Re: var vs def

Thank you Martin and Oliver,

This is now clear to me (I think), correct me if am wrong below:

I created another small example, but this time with no variable in var 
declaration. Here is the example, and I start with initialising a variable 
i with 0

scala> var i = 0

i: Int = 0


scala> var greatWorld3:Boolean = if (i>10) true else { println("Print i:"+i)

     | i += 1

     | greatWorld3 }

Print i:0

greatWorld3: Boolean = false


scala> i

res44: Int = 1


scala> greatWorld3

res45: Boolean = false


Now, I start with a variable i set to 0; and declare a function (I think it 
is still a Function/Method; as when I did not provide *:Boolean* return 
type it complained as being recursive without any return type; do correct 
me if I am getting this wrong).


Given that the right side of '=' sign is only evaluated once, the statement 
is run once therefore the i ends up with 1 value at the end of it. And for 
the same reason, the *greatWorld3* variable/function (ummm... I am still 
confused here if I defined a function or variable...) has the default for 
boolean, which is false.


And at end of the definition the interpreter prints:


greatWorld3: Boolean = false


which might have happened either because:

 a. the interpreter returns the type of the just created function/variable 
by default

OR

 b. Because there is a recursive call to greatWorld3, and the interpreter 
prints the vale of greatWorld3.


Not sure which one these above 2 is applied?


Next I go into function definition, with *def*


I then move on to define a new variable j with 0, and now replace the 
definition of greatWorld3 with a function def as below:


scala> var j = 0

j: Int = 0


scala> def greatWorld3:Boolean = if(j>10) true else {println("Print j:"+j)

     | j += 1

     | greatWorld3 }

greatWorld3: Boolean


scala> greatWorld3

Print j:0

Print j:1

Print j:2

Print j:3

Print j:4

Print j:5

Print j:6

Print j:7

Print j:8

Print j:9

Print j:10

res46: Boolean = true


scala> j

res47: Int = 11

Output for def block, is all ok.


Thanks for your help and time again.


Regard,

S



On Saturday, May 2, 2015 at 2:36:39 PM UTC+1, martin wrote:
>
> On Sat, May 2, 2015 at 12:57 PM, scalanewbie <[email protected] 
> <javascript:>> wrote: 
> > Hello All, 
> > 
> > I am a newbie to scala and getting in grasp with val and def. 
> > 
> > I noticed that I could write : 
> > 
> > scala> var greatWorld2 = println("great world") 
> > 
> > great world 
> > 
> > greatWorld2: Unit = () 
> > 
> > 
> > AND 
> > 
> > 
> > scala> def greatWorld() = println("great world") 
> > 
> > greatWorld: ()Unit 
> > 
> > 
> > Can someone please help me understand the difference? 
> > 
> > 
> > I am aware that var is generally used to define variables and def is 
> used to 
> > define functions. 
> > 
> > 
> > Now, keeping in mind that greatWorld2 is actually a variable and not a 
> > function, I am trying to print it, and I get: 
> > 
> > 
> > scala> greatWorld2 
> > 
> You are probably surprised that you see no output. That's because 
> greatWorld2 returns a unit value (after all, its type is Unit) and the 
> REPL does not print unit values. It could print () but it assumes that 
> you executed the expression for its side effects only. 
>
> Hope this helps 
>
>  - Martin 
>
> > 
> > OR, if I try calling it as a function, it throws an error e.g. below 
> (which 
> > sounds ok as I did not declare this as a function, so was expecting this 
> > anyway). 
> > 
> > 
> > scala> greatWorld2() 
> > 
> > <console>:9: error: Unit does not take parameters 
> > 
> >               greatWorld2() 
> > 
> > 
> > 
> >  Long story short, difference between var and def , in terms of what the 
> > interpreter and or compiler is doing underneath will be very good to 
> > understand. 
> > 
> > 
> > Thanks Guys, 
> > 
> > S 
> > 
> > 
> > 
> > -- 
> > You received this message because you are subscribed to the Google 
> Groups 
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>
>
> -- 
> Martin Odersky 
> EPFL 
>

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