What does this piece of code in C do:
p = (1, 2.1);
What do we know about p?
views:
464answers:
4
+17
A:
The comma operator in C is a sequence point which means that the expressions separated by the comma are executed from left to right. The value of the whole expression is the value of the rightmost expression, in your case 2.1, which gets assigned to the variable p.
Since the expressions in your example don’t have side effects, the use of the comma separator here makes no sense whatsoever.
The parentheses on the other hand are important since the assignment operator (=) binds stronger than the comma operator (it has higher precedence) and would get evaluated before the comma operator without the parentheses. The result would thus be p == 1.
Konrad Rudolph
2009-09-15 16:01:25
In twenty years of writing C code, I have yet to find a really useful non-hackish use for the comma operator.
Graeme Perrow
2009-09-15 16:03:41
You can overload the operator in C++ (but not C, obviously) and Boost makes some beautiful use of that to implement list initializations, such as `list_type mylist = 1, 2, 3, 4, 5`.
Konrad Rudolph
2009-09-15 16:06:27
@Graeme, i bet you sometimes found `for(i = 0; i < n; i++, j--) ...;` to be useful
Johannes Schaub - litb
2009-09-15 16:08:43
@Graeme Perrow -- most of the time I use it is in a for() statement, as in for ( i = 0, j = 0; i < whatever; i++, j++ )... and I end up asking myself "what have I done wrong here" then end up fixing it.
Bob Kaufman
2009-09-15 16:11:34
@litb Yes, I have done that, but I always wince a little as I type it. Unless it's temporary throw-away code, I usually try to find a different way of doing it. That usage would fall under the "hackish" category. Maybe it's just me.
Graeme Perrow
2009-09-15 16:15:35
Most plausible case for a comma operator in the continuation statement is maybe: `int index = 0; for (node *p = head->next; p != tail; p = p->next, ++index) { something }`
Steve Jessop
2009-09-15 16:19:33
Graeme, why would you want non-hackish use if there're plenty of hackish ones?
Michael Krelin - hacker
2009-09-15 16:24:33
The comma operator is a hackish operator pretty much by definition -it's to hack in 2 things where only one is allowed.
Michael Burr
2009-09-15 17:09:16
Michael, so are compounds ;-)
Michael Krelin - hacker
2009-09-15 17:20:11
`if (p = malloc(…), p == NULL)` is nicer to read than `if ((p = malloc()) == NULL)`
Cirno de Bergerac
2009-09-15 18:13:22
why not `if( p = malloc( ... ) )`? I usually use `if( ! (p = malloc( ... )) ) return 0;`
Carson Myers
2009-09-15 19:33:49
Carson Myers, because there's no comma in it! ;-)
Michael Krelin - hacker
2009-09-15 20:53:49
+1
A:
It's a mistake. the comma operator is similar to ;. It does the one, then the other. so (1,2.1) evaluates to 2.1
p will be 2.1 (or 2, if p is an int and needs to be truncated...)
Brian Postow
2009-09-15 16:02:14
uhuh. How often have you seen that functionality and it NOT be a mistake? yes, it couldbe, but HIGHLY unlikely, no?
Brian Postow
2009-09-15 18:30:24
I've seen it once: Here, and it's used as a homework exercise to test knowledge about the comma operator. So far, based on all the cases I've seen, use of the comma operator in this way is not a mistake 100% of the time.
Carson Myers
2009-09-15 19:35:32
99% of the time I've seen it (in CS 1 projects, etc) is people failing to do 2D arrays... arr[3,5] != arr[3][5]...
Brian Postow
2009-09-15 20:08:42
Note, this is not including things like for(i = 0, j = 10; i < j; i++, j--)....
Brian Postow
2009-09-15 20:09:28
A:
all comma seprated expressions will be evaluated from left to right and value of rightmost expression will be returned.
so p will 2.1.
GG
2009-09-15 17:52:55
A:
p = (1, 2.1);
Iam not sure what I know about "p" from the code.
But when I see such code, I'm very sure about the competence of the person who wrote the code :D
codymanix
2009-09-15 18:06:33
if you don't know anything about `p` from that code, it makes a suggestion about the competence of the person reading the code :P
Carson Myers
2009-09-15 19:36:36
But I know that the person who gave me -1 has no humour. At least not what I call humour x-D
codymanix
2009-09-15 23:51:02