Odd C code

bpmurray · 11-02-2006 12:24AM #1

Anyone any idea what this syntax does?

int k = i+++j;

feylya · 11-02-2006 12:25AM

If it worked, I'd imagine it would make k equal to i plus 1 plus j.

bpmurray · 11-02-2006 12:27AM

Of course it works: just curious to see how many folk know or can deduce what it does.

Sandals · 11-02-2006 12:28AM

i don't believe it works.

pH · 11-02-2006 12:40AM

In all fairness it's not as odd as :

#include <stdio.h>
main(t,_,a)char *a;{return!0<t?t<3?main(-79,-13,a+main(-87,1-_,
main(-86,0,a+1)+a)):1,t<_?main(t+1,_,a):3,main(-94,-27+t,a)&&t==2?_<13?
main(2,_+1,"%s %d %d\n"):9:16:t<0?t<-72?main(_,t,
"@n'+,#'/*{}w+/w#cdnr/+,{}r/*de}+,/*{*+,/w{%+,/w#q#n+,/#{l,+,/n{n+,/+#n+,/#\
;#q#n+,/+k#;*+,/'r :'d*'3,}{w+K w'K:'+}e#';dq#'l \
q#'+d'K#!/+k#;q#'r}eKK#}w'r}eKK{nl]'/#;#q#n'){)#}w'){){nl]'/+#n';d}rw' i;# \
){nl]!/n{n#'; r{#w'r nc{nl]'/#{l,+'K {rw' iK{;[{nl]'/w#q#n'wk nw' \
iwk{KK{nl]!/w{%'l##w#' i; :{nl]'/*{q#'ld;r'}{nlwb!/*de}'c \
;;{nl'-{}rw]'/+,}##'*}#nc,',#nw]'/+kd'+e}+;#'rdq#w! nr'/ ') }+}{rl#'{n' ')# \
}'+}##(!!/")
:t<-50?_==*a?putchar(31[a]):main(-65,_,a+1):main((*a=='/')+t,_,a+1)
  :0<t?main(2,2,"%s"):*a=='/'||main(0,main(-61,*a,
"!ek;dc i@bK'(q)-[w]*%n+r3#l,{}:\nuwloca-O;m .vpbks,fxntdCeghiry"),a+1);}

Sandals · 11-02-2006 12:43AM

and what does that do?
re-establish the codes proxy?

bpmurray · 11-02-2006 12:59AM

Sandals wrote:

and what does that do?
re-establish the codes proxy?

Nah - looks familiar. Is it the Christmas one?

PhantomBeaker · 11-02-2006 01:08AM

Right, I'll be able to tell you once I look up C's operator precedence. Specifically if the evaluation pre/post-increment comes before or after addition.

Ok post increment is evaluated first over pre-increment.

So the token +++ would be split into two tokens, ++ and + (which is why I wanted to know the precedence - see references)

So that will evaluate to:

k= (i++) + j;

Which basically should be equivalent to the following statments

k = i + j;
i++;

Check it out. assign some values to i and j, and then check out what the statment does. If I'm right (I'm too tired/not-arsed to compile it myself just now)
if you do

int i = 1;
int j = 1;
int k = i+++j;

if((k == 2) && (i == 2) && (j==1)){
  printf("I got it right. It's i post-increment plus j\n");
} else if ((k == 3) && (i==1) && (j==2)){
  // This shouldn't happen, but covers the possibility that it pre-increments j, so k would be i + j + 1, and j would be j+1. But pre-increment has a lower precedence than post-increment, so it really shouldn't happen if your C implementation is good.
  printf("I got it wrong - it's i plus preincrement j\n");
} else {
  printf("Ok, that totally messed up.\n\nThe real values are: i= %d, j=%d, k=%d\n", i, j, k);
}

I'd say that snippet of code would give you a thorough explanation of what it does.

If I remember to, I'll try it out in the morning. By the looks of things, you should get i=2, j=1 and k=2.

References:
I got the precedence chart by googling "operator precedence c". I got http://www.swansontec.com/sopc.htm

Take care,
Aoife

bpmurray · 11-02-2006 01:21AM

You're right, but for the wrong reasons. The selection of (i++) + j over i + (++j) isn't a question of precedence, but rather about tokenising the line: in this case (as is usual in compilers) the compiler takes the longest string possible, when scanning from left to right, and this is "i++".

The issue of precedence only arises once you have actually identified the operators.

Odd C code

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