5 < x != y < 5

General Usage
1

Hi All,
I am reviewing The Fast Track to julia. In the Basics section Chaining is mentioned. Three lines of code are:

x = y = z = 1 # right-to-left
0 < x < 3 # true
5 < x != y < 5 # false

It’s the last line that I can’t figure out the logic flow. Can anyone provide me with sufficient details as to how I should evaluate this last line? The Fast Track states it is false as per their comment. I get the same result if I code just the third line and set x=1 and y=1. But if I set x=6 and y=2 it evaluates to true. Any help is greatly appreciated.

Thanks!

Jim

1 Like
2

https://docs.julialang.org/en/v1/manual/mathematical-operations/#Operator-Precedence-and-Associativity

they all have the same precedence and are not associative

3

rather this subsection Mathematical Operations and Elementary Functions · The Julia Language

the last line should be equivalent to (5 < x) && ( x != y) && (y < 5)

13 Likes
4

Yes, but from line 1: x = y = z = 1

5

I think the question is more about how to arrive at the answer, than the very specific answer to this specific problem

6

jling, Thank you so much for the link! I should have checked the Manual as well.

Take care, Jim

7

sz73, You have the best answer. Thank you for providing the logic! Now it makes sense to me. The link helps as well.

Take care, Jim.

1 Like
8

I’d suggest the best way to figure this stuff out is to quote the expression and dump it out:

dump(:(5 < x != y < 5))

Then you can reason out why the expression was parsed the way it was after seeing unambiguously how it’s being parsed.

14 Likes
9

Thanks, pbayer! Yes, it’s the logic I’m trying to figure out and sz73’s equivalent logic definitely helps.

Take care, Jim.

10

JMW, I will definitely try the dump feature and see what I come up with. Thanks for the insight!

Jim

11

You can mark his answer as “the answer”, if that helps.

1 Like
12

Thanks, briochemc. Newbie here. Will do.

Take care, Jim.

1 Like
13

Or, more easily perhaps, Meta.@dump 5 < x != y < 5

6 Likes
14

Or even:

julia> Meta.@lower 5 < x != y < 5
:($(Expr(:thunk, CodeInfo(
    @ none within `top-level scope'
1 ─ %1 = 5 < x
└──      goto #5 if not %1
2 ─ %3 = x != y
└──      goto #4 if not %3
3 ─ %5 = y < 5
└──      return %5
4 ─      return false
5 ─      return false
))))

:slight_smile:

One day I will find out what a thunk is. Perhaps not today.

3 Likes
15

Thanks, cormullion! Very helpful.

I had to search for “thunk” and found this: Thunk - Wikipedia. Not sure that helps.

Take care, Jim.

1 Like
16 
dump(:(5 < x != y < 5))
Expr
  head: Symbol comparison
  args: Array{Any}((7,))
    1: Int64 5
    2: Symbol <
    3: Symbol x
    4: Symbol !=
    5: Symbol y
    6: Symbol <
    7: Int64 5

How does this help to understand the behavior of the chaining of the operators?

4 Likes
17

You’re right: in this example, it doesn’t help because the order of operations isn’t discernable from the parse structure.

18

As an aside, I hope people would avoid this in real code:
5 < x != y < 5 # false
which is difficult for the reader to understand without thinking a minute or even wondering if it’s a bug. It is more a brain-teaser to illustrate chaining, than a recommendation for actual code. The following is clearer and more explicit:

Chaining is great when it’s simple and close to what you’d write with math, as in the first examples above:

x = y = z = 1 # right-to-left
0 < x < 3 # true

The slowest compiler is the human reader, so try to be helpful to them, or even yourself if you ever return to the code months/years later.

As a cranky person, I also avoid the idiomatic

x == y && dosomething()

to avoid an if statement like

if x == y; dosomething(); end

I think the if makes intent clearer and doesn’t assume all readers to understand short-circuiting. (I also prefer the semi-colons even if they can be omitted, just to make it easier to read.) [edit: bug fix]

6 Likes
19

apo383,
Makes complete sense! A hacker (of the original sense) might appreciate the most minimal code, but mere mortals (me) need readability and ease of comprehension.