Clojurians Log v2

someone who can give me a hint. I tried (not ( and (not(every? true? args)) (not-any? true? args))

that one is good when doing all the test except [true, true, true] which gives now true but it has to be false

@gdeer81 @gowder

@roelofw you're on the right track but you've made a mess of your logic so lets try to think of this problem in simpler terms

I know im on the right track because most of the tests are a success

I still have difficult to find out which function I need to use

@gdeer81

I'm trying to figure out a good way of talking you through it without giving away the answer

oke

I making dinner right now so It can be that i do not respond quick

so what function would you use if the requirement was to return true if they were all true or all false?

every? Returns true if (pred x) is logical true for every x in coll, else false.

so you would write (or (every? true? args) (every? false? args)) ?

but there is a function that already checks for equality

=

yes, I forget that all of then could be false

yep , 1 = 2 gives false

and 2 = 2 gives true

so far im with you

and you're getting a bunch of booleans so it's even easier because you don't really need a predicate function

oke, you you mean something like a map to compare the item with true or false ?

I was thinking about something like this (and (some true? args) (some? false args))

that sounds like you would need a predicate function which I just hinted you don't need

hmm, I have to do some comparison otherwise I do not know if something is true or false

so I cannot think of a solution without any predicate

im sorry

@gdeer81 I cannot think of a solution that you have in mind

can you give a hint how I can check this [ true, true, true] is all true without a predicate

in my opinion = is also a predicate

or do you mean something like this (map (= true) args)

you don't need to map = since it takes a variable number of arguments

okay I think I'm just making things confusing, lets go back to the thing you put earlier, (and (some true? args) (some false? args))

oke

which cases did that fail on?

moment, I have to try, this is new idea

it fails here :

FAIL in (test1) (1_half_truth_test.clj:7)
Half truth - test 1
expected: (= (half-truth true true true true) false)
  actual: (not (= nil false)) 

did your function return nil?

yes, apperently , which is not so wierd.

Returns the first logical true value of (pred x) for any x in coll,
else nil.  One common idiom is to use a set as pred, for example
this will return :fred if :fred is in the sequence, otherwise nil:
(some #{:fred} coll)  

so some? i returning nill if a item is not found

what did you put?

it was probably because of your and statement, since one of your statements returns nil your and statement returned nil

the code we are talking about

(defn half-truth 
  "Solution to Problem 1: 
   Write a function which takes a variable number of booleans. 
   Your function should return true if some of the parameters
   are true, but not all of the parameters are true. 
   Otherwise your function should return false."
  [& args]
  (and (some true? args) (some false? args)))  

maybe a idea to check if the output is true or false ?

he, with this one all tests of the stanford are green :

(defn half-truth 
  "Solution to Problem 1: 
   Write a function which takes a variable number of booleans. 
   Your function should return true if some of the parameters
   are true, but not all of the parameters are true. 
   Otherwise your function should return false."
  [& args]
  ( true? (and (some true? args) (some false? args))))  

chips, but now working here well :

(half-truth [false true false ])
=> false 

if I read the desription that one schould be true

nope, the answer always is false

it seems that there cannot be a solution that works for all cases

@roelofw look at what you are passing as an argument in that call to half-truth and what the defn is taking as an argument

There's nothing wrong with the code in the body of the defn

chips, the arguments are all seperate

yeah, its good to copy/paste the test cases from the problem so you don't make a typo and craft a solution for a totally different problem

this looks well to me :

(half-truth true true true )
=> false
(half-truth true true false )
=> true
(half-truth false true false )
=> true
(half-truth false false false )
=> false
 

so 2 out of the 4 challenges solved of the standford ones

@roelofw do you know why false false false or true true true was returning nil until you added the true? predicate around the and

?

Read the docs for some and see if you can work out why

@agile_geek yes, see here of the docs of some? Returns the first logical true value of (pred x) for any x in coll,else nil

👍

so if there is nothing that is true when I do (some? true? [false, false, false]) then the answer will be nill

Yes and same if nothing false in the other branch of the and

@roelofw did you get a working solution yet? can I tell you the answer now?

