3  List Comprehensions

3 List Comprehensions

This section starts with a simple example, showing a generator and a filter:

> [X || X <- [1,2,a,3,4,b,5,6], X > 3].

This is read as follows: The list of X such that X is taken from the list [1,2,a,...] and X is greater than 3.

The notation X <- [1,2,a,...] is a generator and the expression X > 3 is a filter.

An additional filter, is_integer(X), can be added to restrict the result to integers:

> [X || X <- [1,2,a,3,4,b,5,6], is_integer(X), X > 3].

Generators can be combined. For example, the Cartesian product of two lists can be written as follows:

> [{X, Y} || X <- [1,2,3], Y <- [a,b]].

The well-known quick sort routine can be written as follows:

sort([Pivot|T]) ->
    sort([ X || X <- T, X < Pivot]) ++
    [Pivot] ++
    sort([ X || X <- T, X >= Pivot]);
sort([]) -> [].

The expression [X || X <- T, X < Pivot] is the list of all elements in T that are less than Pivot.

[X || X <- T, X >= Pivot] is the list of all elements in T that are greater than or equal to Pivot.

A list sorted as follows:

  • The first element in the list is isolated and the list is split into two sublists.
  • The first sublist contains all elements that are smaller than the first element in the list.
  • The second sublist contains all elements that are greater than, or equal to, the first element in the list.
  • Then the sublists are sorted and the results are combined.

The following example generates all permutations of the elements in a list:

perms([]) -> [[]];
perms(L)  -> [[H|T] || H <- L, T <- perms(L--[H])].

This takes H from L in all possible ways. The result is the set of all lists [H|T], where T is the set of all possible permutations of L, with H removed:

> perms([b,u,g]).

Pythagorean triplets are sets of integers {A,B,C} such that A**2 + B**2 = C**2.

The function pyth(N) generates a list of all integers {A,B,C} such that A**2 + B**2 = C**2 and where the sum of the sides is equal to, or less than, N:

pyth(N) ->
    [ {A,B,C} ||
        A <- lists:seq(1,N),
        B <- lists:seq(1,N),
        C <- lists:seq(1,N),
        A+B+C =< N,
        A*A+B*B == C*C 
> pyth(3).
> pyth(11).
> pyth(12).
> pyth(50).

The following code reduces the search space and is more efficient:

pyth1(N) ->
   [{A,B,C} ||
       A <- lists:seq(1,N-2),
       B <- lists:seq(A+1,N-1),
       C <- lists:seq(B+1,N),
       A+B+C =< N,
       A*A+B*B == C*C ].

As an example, list comprehensions can be used to simplify some of the functions in lists.erl:

append(L)   ->  [X || L1 <- L, X <- L1].
map(Fun, L) -> [Fun(X) || X <- L].
filter(Pred, L) -> [X || X <- L, Pred(X)].

The scope rules for variables that occur in list comprehensions are as follows:

  • All variables that occur in a generator pattern are assumed to be "fresh" variables.
  • Any variables that are defined before the list comprehension, and that are used in filters, have the values they had before the list comprehension.
  • Variables cannot be exported from a list comprehension.

As an example of these rules, suppose you want to write the function select, which selects certain elements from a list of tuples. Suppose you write select(X, L) -> [Y || {X, Y} <- L]. with the intention of extracting all tuples from L, where the first item is X.

Compiling this gives the following diagnostic:

./FileName.erl:Line: Warning: variable 'X' shadowed in generate

This diagnostic warns that the variable X in the pattern is not the same as the variable X that occurs in the function head.

Evaluating select gives the following result:

> select(b,[{a,1},{b,2},{c,3},{b,7}]).

This is not the wanted result. To achieve the desired effect, select must be written as follows:

select(X, L) ->  [Y || {X1, Y} <- L, X == X1].

The generator now contains unbound variables and the test has been moved into the filter.

This now works as expected:

> select(b,[{a,1},{b,2},{c,3},{b,7}]).

Also note that a variable in a generator pattern will shadow a variable with the same name bound in a previous generator pattern. For example:

> [{X,Y} || X <- [1,2,3], X=Y <- [a,b,c]].

A consequence of the rules for importing variables into a list comprehensions is that certain pattern matching operations must be moved into the filters and cannot be written directly in the generators.

To illustrate this, do not write as follows:

f(...) ->
    Y = ...
    [ Expression || PatternInvolving Y  <- Expr, ...]

Instead, write as follows:

f(...) ->
    Y = ...
    [ Expression || PatternInvolving Y1  <- Expr, Y == Y1, ...]