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problem about statement

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Join Date: Jul 2004

Posts: 31

Reputation: Sukhbir is an unknown quantity at this point

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Sukhbir

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problem about statement

Sep 17th, 2004

char *ptr="abc";
char *ptr1;
while(*ptr1++=*ptr++); //IT IS WORKING
while((*ptr1)++=(*ptr)++); //ERROR

PLS EXPLAIN ME.

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Join Date: Apr 2004

Posts: 4,443

Reputation: Dave Sinkula has a brilliant future

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Dave Sinkula Dave Sinkula is offline

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long time no c

Re: problem about statement

Sep 17th, 2004

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Originally Posted by Sukhbir

Help with Code Tags

C++ Syntax (Toggle Plain Text)
char *ptr="abc";
 char *ptr1;
 while(*ptr1++=*ptr++);       //IT IS WORKING
 while((*ptr1)++=(*ptr)++);       //ERROR
char *ptr="abc"; char *ptr1; while(*ptr1++=*ptr++); //IT IS WORKING while((*ptr1)++=(*ptr)++); //ERROR
PLS EXPLAIN ME.

[Added [code][/code].]

You haven't initialized ptr1: when you first dereference it, it is already undefined behavior, so "IT IS WORKING" is just plain "luck(?)".

In the "ERROR" line, the result of the ++ operator is not an lvalue, so it cannot be assigned to.

"One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand

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Join Date: Jun 2004

Posts: 436

Reputation: Chainsaw is an unknown quantity at this point

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Chainsaw

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Re: problem about statement

Sep 17th, 2004

Sukhbir, you have a lot of interesting syntax questions, and they are all covered in one chapter of any good C book, because these questions all revolve around the way the C compiler (and thus the C++ compiler) has been specified to work. You can also go into the help for your compiler and look for 'precedence'; there is a nice table that lists the exact order specified by the language.

In your first example,

*ptr1++

is the same as

*(ptr1++)

to the compiler, thus your second example, where you supplied the parens, would be different. In general, when writing code like this that might confuse someone, it is nice to put in the parens explicitly, even if they are not strictly needed by the compiler.

Dave's comments are also right on.