Filtering tuples with lambda function | Together C & C++ | Page 1

hidden bay Jun 8, 2023, 1:05 PM

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I have a task in my HW that require me to create a method that takes a tuple and a lambda function, applies this lambda function on each element of the tuple. at the end the method should return a new tuple containning the elements that returned true in lambda.

This is my tuple in main that i am filtering:

int main(){
        auto tpl = make_tuple(
                [](auto a, auto b) { return a + b; },
                10,
                25,
                42.42,
                "Hello",
                string("there")
        );
        
        auto tpl_type_filtered = filterTuple(tpl, []<typename T>(const T&) { return is_integral_v<T>; } );
        return 0;
}

This is my progress so far:

template <size_t Index = 0, typename... Types,typename... Types2, typename Func>
auto filterTupleHelper(const std::tuple<Types...>& tuple, const Func& func, const std::tuple<Types2...>& container) {
    if constexpr (Index < sizeof...(Types)) {
        if constexpr (func(std::get<Index>(tuple))) {
            return filterTupleHelper<Index + 1>(tuple, func, std::tuple_cat(container, std::make_tuple(std::get<Index>(tuple))));
        } else {
            return filterTupleHelper<Index + 1>(tuple, func, container);
        }
    } else {
        return container;
    }
}

template <typename Tuple, typename Func>
auto filterTuple(const Tuple& tuple, const Func& func) {
    return filterTupleHelper(tuple, func, std::make_tuple());
}

Also the teacher requested the following:
" this function must be protected by a "requires" clause so that it will be only available for tuple params alo "concept" core language keyword usage is FORBIDDEN.
You must use inline requires expressions and clauses.
You can write your own is_tuple/is_tuple_v structs if necessary"

but i dont know why or how to use requires in this case, and i dont get what concept would benifit me. I feel like i am completely lost or missing the point..
I would really apprecaite some help

dull mauveBOT Jun 8, 2023, 1:05 PM

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When your question is answered use !solved to mark the question as resolved.

Remember to ask specific questions, provide necessary details, and reduce your question to its simplest form. For tips on how to ask a good question run !howto ask.

mellow briar Jun 8, 2023, 1:11 PM

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which function exactly do they want to have a "requires"

hidden bay Jun 8, 2023, 1:11 PM

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i think the filter itself

wide mountain Jun 8, 2023, 1:11 PM

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Do we have constexpr lambdas yet? I don't remember.

hidden bay Jun 8, 2023, 1:11 PM

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but i didnt even know why i would use it in this case

mellow briar Jun 8, 2023, 1:12 PM

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which function exactly in your example

hidden bay Jun 8, 2023, 1:12 PM

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filterTuple

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it was supposed to be 1 single function but i implemented 2 functions to apply my logic

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but my logic is not working

hidden bay Jun 8, 2023, 1:12 PM

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wide mountain Do we have `constexpr` lambdas yet? I don't remember.

yes

mellow briar Jun 8, 2023, 1:12 PM

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yes constexpr lambdas exist atleast in c++20 iirc

hidden bay Jun 8, 2023, 1:12 PM

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and i am using 20

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idk if you can understand my logic but

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its i make a container and pass it the helper, helper iterates over each index, if func(element) is true i create a new container which is old cont+new container that has that element

mellow briar Jun 8, 2023, 1:14 PM

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ive never had to apply a requires to a lambda, and i dont understand this assignment

hidden bay Jun 8, 2023, 1:14 PM

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i keep repeating until no more elements exist

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he gave us a hint but i also didnt understand it

mellow briar Jun 8, 2023, 1:14 PM

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they want you to check if each lambda element you pass to the function is a... lambda element?

wary hearth Jun 8, 2023, 1:14 PM

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hidden bay but i didnt even know why i would use it in this case

because the filter tuple should only be allowed on tuples, though you have constrained your function to only tuples in a different way

hidden bay Jun 8, 2023, 1:15 PM

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wary hearth because the filter tuple should only be allowed on tuples, though you have const...

