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Welcome to the Josephus Problem in Algol68 page! Here, you'll find the source code for this program as well as a description of how the program works.
MODE PARSEINT_RESULT = STRUCT(BOOL valid, INT value, STRING leftover); MODE PARSEINTLIST_RESULT = STRUCT(BOOL valid, REF INT values); PROC parse int = (REF STRING s) PARSEINT_RESULT: ( BOOL valid := FALSE; REAL r := 0.0; INT n := 0; STRING leftover; # Associate string with a file # FILE f; associate(f, s); # On end of input, exit if valid number not seen. Otherwise ignore it # on logical file end(f, (REF FILE dummy) BOOL: ( IF NOT valid THEN done FI; TRUE ) ); # Exit if value error # on value error(f, (REF FILE dummy) BOOL: done); # Convert string to real number # get(f, r); # If real number is in range of an integer, convert to integer. Indicate integer is valid if same as real # IF ABS r <= max int THEN n := ENTIER(r); valid := (n = r) FI; # Get leftover string # get(f, leftover); done: close(f); PARSEINT_RESULT(valid, n, leftover) ); PROC usage = VOID: printf(($gl$, "Usage: please input the total number of people and number of people to skip.")); COMMENT Reference: https://en.wikipedia.org/wiki/Josephus_problem#The_general_case Use zero-based index algorithm: g(1, k) = 0 g(m, k) = [g(m - 1, k) + k] MOD m, for m = 2, 3, ..., n Final answer is g(n, k) + 1 to get back to one-based index COMMENT PROC josephus = (INT n, INT k) INT: ( INT g := 0; FOR m FROM 2 TO n DO g := (g + k) MOD m OD; g + 1 ); # Parse 1st and 2nd command-line arguments # INT values; FOR m TO 2 DO STRING s := argv(m + 3); PARSEINT_RESULT result := parse int(s); # If invalid, extra characters, exit # values[m] := value OF result; IF NOT (valid OF result) OR (leftover OF result) /= "" THEN usage; stop FI OD; INT n := values; INT k := values; INT g := josephus(n, k); printf(($gl$, whole(g, 0)))
Josephus Problem in Algol68 was written by:
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Note: The solution shown above is the current solution in the Sample Programs repository as of Jan 31 2023 21:55:33. The solution was first committed on Jan 24 2023 19:58:12. As a result, documentation below may be outdated.
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