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: 2,870	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
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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 .
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08-21-2009, 07:12 AM	#7
BEN10 DESTINY Join Date: Jul 2008 Location: in front of my computer Posts: 803	I think hk_mp5kpdw's post is also wrong then. __________________ HOPE YOU UNDERSTAND....... By associating with wise people you will become wise yourself It's fine to celebrate success but it is more important to heed the lessons of failure We've got to put a lot of money into changing behavior PC specifications- 512MB RAM, Windows XP sp3, 2.79 GHz pentium D. IDE- Microsoft Visual Studio 2008 Express Edition
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08-21-2009, 07:30 AM	#9
BEN10 DESTINY Join Date: Jul 2008 Location: in front of my computer Posts: 803	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....... By associating with wise people you will become wise yourself It's fine to celebrate success but it is more important to heed the lessons of failure We've got to put a lot of money into changing behavior 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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