# pYro's puzzle #1: Increment this.

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 )

``````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:

``````i++ + j; //( i Suffix increment ) + j
``````

rather than:

``````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?

``````i+++++j;
``````
``````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

``````i++++j;
``````

Cheers.

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.

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...

``````i++++j;
``````

...as...

``````( ( i ++ ) ++ ) j ;
``````

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

``````( ( 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...

``````i+++++j;
``````

...is seen as...

``````i ++ ++ + j ;
``````
``````( ( i ++ ) ++ ) + j ;
``````

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 '+'.

pYro_65: I'll convey this information via interpretive dance in puzzle form ...

I'd prefer interpretive dance. A YouTube video will be an acceptable medium.

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