# Facing Problem while appending elements In a list

I was working on a Question which was ,

Input - A list of numbers - [1,2,3,4,5] #example

We have to produce the subsets of this list and display those subsets whose sum is equal to 10

Output Format - [ [1,2] , [2,3] , [4,5,6] ] #example

Means we have to append those subset in a list whose sum is 10 and make a nested list and print it as an output.

here is my code …

``````defmodule Combination do
def combine([]) do
[]
end
tail_combinations = combine(tail)
merged_combinations = Enum.map(
[[]] ++ tail_combinations,
fn c -> c ++ [head] end
)
tail_combinations ++ merged_combinations
end

#Main thing I am doing here, i am Checking the sum of the subsets but unable to append the list of subset into another list and print the final list

def display(lis) do
a=[]
for vale <- Combination.combine(lis) do
if Enum.sum(vale)==6 do
a=a++[vale]
end
end
IO.inspect(a)
end
end
Combination.display([1,2,3,4])
``````

Error :

``````erts@eyrc:~/fr_task0_9999/.vscode\$ elixir practice.exs
warning: variable "a" is unused (if the variable is not meant to be used, prefix it with an underscore)
practice.exs:17: Combination.display/1

[]
``````

Can anyone help me i am new to elixir and i got stuct at this point.
Tell me how can i append the list in another list and print the whole list at once .

thank you

variable are immutable, you would need to use something like Enum.reduce.

``````def display(list) do
list
|> Combination.combine()
|> Enum.reduce([], fn val, acc ->
if Enum.sum(val) == 6 do
[val | acc]
else
acc
end
end)
end
``````

To display the list you could just pipe the result to IO.inspect

``````Combination.display([1,2,3,4]) |> IO.inspect()
``````

I think this should give you what you want but looking at your combine, you might neet to reverse order some things.

I get the following output with your combine and my display:

``````iex> Combination.display([1,2,3,4,5])
[[3, 2, 1], [5, 1], [4, 2]]
``````
6 Likes

Thanks a Lot Brother , i was stuct in this for last 3days

This is a typical code, just not for FP.
There is also a problem of scope, everything inside a do end block, will never make its way to the surrounding block.

3 Likes

I am facing another problem with the output
i.e this

when ever my
input is like - [1,2,3,4,5,6,7] , it gives me like this :-

``````erts@eyrc:~/fr_task0_9999/.vscode\$ elixir practice.exs
[[4, 3, 2, 1], [7, 2, 1], [6, 3, 1], [5, 4, 1], [5, 3, 2], [7, 3], [6, 4]]
``````

And when i input something like this- [3, 5, 2, 7, 4, 2, 3] it gives me this :-

``````erts@eyrc:~/fr_task0_9999/.vscode\$ elixir practice.exs
[
[2, 5, 3],
[2, 5, 3],
[3, 2, 2, 3],
[7, 3],
[3, 4, 3],
[3, 2, 5],
[3, 2, 5],
[3, 7]
]
``````

After puting several list i noticed that when ever my list is having repeating numbers like - [3, 5, 2, 7, 4, 2, 3] this

it shows like second one , but this not happens every time.

my code is this :

``````defmodule Combination do
def combine([]) do
[]
end
tail_combinations = combine(tail)
merged_combinations = Enum.map(
[[]] ++ tail_combinations,
fn c -> c ++ [head] end
)
tail_combinations ++ merged_combinations
end
def display(list,v) do
list
|> Combination.combine()
|> Enum.reduce([], fn val, acc ->
if Enum.sum(val) == v do
[val|acc]
else
acc
end
end)
|>IO.inspect()
end
end
Combination.display([3, 5, 2, 7, 4, 2, 3],10 )

``````

I want the output in linear format like this - [[1,2], [3,4], [5,6]]

Maybe it would be easier if you could explain what your `combine/1` is supposed to be doing. As in what do you expect the outcome to be when calling that function with a list of integers. We might be able to better help if we understand the reason for the function and not just the output

1 Like

basically my combine/1 is creating subset of the given list
actually i have taken this code from internet
and yes i think the output of my combine is affecting the output of my display/1
this is what i get from my combine/1

``````Combination.combine([1,2,3,4])|>IO.inspect()
``````
``````erts@eyrc:~/fr_task0_9999/.vscode\$ elixir practice.exs
[
[4],
[3],
[4, 3],
[2],
[4, 2],
[3, 2],
[4, 3, 2],
[1],
[4, 1],
[3, 1],
[4, 3, 1],
[2, 1],
[4, 2, 1],
[3, 2, 1],
[4, 3, 2, 1]
]
``````
``````Combination.combine([1,2,3])|>IO.inspect()
``````
``````erts@eyrc:~/fr_task0_9999/.vscode\$ elixir practice.exs
[[3], [2], [3, 2], [1], [3, 1], [2, 1], [3, 2, 1]]
``````

So can u tell me why this is happening ??

Are you maybe looking for something like Enum.chunk_every/2?

``````iex> Enum.chunk_every([1, 2, 3, 4, 5, 6], 2)
[[1, 2], [3, 4], [5, 6]]
``````

You’re saying display the subsets with a sum of 10, but the sums you’re outputting is 3, 5, 15?

This is another question related to this

https://elixirforum.com/t/beginner-needs-help-with-sum-of-subsets/50511/78

1 Like

I was just giving u the example of output format

The output is the same. It’s just IEx deciding to put the results on a new line. Probably because the result is too long to put on one line.

no no i want to get the subsets of the list

4+3+2+1=10, 7+2+1=10, 6+3+1=10,…

2+5+3=10, 2+5+3=10, 3+2+2+3=10,…

I don’t see the problem. Both examples give the same correct output, don’t they?

Can someone help me with this error too

You are missing a function… as the error message is saying.

What did You try to solve this task?

already defined sum_of_one (the function showed missing) in another doctest just before this one

Is that a question for e-yantra challenge?

yes it is

There are a significant amount of questions related to this…

You might show some of your code You have tried so far to increase the chance to have an answer. It is supposed to be a personal work.

1 Like