RogerBW's Blog

The Weekly Challenge 195: Frequently Special 18 December 2022

I’ve been doing the Weekly Challenges. The latest involved integer filtering. (Note that this closes today.)

Task 1: Special Integers

You are given a positive integer, $n > 0.

Write a script to print the count of all special integers between 1 and $n.

An integer is special when all of its digits are unique.

Therefore: convert to string, check for uniqueness of characters. (Could have done it as a repeated mod10, but I felt stringy.)

For the uniqueness test, I incremented hashes where that was easy, or used sets where they were readily available.


/specialintegers {
    4 dict begin
    /n exch def
    /o 0 def

For each candidate number:

    1 1 n {
        /f 0 dict def
        /s true def

Convert it to a string and iterate over the characters.

        dup log cvi 1 add string cvs {
            /c exch def

If we've seen it before, mark as non-special and bail out.

            f c known {
                /s false def
            } {

Otherwise record it.

                f c true put
            } ifelse
        } forall

If it's special, increment the output count.

        s {
            /o o 1 add def
        } if
    } for
} bind def

Task 2: Most Frequent Even

You are given a list of numbers, @list.

Write a script to find most frequent even numbers in the list. In case you get more than one even numbers then return the smallest even integer. For all other case, return -1.


sub mostfrequenteven($l) {
    my %f;

Filter for evenness and build a frequency table.

    foreach my $n (@{$l}) {
        if ($n % 2 == 0) {
    if (scalar %f > 0) {

Get the maximum frequency, and sort the keys that have that freqency. The first one is the lowest number.

        my $m = max(values %f);
        my @l = grep {$f{$_} == $m} keys %f;
        @l = sort @l;
        return $l[0];
    } else {

If there were no even numbers, return -1.

        return -1;

Full code on github.

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