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Even

rascal-0.41.2

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) {
|1 >>>>  list[int] result = [];
|2 >>>>  for (int i <- [0..max])
|3 >>>>    if (i % 2 == 0)
|4 >>>>      result += i;
|5 >>>>  return result;
|6 >>>>}
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) {
|1 >>>>  result = [];
|2 >>>>  for (i <- [0..max])
|3 >>>>    if (i % 2 == 0)
|4 >>>>      result += i;
|5 >>>>  return result;
|6 >>>>}
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) {
|1 >>>>  result = [];
|2 >>>>  for (i <- [0..max], i % 2 == 0)
|3 >>>>    result += i;
|4 >>>>  return result;
|5 >>>>}
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) {
|1 >>>>  result = for (i <- [0..max], i % 2 == 0)
|2 >>>>    append i;
|3 >>>>  return result;
|4 >>>>}
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) {
|1 >>>>  return for (i <- [0..max], i % 2 == 0)
|2 >>>>           append i;
|3 >>>>}
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) {
|1 >>>>  return [ i | i <- [0..max], i % 2 == 0];
|2 >>>>}
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.