And because and stops once it's evaluated all the predicates (if they're all truthy) or on the first that is falsey and returns the value from the some clause that's falsey you get nil

yep

@gdeer81 I have this as working solution :

(defn half-truth 
  "Solution to Problem 1: 
   Write a function which takes a variable number of booleans. 
   Your function should return true if some of the parameters
   are true, but not all of the parameters are true. 
   Otherwise your function should return false."
  [& args]
  ( true? (and (some true? args) (some false? args))))  

okay, good now I won't feel bad if I accidentally give away the answer I had in mind

@gdeer81 is it a totally other answer then

I get this idea when you wrote : (or (every? true? args) (every? false? args))

hint 1: this problem can be solved with one function that is in core

yes, and somebody else said it could be done in 4 characters

but there are not much functions with 4 characters 🙂

well then that should narrow it down

write a function that returns all the functions in core that have exactly 4 characters

that is a difficult one

oke, with google I succeed

I can display al functions of core with (dir clojure.core)

there are 72

hmm, filter does not what I expect : (filter (fn[item] (= (count item) 4 )) (dir clojure.core))

just use this as a reference https://gist.github.com/gdeer81/3897e926a7dea79dd070607bcdb5a548

although this probably isn't helping you think about the problem in simpler terms

oke, and now I have to read a lot of pages

no, I see some candidates but no function who looks right

most promising looks not= but if I read the page I do not think its the one

some I tried and it is not the answer

what does the page for not= say?

Same as (not (= obj1 obj2))

hmm, maybe the args could be obj1 and something like [ true , true] as obj2 ?

or maybe something like this one (not= true true true true)

since it is referring to = in the doc string you'd have to read the docs for = to understand how it is supposed to work

I look at the examples and these look very good :

(not= false false false )
=> false
(not= true false false )
=> true
(not= true true true )
=> false  

(not= true false true )
=> true  

so it will be (not= args)

a very short one

or you could just pass in the function itself without having to wrap it since #(not= %&) is the same as just passing in that function itself

nope, this case is failing :

FAIL in (test2) (1_half_truth_test.clj:11)
Half truth - test 2
expected: (= (half-truth true false true) true)
  actual: (not (= false true))
 

it gives false

and because there a two true and one false , t schould give true

replace args with the value and you'll see why

or, what is args?

args is a vector so here [false false]

so when you do (not= args) you are doing (not= [false false])

so I retract my statement above

chips , & args gives a vector and not the input itself 😞

#(not= %&) is not the same as bare not=

take a look at apply @roelofw

@gdeer81 this is also not working :

(defn half-truth 
  "Solution to Problem 1: 
   Write a function which takes a variable number of booleans. 
   Your function should return true if some of the parameters
   are true, but not all of the parameters are true. 
   Otherwise your function should return false."
  [& args]
  #(not= %&))  

it fails with this :

FAIL in (test1) (1_half_truth_test.clj:7)
Half truth - test 1
expected: (= (half-truth true true true true) false)
  actual: (not (= #object[pset1.1_half_truth$half_truth$fn__205 0x1133976 "pset1.1_half_truth$half_truth$fn__205@1133976"] false))  

@roelofw args is being coerced into a sequence in you argument vector in the defn of half-truth

yes, That is what I said earlier, @agile_geek

@roelofw you've gone over your four character limit

yep

but you are calling half-truth with a variable number of arguments and not= takes a variable number of arguments so you don't need to coerce args to a sequence

you don't need to define a new function, just pass in not=

however, if you do want to wrap it in your own function you will have to unroll the args

@agile_geek so I can change [& args] to [ args]

not if you want it in separate fn

as @gdeer81 said

just use not= or use apply to unroll args

so somehing like (not= (apply args)) ????

if you keep your fn and change argument to [arg] it will only expect one arg

No

@roelofw what does the doc string for apply say?

☝️

'(apply function args) `

so (apply (not= ) args)

close

it's the function you're applying not the evaluated function

remember putting a fn in parens makes Clojure evaluate it

@roelofw I'd watch lecture 1B here, you've made a similar mistake a couple times: https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-001-structure-and-interpretation-of-computer-programs-spring-2005/video-lectures/

the function is (not= args) , Right ?

thats the evaluated function

i.e you are calling the fn not=

with the argument args

oke, so it would be (apply (not=) args)

that will give you an arity exception

yes, I does

That's still evaluating not= but with no args

I will take a break, my head is spinning

So what happens if Clojure evaluates this (not=) which gives an arity exception, and then tries to pass the result to (apply <result of calling not=> args)

apply wants a function and some arguments and I cannot see what the function is and what the arguments

What is the name of the function

not=

consider this example (+ [1 2 3])

how could you use apply to make + work with this vector?

there + is the function and [1 2 3 ] as a arguments I think

right

so (apply (+) [ 1 2 3 ] )

What is the reuslt of (+) on its own

what does (+) evaluate to?

but schould this not be in #beginner or #clojure ?

a function ??

