Meet the Zen monks

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In Jeff Borrors classic textbook Q for Mortals you will find frequent references to moments of Zen meditation leading to flashes of insight into the workings of q.

My teacher Myokyo-ni liked to quote Dogen-zenji:

The Great Way is not difficult. It avoids only picking and choosing.
The Do form of the Scan iterator has a pattern I think of as the Zen monks.

 

How many Zen monks does it take to change a lightbulb? Two. One to change it; one not to change it.
The basic pattern is to apply a function and not to apply it. Consider the trim keyword. It must find the spaces in a string, then the continuous spaces from each end. If we had to write trim in q it might be

 

q){b:x<>" ";(b?1b)_ neg[reverse[b]?1b] _ x}" Trim the spaces. " 
"Trim the spaces."

 

We notice the repetitions:

  • both b and reverse[b] are searched for 1b
  • two uses of the Drop operator
We want to do the search/drop thing from both ends of the string.

 

q){x{y _ x}/1 -1*(1 reverse" "<>x)?'1b}" Trim the spaces. " 
"Trim the spaces."

 

Notice the {y _ x} reduction above. Lambda {y f x} commutes a function f by switching its arguments. The pattern R{y f x}/L successively applies a list of left arguments L to an argument R.

Here we use 1 reverse to get the boolean vector and its reversal. I think of this 1 f pattern as the Zen monks.

Here is another use for it, in finding the shape (rows and columns) of a matrix.

 

q)show m:{max[count each x]$'x}string`avoids`picking`and`choosing "avoids " "picking " "and " "choosing" 
q)shp:{count each 1 firstx} / shape of a matrix 
q)shp m 4 8

 

The Zen Buddhist pension plan: A day without work is a day without food. Can you see any other work for the monks?

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This 1 f pattern is crazy, but i love it

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Such a great post - so interesting!

Thanks for sharing @SJT

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Thanks for sharing @SJT !