Picking Out Elements Using a Logical List

[Eddie W. Shore]

I am taking an online class in R Programming language (http://www.edx.org), having a great time. The program BOOLLIST is based on the ability in R to pick out elements using logical elements (TRUE, FALSE).


Example (in R):

vector <- [2, 3, 4, 5]
vector[ c(TRUE, TRUE, FALSE, TRUE) ] returns [2, 3, 5]

The logical vector doesn’t have to be same length as the source vector. If the logical vector has elements than the source vector.

vector <- [1, 2, 3, 4, 5, 6]
vector[ c(TRUE, FALSE, TRUE, FALSE, TRUE, FALSE) ] returns [1, 3, 5]
vector[ c(TRUE, FALSE) ] returns [1, 3, 5] (TRUE, FALSE pattern recycles)

Program BOOLLIST:

Input: BOOLLIST(source list, logical list)

Notes: Use list brackets { }. For the logical list, use 1 for TRUE and 0 for FALSE.

Code:

EXPORT BOOLLIST(LA, LB)
BEGIN

LOCAL LC, n, s, k, j;

//  Initialization
LC≔{ };
j≔1;
s≔SIZE(LA);
b≔SIZE(LB);

// Process
FOR k FROM 1 TO s DO
 
IF LB(j)==1 THEN
LC≔CONCAT(LC,LA(k));
END;

j≔j+1;

IF j>n THEN
j≔1;
END;

END;

RETURN LC;

END;

Examples:

In addition to the examples above that can be tried with BOOLLIST:

BOOLLIST( {4,2,3,6}, {1,0,0,1} ) returns {4, 6}

BOOLLIST( {3,9,6,-1,6}, {1,0} ) returns {3, 6, 6}

http://edspi31415.blogspot.com/2015/08/h...using.html

[Didier Lachieze]

Nice function ! I was expecting it would be very easy to implement it with the current Prime list functions but I've not found a simple solution without a FOR loop.
So, here is another way to do it:

Code:

EXPORT BOOLLIST(LA,LB)
BEGIN
  LOCAL LC,k;
  LC:={};
  FOR k FROM 0 TO SIZE(LA)-1 DO
    IF LB(k MOD SIZE(LB)+1) THEN 
      LC(0):=LA(k+1); 
    END;
  END;
  RETURN LC;
END;

Note: storing to LC(0) is used to add an element to the list LC.

[roadrunner]

Here is a method without a for loop:

Code:


export BOOLLIST3(LA,LB)
BEGIN
 local LC:={};
 local s:=size(LA);
 IF LB(1) THEN LC(0):=LA(1);END; 
 IF s==1 THEN return LC;end;
 LC:=concat(LC,BOOLLIST3(SUB(LA,2,s),CONCAT(SUB(LB,2,SIZE(LB)),LB(1))));
END;

Don't know if it qualifies as simple or not; it runs substantially slower than your version.

[roadrunner]

And another version using no declared variables and no for loop:

Code:

export BOOLLIST4(LA,LB)
BEGIN
 IF LB(1) THEN 
  IF size(LA)==1 THEN return LA(1);end;
  return concat(LA(1),BOOLLIST4(SUB(LA,2,size(LA)),CONCAT(SUB(LB,2,SIZE(LB)),LB(1)))​);
 end;
 if size(LA)==1 then return {};end;
 BOOLLIST4(SUB(LA,2,size(LA)),CONCAT(SUB(LB,2,SIZE(LB)),LB(1)));
END;

[Didier Lachieze]

Nice idea to use the recursion to solve this problem. Here is a variation of your first program:

Code:

EXPORT BOOLLIST5(LA,LB)
BEGIN
  LOCAL LC:={};
  IF LB(1) THEN LC(1):=LA(1); END;
  IF SIZE(LA)==1 THEN RETURN LC; END;
  LB(0):=LB(1);
  CONCAT(LC,BOOLLIST5(tail(LA),tail(LB)));
END;

[Thomas Klemm]

(08-17-2015 10:56 AM)Didier Lachieze Wrote:  Nice idea to use the recursion to solve this problem.

[roadrunner]

(08-17-2015 10:56 AM)Didier Lachieze Wrote:  Here is a variation of your first program

I was not aware of the tail function (nor the head function). Thank you for sharing.