How to understand this simple piece of code?

Here is the code,

Code:

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void reverseStr(char* l, char* r)
{
        char* p = l;
        char t;
        int len = 0;

        while(*p)
        {
                *r++ = *p++;
                len++;
        }

        *r = '\0';
       
        p = r-len;
        r = r-1;

        while(p<r)
        {
                t = *r;
                *r-- = *p;
                *p++ = t;               
        }

}

int main(int argc, char* argv[])
{
        char s[24] = "abcdefg";
        char r[24];

    reverseStr(s, r);

        cout<<r<<endl;
        return 0;
}

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The output is "gfedcba". But I would expect it to be "dcba". Take a look at
while(p<r)
{
t = *r;
*r-- = *p;
*p++ = t;
}
r eventually will move to the middle of "gfedcba", which is "dcba". How come r points to 'g' finally? Thanks.

The debugger reveals all.

Quote:

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Originally Posted by GCDEF

The debugger reveals all.

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+1 Yeah Larry, why ask a question when a couple of minutes in a debugger will tell you everything?

Quote:

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r eventually will move to the middle of "gfedcba", which is "dcba". How come r points to 'g' finally? Thanks.

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To which 'r' are you referring - r in reverse() or r in main()?

Quote:

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Originally Posted by LarryChen

r eventually will move to the middle of "gfedcba", which is "dcba". How come r points to 'g' finally? Thanks.

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r is passed by value, so any changes to r itself will not be seen at the end of the call: Even if in "reverseStr", r points to the middle of the string, the one in main will be unchanged.

Quote:

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Originally Posted by 2kaud

To which 'r' are you referring - r in reverse() or r in main()?

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there's an r in main() ? :p

Quote:

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Originally Posted by oreubens

there's an r in main() ? :p

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:) :rolleyes:

Quote:

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Originally Posted by monarch_dodra

r is passed by value, so any changes to r itself will not be seen at the end of the call: Even if in "reverseStr", r points to the middle of the string, the one in main will be unchanged.

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Thanks for your reply. I agree that r in reverseStr points to the middle of the string. But how come the r in main points to the beginning of the string? That is really my question.

Quote:

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Originally Posted by 2kaud

To which 'r' are you referring - r in reverse() or r in main()?

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The r in reverse points to the middle of the string but the r in main points to the beginning of the string. I don't understand why? Thanks.

Quote:

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Originally Posted by LarryChen

The r in reverse points to the middle of the string but the r in main points to the beginning of the string. I don't understand why?

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Set a breakpoint at the beginning of the reverseStr function.
Then press F5.
Then debug your code using F10 and you will understand it!

Quote:

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Originally Posted by LarryChen

The r in reverse points to the middle of the string but the r in main points to the beginning of the string. I don't understand why? Thanks.

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Have you tried debugging? r in reverse points to the beginning of an unitialized 24 byte char array. I and I assume others don't know what you mean by points to the middle of the string or why you think that.

Quote:

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Originally Posted by LarryChen

The r in reverse points to the middle of the string but the r in main points to the beginning of the string. I don't understand why? Thanks.

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Are you kidding? I mean, seriously? The answer is in the reply you *just* quoted:

Quote:

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Originally Posted by monarch_dodra

r is passed by value, so any changes to r itself will not be seen at the end of the call: Even if in "reverseStr", r points to the middle of the string, the one in main will be unchanged.

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How can you not understand this?

Quote:

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Originally Posted by GCDEF

Have you tried debugging? r in reverse points to the beginning of an unitialized 24 byte char array. I and I assume others don't know what you mean by points to the middle of the string or why you think that.

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Yes, I did debug. Now I think I understand why. Within the function reverse, r does point to the middle of the string. But since r is passed by value, it won't change the address of r in main. r in main still points to the beginning of an uninitialized array. Silly me. Thanks a lot.

Quote:

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Originally Posted by LarryChen

Yes, I did debug. Now I think I understand why. Within the function reverse, r does point to the middle of the string. But since r is passed by value, it won't change the address of r in main. r in main still points to the beginning of an uninitialized array. Silly me. Thanks a lot.

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Any particular reason you didn't start answering the question by stepping through with the debugger first?