# 3 List Comprehensions

### 3.1 Simple Examples

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].
[a,4,b,5,6]
```

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, integer(X), can be added to restrict the result to integers:

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

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]].
[{1,a},{1,b},{2,a},{2,b},{3,a},{3,b}]
```

### 3.2 Quick Sort

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.

### 3.3 Permutations

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]).
[[b,u,g],[b,g,u],[u,b,g],[u,g,b],[g,b,u],[g,u,b]]
```

### 3.4 Pythagorean Triplets

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). [{3,4,5},{4,3,5}] > pyth(50). [{3,4,5}, {4,3,5}, {5,12,13}, {6,8,10}, {8,6,10}, {8,15,17}, {9,12,15}, {12,5,13}, {12,9,15}, {12,16,20}, {15,8,17}, {16,12,20}]

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 ].

### 3.5 Simplifications With List Comprehensions

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)].

### 3.6 Variable Bindings in List Comprehensions

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}]).
[1,2,3,7]
```

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}]).
[2,7]
```

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, ...] ...