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Welcome to the Factorial 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); 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 a non-negative integer")); # Allow answer to be up to 201 digits (googol squared!) # PR precision=201 PR PROC factorial = (INT n) LONG LONG INT: ( # Multiply from 1 through n (note that 0! = 1) # LONG LONG INT fact := 1; FOR k FROM 2 TO n DO # Exit if next multiplication will cause an overlow # IF fact > long long max int / k THEN putf(stand error, ($gl$, "Overflow!")); stop FI; fact *:= k OD; fact ); # Parse 1st command-line argument # STRING s := argv(4); PARSEINT_RESULT result := parse int(s); # If invalid, extra characters, or negative, exit # INT n := value OF result; IF NOT (valid OF result) OR (leftover OF result) /= "" OR n < 0 THEN usage; stop FI; # Calculate and display factorial # LONG LONG INT fact := factorial(n); print((whole(fact, 0), newline))
Factorial 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 30 2023 18:32:37. The solution was first committed on Jan 21 2023 16:17:56. As a result, documentation below may be outdated.
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