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| -rw-r--r-- | challenge-140/james-smith/README.md | 43 |
1 files changed, 22 insertions, 21 deletions
diff --git a/challenge-140/james-smith/README.md b/challenge-140/james-smith/README.md index c80fdc8c5e..8689d712fd 100644 --- a/challenge-140/james-smith/README.md +++ b/challenge-140/james-smith/README.md @@ -104,38 +104,39 @@ with output: ## The solution -Obviously there are two parts to this - a first pass which finds all the numbers and a second pass which counts to find the `$k`th element. +This is written as a 1-liner as so: ```perl sub get_num { - my($i,$j,$k,$t,%h) = @_; - $t=$_, map { $h{$t*$_}++ } 1..$j for 1..$i; - $k-=$h{$_}, ($k<1) && (return $_) for sort { $a<=>$b } keys %h; + my$t; + (sort{$a<=>$b}map{++$t;map{$t*$_}1..$_[0]}1..$_[1])[$_[2]-1]; } ``` -Here we do some *naughty* code (as in challenge 1), using `,` to perform multiple commands in one line; using `map` to perform a `for` -loop (altering values & ignoring the result) and using `&&` to simulate an `if` statement. +We loop through the each row for each column and create an array of the results -In this function each of these is written as a single line. We can expand each of these functions out to see how the algorithm works: +```perl +map{++$t;map{$t*$_}1..$i}1..$j +``` + +We sort the resultant array which gives a list. We wrap this in `()` to convert it into an array. + +We then and take the `$k-1` element of the array (the array is `0` based, where the question is `1` based) + +There is one bit of *naughty* where we have two statements in the outer `map` because we can't refer to both the value of the inner and outer loop as they would both be `$_`. + +We can expand the procedure out to a more readable form: ```perl sub get_num { - my($i,$j,$k,$t,%h) = @_; - for $t (1..$i) { - $h{$t*$_}++ for 1..$j; - } - for (sort {$a<=>$b} keys %h) { - $k -= $h{$_}; - return $_ if $k<1; + my($i,$j,$k,@A) = @_; + foreach my $t (1..$j) { + push @A,map{$t*$_} 1..$i; } + @A = sort @A; + return $A[ $k-1 ]; } ``` -## Notes - * In the `my` statement we initalise the first 3 parameters with the values passed in, the remaining 2 values `$t` and `%h` are not assigned a value. - * The first `for` loop (`for` can be used in place of `foreach` in perl, simply stores the numbers as keys to a hash, whose values are the "frequency" of the number occuring. - * The second one finds the answer. We first thing we do is sort the numbers into order as the keys of the hash are un-ordered. - * Rather than working up to `$k` we can work down from it to `0`. So we subtract the frequency of the current number and if the - value is less than `1` then we know this is the number we are looking for and return it's value. - * Note we always return in the `for` loop unless there is no answer - so don't need a return at the end. + +where we do each of the code blocks above as separate statements, and avoid the double `map` by using `for`/`push` to create the array. |