Sunday, April 24, 2016

Always study the standard libraries (2)



Here we have another example of the importance of the standard libraries. It all started with this question here, posted at StackOverflow:

The code given by the OP could be fixed, but it had some problems. The main one is the fact it is too particular. It had lines like 

while row.length >= 13

This makes it specific to solve the problem proposed for a single value (13). This is something we must avoid at all costs. Never write code tar particular. It is not worth the effort. In a purely theoretical problem like this one can't see this easily. But if you do that in a system designed for production you'll regret it the first time you need to perform a maintenance.

Then I suggested this code, more generic but basically with the same structure:

sequence = "73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450"

## This will remove all \n
seq = sequence.tr("\n","")

def multiply_arr(a)
    prod = 1
    a.each do |f|
        prod = prod * f.to_i    
    end
    prod
end

def max_product(s,n)
    max = -1
    lim = s.length - n
    (0..lim).each do |pos|
        ns = s.slice(pos,n)
        arr = ns.each_char.to_a
        prod = multiply_arr(arr)
        max = (prod > max) ? prod : max 
    end
    max
end

Good code. Works fine. It is more generic than the code given by the OP. But...

The user Jörg W. Mittag proposed the following code:

def max_product_subsequence(sequence, subsequence_length)
    sequence.
        gsub(/\s+/, ''). 
        each_char.                    
        map(&:to_i).                     
        each_cons(subsequence_length).    
        map {|subseq| subseq.reduce(:*) }. 
        max                              
end

As you may see, this code does the same mine does, but is is much shorter and much more efficient. I benchmarked it and the execution time is much better.

This is a great example of the importance of the standard libraries. Core Ruby provides wonderful functionalities, if you know it well and apply whenever it is possible.

Ah... before I forget... People say good programmers are always lazy. They need to be lazy in order to try to always find the easiest way to do things. But this does not include being lazy to rewrite code.

Never believe your code is good. Always try to find another way to rewrite it.