(fn? (+))

vs (fn? +)

heading to lunch now, keep at it, you're almost there

I do not have that feeling

so if (+) evaluates to 0

yes, I saw it on your code

then this (apply (+) [1 2 3])

is same as (apply 0 [1 2 3])

I think this will be (+ 1 2 3) at the end

so something is still not right

the + has to stay

no, it will be (0 1 2 3) apply only sees the substituted value

apply is not easy to understand

It is when you get the hang of it. At the moment you are passing it the result of evaluating a function, not the function itself

yeah but understanding apply is fundamental to lisp, understand it and other things will make sense too

i.e. in the + example you were evaluating + (+) which is 0 and then passing that to apply but 0 is not a function

hence the class cast exception in Clojurebot ☝️

what I want is something like this (fn [ acc item] ( acc + item)) Like I will do when using a map or reduce

@roelofw do you know other languages?

You could use reduce but apply is easier

a little bit ruby

@agile_geek that part I understand now

but I do not see how I can make it work with apply at this moment, sorry

apply is like the splat operator not_equal(*args)

So you need to pass apply the unevaluated +

As you evaluate + using parens (+), how would you pass + not evaluated?

oke, so without the ()

Yep

so it would be `(apply + [1 2 3])

oke, so back to our code

So in your half-truth example you can use apply with not=

(apply not= args) ?

Try it

what does (apply not= [true true false]) give you

all test provided by the standford challenges are green

So now you know a way of calling a function that takes separate multiple arguments when you have a collection of arguments

yes, I learned a lot today from you and some others

now time for the dishes 😞

and tomorrow try to find out how to find the nth row of a pascal triangle

but I noticed how more you know of clojure how shorter the code it seems

All thanks and have a good day/evening

@roelofw I was going to ask you to now try and solve it with set theory

the same exercise with a set

?

@gdeer81

yeah your function rolls up the arguments into a collection, so what happens if you called (set args) ?

( count (set(args) > 1) out if my head

exactly right

so if there are more then 1 arg then there is one or more true or false

if there is only 1 the seq contains only true or false

okay, I updated my statement

@gdeer81 happy with the solution

the readability could be improved. how many items does the set have every time its suppose to return true?

more then1

but it will never have 3

nope,, its 1 or 2

1 if the items are all true or false, 2 if both

so what is more readable (= 2 (count (set args))) or (> (count (set args)) 1) ?

I think the first

ofc things look a bit different with 0 or 1 args

well this is assuming that this is being called with (defn [& args] ...) so the args are always being rolled up

I have asummed the same

So I have earned a C today 🙂

the point is that you've come away from this experience knowing more than you knew when you came in

yep, today I did

then today you get an A

learned to read stack traces and where the error can be found

learned apply

learned not=

I hope I can use it later on when needed

and you've learned when Clojure "rolls up" arguments and what that means

that will be the biggest challenge on the standford and 4clojure challenges

You've also learned the difference between a function and a function evaluation

that is why my head is spinning

😛

learned how to refractor functions so I can test them

I think it's confusing because as a beginner you're taught to always hug your functions

hug my functions ??

so (apply + [1 2 3]) looks weird because you want to hug that plus sign (+)

oke

this is what tripped me up when I started learning the -> macro

oke, I use the ->> on my first attempt for a website but there I had not make this mistake

this was quite a thorough code review

yep

with a lot of explanation , I wonder if some of this schould not be in beginners or the clojure channel

Now with 4clojure I have to reimplement zipmap and with the stanford challenges I have to find the nth row of a pascal triangle

both not easy

but now time for family

@gdeer81 if you want and have time , do you want to review the last version of my paintings website : https://github.com/rwobben/paintings

yeah I'll take a look and see if I notice any glaring issues

thanks, do it only if you have time