-letter-word-list, even though there are longer words.
As humans we’re good at sorting things into an unknown number of categories, and we have to unlearn that for programming
Would one not just use a dict/hashmap with int keys labelling lengths and the list of strings as the values?
A programmer might, as trained/conditioned by the limits of programming languages.
A human would intuitively not, these are meaningless and/or convoluted concepts to the untrained human.
I like the implication that programmers aren’t humans, but a sort of alien being naturally apt at algorithmic thinking, while puny humans are an irrational species that needs to undergo training from the mighty race of programmers if they hope to get into the field brought us here by the aliens
Found the Python guy.
‘List’ is the correct abstract term for any data structure that holds a given number of values in an order, regardless of the implementation. So Python’s List, or C++'s Array or Vector, or a Linked List are all considered lists in the abstract sense.
I did use ‘list’ and forgot it is called an ‘array’ or ‘vector’ in other languages. So sure, close enough :-)
Because those are limited to Python? 😜
This makes a ton of sense and I think you probably solved this mystery for me.
“Oh I need to iterate over something, and keep track of new information as I do it, therefore I should be able to create ‘dynamic variables’ as I progress.”
Yep, what you failed to realise at the time is you’ve just invented a dynamic data structure like a list or a dictionary.
I remember wondering this when I was first trying to self learn. It’s because I needed a map (or list + struct or something) and was such a noob I didn’t know what maps were. Whatever material I was learning from wasn’t good enough, especially for winging things. Plus I was trying to learn C++ and maps aren’t quite so built into the language as they are with a better first language like Python.
In lower level languages like C/C++ the reason becomes much more apparent when you learn about memory allocation and management (as a bonus it also really helps to understand how OS’s handle memory). Dynamically declaring variables in a loop would mean you need to allocate a chunk of memory for each variable that’s generated on the fly, most of, if not all of the dynamically declared variables would not even use most of their allocated memory resulting in a ton of extra overhead and wasted space within memory. An array is usually the answer when someone asks how to dynamically define variables. With an array you allocate the space needed in memory and can iterate across it block by block resulting in more control and efficiency within your reserved memory block. Linked lists are also a fun thing to look into when you aren’t sure how big your array needs to be. It’s a hard question to answer in a 100 level class because the answer actually goes pretty deep into low level programming, operating system and hardware principles.
I distinctly remember having the same experience. For some reason I believed dynamic variable naming was a good idea. What was I on??
Found the creator of PHP
What are talking about? $$ is the way to go :D
I can’t remember the exact use case but I did submit a PR at a previous job that used $$ and not a single senior developer questioned it
Piiiitchforks! Getcher pitchforks heeeere!
Every time we see this in our legacy code we yell out: dolla-dolla bills 'yall!
The answer is writing a program that writes the variables dynamically to a file and including that file into the source file that uses them.
No need to thank me folks I’m just trying to make the world a better place.
Or just use a dictionary/hashmap if it’s all in the same program
Or in JavaScript you can build a string that creates the name of a variable in a sub loop by concatenating ““variable”+$i” and passing that value into a variable that is then read as the name of an incremental variable that a value is then passed to.
This has the advantage of being both extremely unwieldy and highly inscrutable, and there’s a small chance it will make your coworkers send you death threats in slack.
JavaScript: They were so focused on whether or not they could that they forgot to consider whether or not they should
I once created a system to automatically load and execute arbitrary JS files and any exported functions in them as part of a server
I see you’re not ready for the ways of the JavaScript.
Is anyone really?
I think it should work in Clojure
(doseq [i (range 10)] `(def ~(str "var-" i) i))
Everything goes with macros :D
That rhymes! :D
The reason I hate HTML: I’ve seen smart, reasonable people do this with IDs, and I’m not 100% sure they’re wrong.
That’s because it makes sense when dynamically creating HTML. HTML is not a programming language, it’s simply markup - so if you want to generate some block of HTML in a loop and later access that block of HTML in JS (e.g. to interact with the UI separate from creating it in the first place), it’s a completely reasonable thing to do.
Agreed.
Also, HTML is only meant to be read by a browser’s interpreter which has no problem keeping track of variable names.
So long as the ID never repeats, it’s cool
It would probably be way easier than expected to do so in javascript. It would also be awful.
for(var i=0; i \< numLoops; i++){ globalThis['example'+i] = doStuff(i); }
for(var i = 0; i < numLoops; i++) { eval("var_" + i + " = " + i + ";"); }
eval
is a path to powers many would consider to be… unnaturalIs it possible to learn this power?
Not from ES6
In R:
assign("x", value)
Within a loop could be:
for(i in 1:10){ assign(paste0("listNum", i), list(i, someStringVector[i], i:(i+20), i*value)) }``` And you can also use get() in the same way to dynamically retrieve a variable. I've gone so far into coding debauchery that I've dynamically assigned variables from dynamically retrieved ones, and I've done so fairly frequently.
Legit thank you.
I’ve wondered on the right way to do this in R… Too many times.
I recently saw a SO answer suggesting to use PHP as an alternative to the c preprocessor.
PHP variable variables
deleted by creator
USE ASSOCIATIVE ARRAY, OR SOME KIND OF MAPPING DATA STRUCTURE
So is that a no?
Array, and put the names or hash or id in either another array or the same array as a tuple.