i didnt get it

wary hearth Jun 8, 2023, 1:15 PM

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you constrained filter tuple to only work on tuple

mellow briar Jun 8, 2023, 1:16 PM

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template <typename Tuple, typename Func> auto filterTuple(const Tuple& tuple, const Func& func) { return filterTupleHelper(tuple, func, std::make_tuple()); } is this the function you want to constrain?

wary hearth Jun 8, 2023, 1:16 PM

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you did it "wrong" by not following instructions on how to do that

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currently your filterTuple sort of takes anything as first argument

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but it doesn't actually allow anything in, because you forward that argument into your helper that explicitly does not allow anything that isn't a specialization of std::tuple

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which is not what the instructions were requiring you to do

wary hearth Jun 8, 2023, 1:17 PM

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mellow briar `template <typename Tuple, typename Func> auto filterTuple(const Tuple& tuple, c...

basically what the instructions require, but not what he did

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so yes

hidden bay Jun 8, 2023, 1:17 PM

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auto filterTupleHelper(const std::tuple<Types...>& tuple,

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you mean this

wary hearth Jun 8, 2023, 1:18 PM

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well, that's the first argument of your helper

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and it has to be a specialization of std::tuple because of how you've set it up

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but again that doesn't follow the instructions

hidden bay Jun 8, 2023, 1:19 PM

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should i start from scratch

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i thought my logic atleast is correct but its not working at all

wary hearth Jun 8, 2023, 1:19 PM

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I haven't actually read the rest of what you wrote

mellow briar Jun 8, 2023, 1:21 PM

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lol i am also lost. is the problem that your logic is not working or that you dont know how to smack on a requires on this

wary hearth Jun 8, 2023, 1:22 PM

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checked what you wrote, you don't need to start from scratch I would say

hidden bay Jun 8, 2023, 1:22 PM

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mellow briar lol i am also lost. is the problem that your logic is not working or that you do...

logic

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fuck require i can deal with it later

wary hearth Jun 8, 2023, 1:23 PM

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but if you feel like you need to reset yourself you could restart

hidden bay Jun 8, 2023, 1:23 PM

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just logic wise i cannot figure it out at all

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i want to be able to call this function with a tuple and a function, and return a new tuple with elements that passed the test of the function

hidden bay Jun 8, 2023, 1:23 PM

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wary hearth checked what you wrote, you don't need to start from scratch I would say

could you spot the issue tho? i cannot at all

mellow briar Jun 8, 2023, 1:24 PM

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id start over if you have no clue whats wrong =P

hidden bay Jun 8, 2023, 1:25 PM

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i will do that i guess

wary hearth Jun 8, 2023, 1:25 PM

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hidden bay could you spot the issue tho? i cannot at all

well first thing first, does your code compile

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because it almost definitely will not compile

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because the way you used constant expression is wrong

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which is a different problem from just the pure logic of what you're trying to achieve

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like this if constexpr (func(std::get<Index>(tuple))) has no change to work because tuple is not usable in a constant expression

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nor func

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actually the more I think about this, the weirder it sounds

hidden bay Jun 8, 2023, 1:29 PM

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idk man i am starting from scratch

hidden bay Jun 8, 2023, 1:29 PM

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wary hearth actually the more I think about this, the weirder it sounds

its such a weird thing, it should be much easier than this

wary hearth Jun 8, 2023, 1:29 PM

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what sort of homework/course/uni are you in that already teaches c++20 and forces you to work on that kind of constexpr stuff

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no, it's not easy

hidden bay Jun 8, 2023, 1:29 PM

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its advanced c++

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elective course i am taking as an ug

wary hearth Jun 8, 2023, 1:29 PM

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if the type of your returned tuple depends on the predicate being satisfied, you're basically forced into nttp

hidden bay Jun 8, 2023, 1:29 PM

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class mixed with master students and its pretty fucking hardcore

