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f5deffb | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 | \ Return-stack words and DO ... LOOP. Halts with 45 18 10 55 7 3 3 10 on the stack
\ (bottom to top).
\ Sum of the indices 0 .. n-1, using a DO ... LOOP and I.
: TRI ( n -- 0+1+...+n-1 ) 0 SWAP 0 DO I + LOOP ;
\ Sum by recursion, parking n on the return-data stack across the recursive call.
: SUMR ( n -- 0+1+...+n ) DUP 0= IF EXIT THEN DUP >R 1 - RECURSE R> + ;
\ A word that works on a value another word left on the return-data stack.
: BUMP ( -- ) ( R: x -- x+1 ) R> 1 + >R ;
\ EXIT inside a loop leaves the loop parameters (index, limit) on the return-data stack.
: FIND3 ( -- 3 ) 10 0 DO I 3 = IF I EXIT THEN LOOP 99 ;
10 TRI \ 45
0 3 0 DO 4 0 DO I J * + LOOP LOOP \ 18: nested loops, J is the outer index
0 5 1 DO I TRI + LOOP \ 10: a loop calling a word with its own loop
10 SUMR \ 55
5 >R BUMP BUMP R> \ 7
FIND3 R> R> \ 3, then the leftover index 3 and limit 10
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