snapsvg

2014-12-08

Day 8: Mindset

It doesn't matter what language you start in. The language doesn't help. The problem is you; you're the new developer, the inexperienced green sapling; you're the one with no instinct, no sense of smell, and no idea where to begin. You probably don't even have a problem you want solving.

Whenever we solve a problem we draw on our knowledge and experience to solve it. Knowledge and experience differ like theory and practice do. Knowledge is the theory. You can know something because you were told it, and it stuck. Arguably, the best way to know something is to understand it; then you know why it is the case, and what you really know is more general, more applicable, and hence more useful. Experience is practice; you've done this before. Experience is the sort of knowledge you need in order to produce a good solution to a problem, because experience tells you what the next problem is, and how to avoid it now.

Experience alters your thought process.

Today's example comes from irc.freenode.com#perl, where we see a green programmer trying to solve a problem:

Report the powers of two that sum to produce a given integer

That is, break down an integer into the powers of two from which it is composed.

Scroll no further if you wish to solve it yourself. In Perl.

No language can provide you, up front, with the knowledge you need to answer this question. Most languages have for loops and while loops, and something that can raise 2 to a power. But that's all you know. You have a few bits of theory, but no experience to draw upon. So your thought process goes something like this:

  • I can take a number n and find the nth power of two 2 ** $n
  • I can store a value and compare it to my target num $total > $num
  • I can loop an indefinite number of times with while
  • The biggest power of two less than num is definitely part of it

You reach the conclusion, using knowledge, that you can subtract ever-decreasing numbers from your target, in a loop. Any number that leaves you with a positive number simply means you can repeat the process with the new number, having remembered that particular power of two.

use 5.010;
use strict;
use warnings;

my $num = shift;
my $power = 0;

$power++ until 2 ** $power > $num;
$power--;

while ( $power ) {
  if ($num - (2**$power) >= 0) {
    say "$power (" . (2**$power) . ")";
    $num -= 2 ** $power;
  }

  $power--;
}
4 (16)
2 (4)

Reasonable. Now here's my thought process:

  • They want all powers of two that come together to sum a number
  • That's how binary works
  • We can ask the binary representation of num for all the on bits
  • The positions of those on-bits are the answer.

So we write that.

say for grep { $_ } map { 2 ** $i++ * $_ } reverse split //, sprintf "%b", shift

This is a one-liner. Try it in perl -E'...' 20, in place of the ....

4
16

OK we'll break it down, but you'll see that each section maps roughly to each of the items in that list.

"They want all powers of two"

The answer is going to be a list. say for LIST, and we have to construct LIST. The powers of two have a test for validity, so there's probably a grep. say for grep { CONDITION } LIST.

We should really build an array for LIST, and use it at the end.

use 5.010;

my @bits;
...

say for @bits;

"That's how binary works"

Getting the binary representation of a number is easy; sprintf "%b", EXPR. In the one-liner we used shift to take the first command-line argument. We can put $num here and save the result of sprintf instead of using it directly.

my $num = shift;
my $binary = sprintf "%b", $num;

"We can ask the binary representation for all the on bits"

How? This is a two-parter. First you have to turn the string into bits. Then you have to find the on-bits.

Turning the string into bits is easy - you split it on the gap between characters:

my @bits = split //, $binary;

Not obvious is the finding the on-bits. See, we don't want the actual bits themselves; all the on-bits are 1, so finding them all would simply tell us how many there are. We actually want to know where they are.

Trouble is, sprintf gives us 10100 for 20. The first bit is the high bit, but that has the smallest offset, i.e. it's the 0th digit in that string. And the other 1 is the 2th digit. Knowledge tells us that our 20 working example should report 4 and 16; but 2 ** 0 is neither of those, even though 2 ** 2 is.

The answer to this is actually in the original solution: we have to work backwards, biggest number last. That's why we reverse it.

my @bits = reverse split //, $binary;

"The positions of those on-bits are the answer"

In the final solution I report the powers of two, not the numbers we raise two to, and the positions are the numbers to raise two to, not the power of two to that. Clear?

The positions of the on-bits are found using a bit of a naughty map, which uses a counter outside its scope. map should really not have side-effects. We can work around this in a proper script, however.

By iterating through the bits and incrementing a counter as we go, we can determine the value that this bit represents.

2 ** $i++

$i++

of course returns the value of $ibefore incrementing it, meaning it starts off undefined. We can't have that.

my $i = 0;

Now we can produce a list of all those values:

map { 2 ** $i++ } @bits;

Plug this into say for debugging purposes:

say for map { 2 ** $i++ } @bits;
1
2
4
8
16

We've lost information - what happened to the fact some of the bits were turned off? Although I had this in knowledge, it was experience that reminded me that I can multiply:

map { 2 ** $i++ * $_ } @bits;

That's better - we also should always use $_ in a map because map is supposed to transform $_.

0
0
4
0
16

Now we have something we can grep: $_ itself!

my @powers = map { 2 ** $i++ * $_ } @bits;
say for grep { $_ } @powers;

This collects all powers, but only reports those with a nonzero value.

We can fix the $i situation by using keys on @bits. keys on an array returns the list of indices, even though they're not really keys.

map { 2 ** $_ * $bits[$_] } keys @bits

This uses $_ in place of $i (0 to 4), but now that $_ is the index, we have to get the actual bit value by looking it up in @bits.

Answers on a postcard, please

Here's the final script, then

use 5.010;
  use strict;
  use warnings;

  my $num = shift;
  my $binary = sprintf "%b", $num;
  my @bits = reverse split //, $binary;

  my @powers = map { 2 ** $_ * $bits[$_] } keys @bits;

  say for grep { $_ } @powers;