# Even

rascal-0.33.7

#### Synopsis​

Produce a list of even numbers.

#### Description​

Let's write a function that generates all the even numbers in a list up to a certain maximum. We will do it in a few alternative ways: from very imperative to very declarative and some steps in between.

``rascal>list[int] even0(int max) {>>>>>>>  list[int] result = [];>>>>>>>  for (int i <- [0..max])>>>>>>>    if (i % 2 == 0)>>>>>>>      result += i;>>>>>>>  return result;>>>>>>>}list[int] (int): function(|prompt:///|(0,135,<1,0>,<7,1>))rascal>even0(25);list[int]: [0,2,4,6,8,10,12,14,16,18,20,22,24]``

Now lets remove the temporary type declarations:

``rascal>list[int] even1(int max) {>>>>>>>  result = [];>>>>>>>  for (i <- [0..max])>>>>>>>    if (i % 2 == 0)>>>>>>>      result += i;>>>>>>>  return result;>>>>>>>}list[int] (int): function(|prompt:///|(0,121,<1,0>,<7,1>))rascal>even1(25);list[int]: [0,2,4,6,8,10,12,14,16,18,20,22,24]``

To make the code shorter, we can inline the condition in the for loop:

``rascal>list[int] even2(int max) {>>>>>>>  result = [];>>>>>>>  for (i <- [0..max], i % 2 == 0)>>>>>>>    result += i;>>>>>>>  return result;>>>>>>>}list[int] (int): function(|prompt:///|(0,111,<1,0>,<6,1>))rascal>even2(25);list[int]: [0,2,4,6,8,10,12,14,16,18,20,22,24]``

In fact, for loops may produce lists as values, using the append statement:

``rascal>list[int] even3(int max) {>>>>>>>  result = for (i <- [0..max], i % 2 == 0)>>>>>>>    append i;>>>>>>>  return result;>>>>>>>}list[int] (int): function(|prompt:///|(0,102,<1,0>,<5,1>))rascal>even3(25);list[int]: [0,2,4,6,8,10,12,14,16,18,20,22,24]``

So now, the result temporary is not necessary anymore:

``rascal>list[int] even4(int max) {>>>>>>>  return for (i <- [0..max], i % 2 == 0)>>>>>>>           append i;>>>>>>>}list[int] (int): function(|prompt:///|(0,90,<1,0>,<4,1>))rascal>even4(25);list[int]: [0,2,4,6,8,10,12,14,16,18,20,22,24]``

This code is actually very close to a list comprehension already:

``rascal>list[int] even5(int max) {>>>>>>>  return [ i | i <- [0..max], i % 2 == 0];>>>>>>>}list[int] (int): function(|prompt:///|(0,71,<1,0>,<3,1>))rascal>even5(25);list[int]: [0,2,4,6,8,10,12,14,16,18,20,22,24]``

And now we can just define `even` using an expression only:

``rascal>list[int] even6(int max) = [i | i <- [0..max], i % 2 == 0];list[int] (int): function(|prompt:///|(0,59,<1,0>,<1,59>))rascal>even6(25);list[int]: [0,2,4,6,8,10,12,14,16,18,20,22,24]``

Or, perhaps we prefer creating a set instead of a list:

``rascal>set[int] even7(int max) = {i | i <- [0..max], i % 2 == 0};set[int] (int): function(|prompt:///|(0,58,<1,0>,<1,58>))rascal>even7(25);set[int]: {10,16,8,14,20,2,4,6,24,12,22,18,0}``

#### Benefits​

• You can program in for loops and use temporary variables if you like.
• Comprehensions are shorter and more powerful.
• There are comprehensions for lists, sets, and maps

#### Pitfalls​

• Trainwreck alert: if you start putting too many conditions in a single for loop or comprehension the code may become unreadable.