expression evaluation

lazy_hack · 08-21-2009, 05:12 AM

Hi,

Although the expression *((a<20)?&b:&c)=30; works but

((a<20)?b:c)=30 gives

error: invalid lvalue in assignment.

what could be the reason for it? Please need sum urgent help.

Thanks in advance.

hk_mp5kpdw · 08-21-2009, 05:31 AM

The result of the left hand side of:

Code:

((a<20)?b:c)=30

is a value (the value stored in either b or c). Let's say for example that b is 20 and c is 30. If a is 10 then the result of the left-hand side is 20 and you are effectively trying to say:

Code:

20=30

which would of course give you that error.

lazy_hack · 08-21-2009, 05:38 AM

full program:

Code:

main()
{
  int a=10,b,c;
  ((a<20)?b:c)=30;
  printf("%d",b);
 }

this also gives error invalid lvalue in assignment.

but if i replace the line with *((a<20)?&b:&c)=30; it works.

please explain also why the second one works .

**laserlight** · 08-21-2009, 05:50 AM

Quote:

Originally Posted by lazy_hack

but if i replace the line with *((a<20)?&b:&c)=30; it works.

please explain also why the second one works .

You are assigning to what the corresponding pointer points to, and that is not a problem. Where did you come across this code, anyway? It looks like a lazy hack, or rather, bad style.

BEN10 · 08-21-2009, 07:02 AM

Quote:

Originally Posted by lazy_hack

full program:

Code:

main()
{
  int a=10,b,c;
  ((a<20)?b:c)=30;
  printf("%d",b);
 }

this also gives error invalid lvalue in assignment.

but if i replace the line with *((a<20)?&b:&c)=30; it works.

please explain also why the second one works .

In the code above b and c are having garbage values, that's why as told to you in post#2, it'll give you error, while using & you are directly "attacking" in the address, thus no error.

**laserlight** · 08-21-2009, 07:07 AM

Quote:

Originally Posted by BEN10

In the code above b and c are having garbage values, that's why as told to you in post#2, it'll give you error, while using & you are directly "attacking" in the address, thus no error.

That is off the mark. That b and c were not initialised does not matter, since the intention is to assign a value to them. What matters is that the result of the conditional operator is not an lvalue, hence the attempted assignment does not work.

BEN10 · 08-21-2009, 07:12 AM

I think hk_mp5kpdw's post is also wrong then.

**laserlight** · 08-21-2009, 07:26 AM

Quote:

Originally Posted by BEN10

I think hk_mp5kpdw's post is also wrong then.

What do you find wrong about it? hk_mp5kpdw made the point that the result of the conditional operator is not a variable that can be assigned to, but a value that cannot be assigned to.

BEN10 · 08-21-2009, 07:30 AM

From hk_mp5kpdw's post, let b has a value of 12345 in the code given by the OP, then this will happen

Code:

12345=30;

That's why the error. This is what I said in my previous post.

roaan · 08-21-2009, 07:34 AM

Quote:

Originally Posted by BEN10

In the code above b and c are having garbage values, that's why as told to you in post#2, it'll give you error, while using & you are directly "attacking" in the address, thus no error.

No you said this. But as laserlight pointed out that the uninitialized b and c are not a problem here. The problem is the assignment being checked

for example as in your case

12345 = 30 // this is wrong

**laserlight** · 08-21-2009, 08:06 AM

Quote:

Originally Posted by BEN10

That's why the error. This is what I said in my previous post.

Fair enough: looking at your post #5 again, I see that I missed the phrase "as told to you in post#2". It would have been clearer if you had given an example there and then, or at least suggested the scenario where 20 and 30 were the "garbage" values of b and c respectively.

lazy_hack · 08-21-2009, 09:37 AM

thanks for the explanation...

"the result of the conditional operator is not an lvalue" line is the crux i believe...

the code was from test ur c skills book....just anther intrsting book..

08-21-2009, 05:12 AM	#1
lazy_hack Registered User Join Date: Aug 2009 Posts: 5	expression evaluation Hi, Although the expression *((a<20)?&b:&c)=30; works but ((a<20)?b:c)=30 gives error: invalid lvalue in assignment. what could be the reason for it? Please need sum urgent help. Thanks in advance.
lazy_hack is offline

08-21-2009, 05:31 AM	#2
hk_mp5kpdw Registered User Join Date: Jan 2002 Location: Northern Virginia/Washington DC Metropolitan Area Posts: 2,787	The result of the left hand side of: Code: ((a<20)?b:c)=30 is a value (the value stored in either b or c). Let's say for example that b is 20 and c is 30. If a is 10 then the result of the left-hand side is 20 and you are effectively trying to say: Code: 20=30 which would of course give you that error. __________________ On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question. --Charles Babbage, 1792-1871 09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
hk_mp5kpdw is offline

08-21-2009, 05:38 AM	#3
lazy_hack Registered User Join Date: Aug 2009 Posts: 5	Help in assigment stmt. full program: Code: main() { int a=10,b,c; ((a<20)?b:c)=30; printf("%d",b); } this also gives error invalid lvalue in assignment. but if i replace the line with *((a<20)?&b:&c)=30; it works. please explain also why the second one works .
lazy_hack is offline

08-21-2009, 07:12 AM	#7
BEN10 DESTINY Join Date: Jul 2008 Location: in front of my computer Posts: 656	I think hk_mp5kpdw's post is also wrong then. __________________ HOPE YOU UNDERSTAND....... for( ; ; ) printf("If you can't make it good, at least make it look good"); PC specifications- 512MB RAM, Windows XP sp3, 2.79 GHz pentium D. IDE- Microsoft Visual Studio 2008 Express Edition
BEN10 is offline

08-21-2009, 07:30 AM	#9
BEN10 DESTINY Join Date: Jul 2008 Location: in front of my computer Posts: 656	From hk_mp5kpdw's post, let b has a value of 12345 in the code given by the OP, then this will happen Code: 12345=30; That's why the error. This is what I said in my previous post. __________________ HOPE YOU UNDERSTAND....... for( ; ; ) printf("If you can't make it good, at least make it look good"); PC specifications- 512MB RAM, Windows XP sp3, 2.79 GHz pentium D. IDE- Microsoft Visual Studio 2008 Express Edition
BEN10 is offline

08-21-2009, 09:37 AM	#12
lazy_hack Registered User Join Date: Aug 2009 Posts: 5	thanks for the explanation... "the result of the conditional operator is not an lvalue" line is the crux i believe... the code was from test ur c skills book....just anther intrsting book..
lazy_hack is offline

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