66 lines
1.6 KiB
Forth
66 lines
1.6 KiB
Forth
# palindromic-numbers.retro
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## Description
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December: Palindromic numbers
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Palindromic Numbers: The programming challenge for the Saturday, Dec. 19
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Silicon Valley Forth Interest Group meeting. Numbers the same forward
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and backward as for RADAR but with numbers. Such as 88 and 666 and
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12321.
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Program a generator or a filter to find all palindromic integers from
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1 (yes, it is) to 99999. Please confirm your interest to Bill Ragsdale,
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bill@billragsdale.cc
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## Code & Commentary
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Well this is really easy. I begin by creating an array of potential
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values.
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~~~
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#100000 [ I n:inc , ]
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'Potentials d:create over , indexed-times
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~~~
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Then a simple `palindrome?` word to convert the number to a string and
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return a flag indicating if it's a palindrome.
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~~~
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:palindrome? n:to-string dup s:reverse s:eq? ;
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~~~
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Use `palindrome?` to filter out invalid values.
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~~~
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&Potentials [ palindrome? ] a:filter
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~~~
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And finally, display the palindromes.
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~~~
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[ n:put nl ] a:for-each
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~~~
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## Notes
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*Numeric Bases*
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While Retro only supports decimal values by default, if you have
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extended `n:to-string` to support a `Base`, this will still work with
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other bases.
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*Memory Use*
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In my tests, this isn't memory efficient, as I'm keeping both the list
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of potentials and the results in memory, so it ends up consuming about
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101k memory locations. If the results don't need to be kept, a much
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smaller solution would look like:
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:palindrome? n:to-string dup s:reverse s:eq? ;
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:value I n:inc dup ;
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:process [ n:put nl ] &drop choose ;
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#100000 [ value palindrome? process ] indexed-times
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This only uses 74 memory locations.
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