Avoiding 'null' output in lists

[Fulvio Gesmundo]

Hi,

I often have the following problem (not really a problem, just a slight annoyance). Just as a silly example, consider the line

apply( 10, i -> ( if i%2 == 0 then i))

that outputs {0, , 2 , , 4 , , 6 , , 8 , }, where the gaps are there because when the if statement is not satisfied a "null" gets spit out.

Now, I have a way to remove all the "null" by doing toList(set ( nameofthelist) - {null} )) but I feel that it is not the right way to do it; also, sometimes the order of the elements in the list is important, and using "set" messes it up.

So my question is: is there a way to make a list like that, but in a way that the output is directly {2,4,6,8}?

Thanks

Fulvio

[Greg]

Dear Fulvio,

To achieve your goal, I would combine a "for" loop with the "continue" function:

i1 : for i to 10 list if i % 2 == 0 then i else continue

o1 = {0, 2, 4, 6, 8, 10}

o1 : List

Alternatively, you could use the "select" command:

i2 : select(10, i -> i % 2 == 0)

o2 = {0, 2, 4, 6, 8}

o2 : List

Greg

[Fulvio Gesmundo]

This is very helpful. I like both the options. Thank you

[Luke Oeding]

Dear Greg,

Is there a way to do this using the apply command? I run into an error:

i1 : : toList apply(1..6, i-> if i == 1 then 1 else continue )

stdio:1:1:(3): error: syntax error at ':'

i1 : stdio:266:57:(3): error: unhandled continue command

stdio:1:37:(3): error: unhandled continue command, with argument

On Monday, September 11, 2017 at 2:08:12 PM UTC+2, Greg wrote:

[Greg]

No.  As I understand it, the BasicList returned by the apply method always has the same number of elements as the input one.

The for loop is much more flexible.

[Luke Oeding]

Ok, thanks!