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nttp?

wary hearth Jun 8, 2023, 1:30 PM

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yeah so if you do not know what those are, you're sort of screwed

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non-type template parameters

hidden bay Jun 8, 2023, 1:30 PM

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i might have missed a class

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ill check if it was given before

wary hearth Jun 8, 2023, 1:31 PM

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what's your exact homework instruction/prompt for this function you need to write

mellow briar Jun 8, 2023, 1:31 PM

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what youre gonna have to do is make a function that appends one element to a tuple, then you can if constexpr branch on thhe result of the tuple element evalutaiton function to either append that element or recursively call filter on the next element index

wary hearth Jun 8, 2023, 1:31 PM

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the type of the "function" depends on the "input" value

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the only way to get that to work is if the "input" value is a constant expression you can stuff as template parameter

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unless the return type doesn't actually depend on the value, and you're meant to return a tuple with different values where the predicate wasn't satisfied

wary hearth Jun 8, 2023, 1:32 PM

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mellow briar what youre gonna have to do is make a function that appends one element to a tup...

that's exactly what he tried

wary hearth Jun 8, 2023, 1:33 PM

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hidden bay I have a task in my HW that require me to create a method that takes a tuple and...

I reread again, but if your "lambda predicate" does not actually depend on the value, but only on the type of the tuple element, then it's a different story

mellow briar Jun 8, 2023, 1:34 PM

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oh the value does not even matter here

wary hearth Jun 8, 2023, 1:35 PM

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what I was talking about was if you need something like

auto is_even = [](auto value) { return value % 2 == 0; };
auto result = filterTuple(std::make_tuple(0, 1, 2, 3), is_even);

then this has no chance of working if you don't understand how to use values as template arguments

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aka nttp's

mellow briar Jun 8, 2023, 1:36 PM

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that tuple isnt gonna be constexpr

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but if its just the type he checks it should work?

wary hearth Jun 8, 2023, 1:37 PM

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it can work but he'll have to make some amendment to his implementation

hidden bay Jun 8, 2023, 1:37 PM

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it shouldnt be just type, it should be based on the lambda

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if lambda is true then that element should be added

mellow briar Jun 8, 2023, 1:38 PM

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ah then as sly said, youre gonna have to pass it as a template parameter value

wary hearth Jun 8, 2023, 1:38 PM

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then there's a whole bunch of things you do not know that is required to get this to work

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also strings will pretty much not be supported

hidden bay Jun 8, 2023, 1:39 PM

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man fml what can i do

wary hearth Jun 8, 2023, 1:39 PM

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by that I mean specifically std::string

hidden bay Jun 8, 2023, 1:39 PM

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i can work with const char *

wary hearth Jun 8, 2023, 1:39 PM

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there's an argument for supporting other kind of strings

mellow briar Jun 8, 2023, 1:39 PM

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this has gone into the "probably not worth it " territory lol

wary hearth Jun 8, 2023, 1:39 PM

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there are restrictions for those as well technically, cause a char const* as nttp cannot refer to a string literal

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unless they changed that rule

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I'll admit I don't exactly check that specific rule all the time

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and how it changes/does not change

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but the basic thing is still that you need to know how to use nttp's

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which isn't the end of the world, but still awkward/weird if you've never done it before

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the most common/well-known example being the size of an std::array

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std::array<int, 10> uses an nttp as second parameter

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you passed in a value

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that value, as a template parameter, can be used to change the return type of a template function

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which is something you need to be able to do for this homework

hidden bay Jun 8, 2023, 1:43 PM

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mellow briar this has gone into the "probably not worth it " territory lol

hahahaha

wary hearth Jun 8, 2023, 1:43 PM

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unless I misunderstood what the homework was about

hidden bay Jun 8, 2023, 1:43 PM

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i did something

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its not working but logically should be 100% true

