TIMTOWTDI, but PDL is not about explicit loops. Not how it works. Consider this vector:

pdl> p $v = pdl split '', '30373'
[3 0 3 7 3]

[download]

Now then e.g. the cumulative sum or running total:

pdl> p cumusumover $v
[3 3 6 13 16]

[download]

Not very useful, but I have a vague feeling that "cumulative maximum" would be handy if someone is looking at a forest. You could type 

pdl> ?? cumulative

[download]

to investigate what's available. OK, then, consider "strictly lower triangular matrix" or whatever it's called in mathematics if non-zero members are all 1's:

pdl> p $SLTM

[
 [0 0 0 0 0]
 [1 0 0 0 0]
 [1 1 0 0 0]
 [1 1 1 0 0]
 [1 1 1 1 0]
]

[download]

Hm-m, looks interesting. Not sure about forestry, but what if it's multiplied by our vector?

pdl> p $SLTM * $v

[
 [0 0 0 0 0]
 [3 0 0 0 0]
 [3 0 0 0 0]
 [3 0 3 0 0]
 [3 0 3 7 0]
]

[download]

That's even more interesting! What could we use this result for? What about computing maximum over rows?

pdl> p maxover $SLTM * $v
[0 3 3 3 7]

[download]

Now that's what I call interesting! Isn't it cumulative maximum of all elements before current for original vector? Let's check it without further ado:

pdl> p $v > maxover $SLTM * $v
[1 0 0 1 0]

[download]

But wait, that's exactly the answer we were looking for -- what trees are seen if forest patch is looked at from the left!

And all the rest was just further expansion of investigation performed above.

In reply to Re: first stumbling steps in PDL by Anonymous Monk
in thread first stumbling steps in PDL by Discipulus

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