explanation of x('[min;+])/: x

[jainbhavya_p24]

Hi KDB gurus, can anyone please explain the meaning of the expression: x('[min;+])/: x, especially the function composition '[min;+] and its use with /:, here x is a matrix. TIA!

[kadir_kilicoglu]

Hello Bhavya,

/: is the each-right operator, you can see the documentation at  https://code.kx.com/q/ref/maps/#each-left-and-each-right 

The ' is the compose operator, hence you are composing a function over something.

Finally, we can read the function  x('[min;+])/: x as "find the minimum element of x and add this to all elements in x. 

Hope it helps, regards.

[george_winton_h]

It only makes sense for square matrices. Here are some equivalents which I hope provide clarity:

x {min x+y}/: x

{min x+y}[x;] each x

(min +[x;]@) each x

(math)

r_{i,*} = min_j x_{i,j} + x_{j,*}

(python)

result = []

for r in x:

    candidates = []

    for i in range(len(r)):

        candidates.append([v+r[i] for v in x[i]])

    result.append(min(candidates)) # lexographical min

result

[jainbhavya_p24]

Thanks a lot for explaining with the alternate forms of the expression. It is indeed min plus matrix multiplication used in finding the shortest distance between nodes(ref: https://code.kx.com/q/kb/lp/). 

I am more interested in understanding the idea behind the original expression, basically '[min;+] part in x('[min;x])/: x, as in why and how it is expanding to "min each x +/: x".

All help appreciated!

[JFSawyer88]

The usage of the single quote here is as the compose operator (https://code.kx.com/q/ref/compose/), which takes two functions and composes them into one, applying the second argument followed by the first.

In this specific case '[min;+] is a function of rank two (matching +), which sums the two inputs and returns the min of the result. 

In fact, compositions have a special type in q: https://code.kx.com/q/basics/datatypes/

q)type ('[min;+])

105h

[TerryLynch]

It's the same as

{min each x+/:x}

Could possibly be an implementation of min-plus matrix multiplication? Not sure on that. 

Terry