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### Topic: pYro's puzzle #1: Increment this. (Read 586 times)previous topic - next topic

#### pYro_65

##### Jan 09, 2013, 02:40 pmLast Edit: Nov 12, 2013, 08:28 pm by pYro_65 Reason: 1
Hi all,

Over the years I've encountered some great topics regarding C++ that I found interesting enough to archive in my 'box o stuff'.
As I have nothing better to do on a Wednesday night, I'll convey this information via interpretive dance in puzzle form for anybody else looking for a time passer.If you know the answer hold off a little ( or for a few posts between Q & A ) for people to have a crack. If people like these sorts of things, I'll post more.

And before anyone asks, I'm not trying to get someone to do my homework.

#1 Increment This

For the first entry, here is my own extension to a common puzzle. This requires a short preface.

Take the code below, what is printed to the serial monitor? ( not the question )

Code: [Select]
`int i = 0;int j = 1;Serial.println( i+++j, DEC );`

For those of you who haven't seen this, it is a classic example of 'order of evaluation', without looking at associativity, operator precedence defines how this is evaluated.

http://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B#Operator_precedence

 Precedence Operator 1 ++ ( Suffix increment ) 2 ++ ( Prefix increment ) 2 + ( Unary plus )

So...
The suffix operator has precedence, and the code is:
Code: [Select]
`i++ + j; //( i Suffix increment ) + j`

rather than:

Code: [Select]
`i + ++j; //i + ( j prefix increment )`

As you can see operator precedence explains why this works... Or does it.

Why does the code below fail to compile while the two equivalent versions beneath it compile fine?
Code: [Select]
`i+++++j;`
Code: [Select]
`i++ + ++j;i+++ ++j;`

Hint #1: the answer is not related to the spaces in the code, if you believe this is the answer, explain why the spaces didn't cause error with the version: 'i+++j'

Hint #2
Code: [Select]
`i++++j;`

Cheers.

#### pYro_65

#1
##### Jan 10, 2013, 07:46 am

I'll bump it back up for others to see anyway. This may be difficult, but I thought it was a good one for a first, the next one I have planned should be a little easier.

#2
##### Jan 10, 2013, 08:14 amLast Edit: Jan 10, 2013, 08:16 am by Coding Badly Reason: 1
I'll play along.

Hint #2 fails to compile because of the way C combines characters into diagraphs to create a lexical unit and operator precedence.  The compiler sees this...

Code: [Select]
`i++++j;`

...as...

Code: [Select]
`( ( i ++ ) ++ ) j ;`

From the perspective of the outer operator (the second ++), the expression is...

Code: [Select]
`( ( i + 1 ) ++ ) j ;`

...which fails to compile because the increment operator requires a variable (an "lvalue") as the operand.

The five pluses is just an extension of the above...

Code: [Select]
`i+++++j;`

...is seen as...

Code: [Select]
`i ++ ++ + j ;`
Code: [Select]
`( ( i ++ ) ++ ) + j ;`

#### pYro_65

#3
##### Jan 10, 2013, 08:25 am
Yup, you got it, nice answer too.

C or C++ hasn't even come into play when this occurs, this happens even before macros are expanded.
The tokenizer creates the largest possible token from the source input, so the token '++' is considered before '+'.

#### Nick Gammon

#4
##### Jan 10, 2013, 08:59 am

I'll convey this information via interpretive dance in puzzle form ...

I'd prefer interpretive dance. A YouTube video will be an acceptable medium.
Please post technical questions on the forum, not by personal message. Thanks!

http://www.gammon.com.au/electronics

#5
##### Jan 10, 2013, 09:09 am

Using this dance troop... http://www.military.com/video/forces/navy/gangnam-style-usna-spirit-spot/1845842268001/

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