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template<int Index = 0,typename ...Args>
auto filterhelper(std::tuple<Args...> tpl,auto func,auto container){
    if constexpr (Index < sizeof...(Args)) {
        if(func(std::get<Index>(tpl))){
            auto newElementtuple= make_tuple<decltype(std::get<Index>(tpl))>(std::get<Index>(tpl));
            auto newContainer=std::tuple_cat(container,newElementtuple);
            return filterhelper<Index+1>(tpl,func,newContainer);
        }
    }
    else{
        return container;
    }
    return filterhelper<Index+1>(tpl,func,container);
}

auto filtertuple(const auto& tpl,const auto& func){
    return filterhelper<0>(tpl,func,std::make_tuple<>());

}```

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almost same approach just cleaned it up i think

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i fucking hate mylife man

wary hearth Jun 8, 2023, 1:45 PM

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;asm -std=c++20

template <int I>
auto foo()
{
  if constexpr (I % 2 == 0)
    return 0.0f;
  else
    return 0u;
}

#include <type_traits>
static_assert(std::is_same_v<decltype(foo<0>()), float>);
static_assert(std::is_same_v<decltype(foo<1>()), unsigned>);

mild heraldBOT Jun 8, 2023, 1:45 PM

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Compilation successful

No assembly generated.

mellow briar Jun 8, 2023, 1:47 PM

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problem is i dont think std tuple is supported as a nttp

hidden bay Jun 8, 2023, 1:47 PM

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i get this error : fatal error: template instantiation depth exceeds maximum of 900

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its fatal i have never seen this

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be cautious please

wary hearth Jun 8, 2023, 1:48 PM

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hidden bay ```c++ template<int Index = 0,typename ...Args> auto filterhelper(std::tuple<Arg...

doesn't look right tbh, there are problems with how the return type is dealt with

wary hearth Jun 8, 2023, 1:48 PM

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mellow briar problem is i dont think std tuple is supported as a nttp

depends what you stuff in it, but it works

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probably works at least

hidden bay Jun 8, 2023, 1:49 PM

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wary hearth doesn't look right tbh, there are problems with how the return type is dealt wit...

how

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oh fuck hold on

wary hearth Jun 8, 2023, 1:49 PM

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because the return type of a template function cannot change based on runtime values

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because it's still a function at the end

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what do you think should happen if your if constexpr evals true but the if nested in it evals true/false

hidden bay Jun 8, 2023, 1:51 PM

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is there any suggestions what to do

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how do you filter a tuple, its impossible that this doesnt exist on the internet

wary hearth Jun 8, 2023, 1:52 PM

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I guess to make it more obvious you'd rewrite the thing as

template<int Index = 0,typename ...Args>
auto filterhelper(std::tuple<Args...> tpl,auto func,auto container){
    if constexpr (!Index < sizeof...(Args)) {
        return container;
    }
    else {
        if(func(std::get<Index>(tpl))){
            auto newElementtuple= make_tuple<decltype(std::get<Index>(tpl))>(std::get<Index>(tpl));
            auto newContainer=std::tuple_cat(container,newElementtuple);
            return filterhelper<Index+1>(tpl,func,newContainer);
        }
        else{
            return filterhelper<Index+1>(tpl,func,container);
        }
    }
}

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I told you, you go and learn how to deal with nttp

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and the restrictions on constant expressions

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vs function parameters

hidden bay Jun 8, 2023, 1:57 PM

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this compiles but it always returns empty container, it never reach else statement

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i tried to change position of ! in the if but it creates a new error on filterhelper<Index+1> line

wary hearth Jun 8, 2023, 1:58 PM

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I never said what I just posted would work

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more like, I was saying it wouldn't work

wary hearth Jun 8, 2023, 2:01 PM

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mellow briar problem is i dont think std tuple is supported as a nttp

double checked, and no it isn't supported as nttp

dull mauveBOT Jun 8, 2023, 2:06 PM

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@hidden bay Has your question been resolved? If so, run !solved :)

#Filtering tuples with lambda function