How to use a variable as pass by reference in Erlang?

Question

Why is my output not getting reflected in Lst1?

-module(pmap). 
-export([start/0,test/2]). 

test(Lst1,0) ->
   {ok, [Temp]} = io:fread( "Input the edge weight  ", "~d" ),
   lists:append([Lst1,[Temp]]),
   io:fwrite("~w~n",[Lst1]);

test(Lst1,V) ->
   {ok, [Temp]} = io:fread( "Input the edge weight  ", "~d" ),
   lists:append([Lst1,[Temp]]),
   test(Lst1, V-1).

start() -> 
   {ok, [V]} = io:fread( "Input the number of vertices your graph has  ", "~d" ),
   Lst1 = [],
   test(Lst1,V).

So, my Lst1 is printing [], whereas I expect it to print, suppose, [1,2,3] if I provide input 1,2,3.

score 4 · Answer 1 · answered Nov 07 '20 at 15:44

4

Because Erlang variables are immutable and can't be changed at all. lists:append returns a new list which you throw away.

answered Nov 07 '20 at 15:44

Alexey Romanov

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Thanks for your comment, What is the alternative option to achieve this? I am very new to Erlang. – Manika Sharma Nov 07 '20 at 15:46
1

Your `test` function should return the new list. Brujo Benavides' answer shows some ways to do that. – Alexey Romanov Nov 07 '20 at 20:40

score 3 · Accepted Answer · answered Nov 07 '20 at 18:20

You're not using the result of lists:append/2, as @Alexey Romanov correctly pointed out.

This is how I would fix your code…

-module(pmap). 
-export([start/0,test/2]). 

test(Lst1,0) ->
    {ok, [Temp]} = io:fread( "Input the edge weight  ", "~d" ),
    Lst2 = lists:append([Lst1,[Temp]]),
    io:fwrite("~w~n",[Lst2]),
    Lst2;
test(Lst1,V) ->
    {ok, [Temp]} = io:fread( "Input the edge weight  ", "~d" ),
    Lst2 = lists:append([Lst1,[Temp]]),
    test(Lst2, V-1).

start() -> 
   {ok, [V]} = io:fread( "Input the number of vertices your graph has  ", "~d" ),
   Lst1 = [],
   test(Lst1,V).

But actually, a more idiomatic code to achieve the same result would be…

-module(pmap). 
-export([start/0,test/2]). 

test(Lst1,0) ->
    {ok, [Temp]} = io:fread( "Input the edge weight  ", "~d" ),
    Lst2 = lists:reverse([Temp|Lst1]),
    io:fwrite("~w~n",[Lst2]),
    Lst2;
test(Lst1,V) ->
    {ok, [Temp]} = io:fread( "Input the edge weight  ", "~d" ),
    test([Temp | Lst1], V-1).

start() -> 
   {ok, [V]} = io:fread( "Input the number of vertices your graph has  ", "~d" ),
   Lst1 = [],
   test(Lst1,V).

How to use a variable as pass by reference in Erlang?

2 Answers2