--- /dev/null
+! Copyright (c) 2008 Aaron Schaefer.
+! See http://factorcode.org/license.txt for BSD license.
+USING: kernel math math.parser sequences strings ;
+IN: project-euler.040
+
+! http://projecteuler.net/index.php?section=problems&id=40
+
+! DESCRIPTION
+! -----------
+
+! An irrational decimal fraction is created by concatenating the positive
+! integers:
+
+! 0.123456789101112131415161718192021...
+
+! It can be seen that the 12th digit of the fractional part is 1.
+
+! If dn represents the nth digit of the fractional part, find the value of the
+! following expression.
+
+! d1 × d10 × d100 × d1000 × d10000 × d100000 × d1000000
+
+
+! SOLUTION
+! --------
+
+<PRIVATE
+
+: (concat-upto) ( n limit str -- str )
+ 2dup length > [
+ pick number>string over push-all rot 1+ -rot (concat-upto)
+ ] [
+ 2nip
+ ] if ;
+
+: concat-upto ( n -- str )
+ SBUF" " clone 1 -rot (concat-upto) ;
+
+: nth-integer ( n str -- m )
+ [ 1- ] dip nth 1string 10 string>integer ;
+
+PRIVATE>
+
+: euler040 ( -- answer )
+ 1000000 concat-upto { 1 10 100 1000 10000 100000 1000000 }
+ [ swap nth-integer ] with map product ;
+
+! [ euler040 ] 100 ave-time
+! 1002 ms run / 43 ms GC ave time - 100 trials
+
+MAIN: euler040
! Basically, this makes an array of 1000000 zeros, recursively creates
! primitive triples using the three transforms and then increments the array at
! index [a+b+c] by one for each triple's sum AND its multiples under 1000000
-! (to account for non-primitive triples). The answer is just the number of
-! indexes that equal one.
+! (to account for non-primitive triples). The answer is just the total number
+! of indexes that are equal to one.
SYMBOL: p-count
project-euler.025 project-euler.026 project-euler.027 project-euler.028
project-euler.029 project-euler.030 project-euler.031 project-euler.032
project-euler.033 project-euler.034 project-euler.035 project-euler.036
- project-euler.037 project-euler.038 project-euler.039 project-euler.067
- project-euler.075 project-euler.134 project-euler.169 project-euler.173
- project-euler.175 ;
+ project-euler.037 project-euler.038 project-euler.039 project-euler.040
+ project-euler.067 project-euler.075 project-euler.134 project-euler.169
+ project-euler.173 project-euler.175 ;
IN: project-euler
<PRIVATE