Interpretation of the calculator "eatspaces" function

RufusDeRed · April 5th, 2009, 04:47 AM

Hello,

Can someone please explain how the following term:

Code:

( *(str+i) = *(str+j++) )

evaluates when used in the following line

Code:

	while ( (*(str+i) = *(str+j++) )!= '\0' )

in the "eatspaces" function on page 302 of this book? Note that this is a function which is used in the Calculator Example, which is Ex6_09.

At first glance, I thought it was assigning the value of the pointer at *(str+j++) to *(str+i), and then checking that this wasn't the end of the line... but that doesn't seem to be quite correct. I say that's not quite correct because when I try to spread these commands over a few lines, it doesn't give the same result.

This means one of two things:
(1) My attempt to spread the code out over two lines is rubbish, with a bug in it, or
(2) My interpretation of this line of code is faulty.

Can someone please let me know which of the two problems it is?

Cheers,
- Steve.

Old Pedant · April 5th, 2009, 03:44 PM

Don't have the book, but that code doesn't really make sense to me.

Since no place there does the value of i get changed, that sure looks to me like *ALL* the characters starting at j get copied into the SINGLE character at i, from position j to the end of the string.

Sure looks like a typo/error to me.

What are i and j initialized to??

I could see *THIS* code being used to "eat spaces":

Code:

int i = ... find a space in the str string...
int j = i;
while ( (*(str+i) = *(str+j++) ) == ' ' );
while ( (*(str+(++i)) = *(str+j++) ) != '\0' ' );

being used to remove spaces.

Incidentally, I personally find that notation distasteful. This notation is 100% equivalent (according to the laws of C and C++) and, I think, is clearer:

Code:

int i = ... find a space in the str string...
int j = i;
while ( ( str[i] = str[j++] ) == ' ' );
while ( ( str[++i] = str[j++] ) != '\0' );

sudhanshu631 · May 20th, 2009, 02:09 AM

yup yup yup