not getting the desired behavior

Community Support
1

Hi all,

for a sample input: x:-1 2 3 4 -1 6 7 8 9 -1

why does this work -> 0 (min+)\x, and this doesn’t (min+)x?

Thanks.

2

Before others can understand your question, would you please specify what is considered “desired behaviour” in the first place?

@see http://www.catb.org/~esr/faqs/smart-questions.html

3

sorry! I was in a bit hurry. Thanks for sharing smart questions link.

So, the question is:

For sample input, x::-1 2 3 4 -1 6 7 8 9 -1, I am expecting same output for both the following expressions,

1) 0 (min+)\x

output -> -1 2 3 4 -1 6 7 8 9 -1

2) (min+)x

output -> -1

Why is the output for 2) not the same as 1)? Where is the thought process going wrong?

Thanks,

4

the first example uses the ‘do’ syntax: https://code.kx.com/q/ref/accumulators/#do

so you are running the function (min+) (which is a curiosity to me) zero times.  the result is thus the same as the input.

the second example runs (min+) which is the same as ‘min sums’ on your input.  the result is thus -1

i think the question is, what did you expect to happen and why do you think they should produce the same output?

5

It’s not ‘min sums x’ but ‘min x’

It appears the interpreter doesn’t like the composition & ‘sums’ is not evaluated

q)x:-1 2 3 4 -1 6 7 8 9 -1
q)(min+)x
-1
q)min(+)x
-1
q)(max+)x
9
q)max(+)x
37
q)((::)+)x
-1 2 3 4 -1 6 7 8 9 -1
q)
q)parse"(min+\)x"
((;+);min)
`x
q)x:20000000#x
q)\ts (+)x
83 268435648
q)\ts (min+)x
17 848
q)\ts min x
18 512

 

6

as your ‘parse’ reveals, it is actually ‘sums min’:

q)0N!parse “sums min”;
(+;min)

and since sums on an atom returns the original atom, the result is ‘min’:

q)sums min
min

7

Thanks Nick, Jason for your explanations! I was trying to understand composition of functions, since the left side of the 2 functions should be a monadic/unary, I randomly picked min. Even for - 0 (min+)\ x, I was expecting the result to be same as (+)x since, I expected it to sum the first two numbers, take min, then use that result to sum with the next number, and so on. I even have question on that.

Why I expected the sample output was:

for a diadic function, f, eg, +, “(f)x” and “0 f\ x” should be result in same output. What I realized is, (min+) is not same as (min+), since it would be evaluated right to left, so (+), and then joining it with min would result in different kind of composition, as pointed by Jason in parse result.

PS: Still thinking why its translating to sums min? Is it bug?