CODEWITHSUNDEEP
javacpointers
Martin Moore
Martin Moore

Reputation: 733

Converting C to Java - Pointer query

I'm converting a C prog that parses a text string and produces human readable o/p.

Got it nearly done, but having a problem understanding the difference in 

*char_ptr++

and
char_ptr-- in 

 token[i++] = c = toupper(*char_ptr++);
 if (c == '\0')
   {
     char_ptr--;
     return( 0 );
   }

Am I correct in thinking that *char_ptr++ will effectivley point to the next char in the 'string'? If so, what does char_ptr-- do?

Thanks.

Upvotes: 2

Views: 223

Answers (5)

Dave
Dave

Reputation: 11162

Because you're converting to java, you're going to have to remove the pointers, Java doesn't support them.

token[i++] = c = toupper(*char_ptr++);
if (c == '\0')
{
    char_ptr--;
    return( 0 );
}

Likely had some sort of declaration above it saying:

char* char_ptr = some_array;

Instead, that'll be 

int pos = 0;

And the above code becomes:

token[i++] = c = toupper(some_array[pos++]);
if (c == '\0')
{
    pos--;
    return( 0 );
}

Upvotes: 1

Blagovest Buyukliev
Blagovest Buyukliev

Reputation: 43508

Regard *char_ptr++ as:

char tmp = *char_ptr;
char_ptr++;
c = toupper(tmp);

So it effectively fetches the current character, and advances the pointer by one. Because the ++ operator has a higher precedence than the unary *, such an expression is evaluated in the order *(char_ptr++).

The incrementation is applied first, but since the postfix ++ operator returns the result prior to the manipulation, the * dereferencing operator is applied on the old address.

char_ptr-- simply decreases the pointer by one.

Upvotes: 3

0xCAFEBABE
0xCAFEBABE

Reputation: 5666

This question is solved by precedence rules. The expression

*char_ptr++

is resolved to

*(char_ptr++)

meaning "The character after the character that ptr is pointing to". So when c equals a '\0' (zero), the pointer gets decremented, so it points to the previous character. It practically decrements to the previous character whenever there's a 0 encountered, so that char_ptr points to whatever it pointed to during the last run of the block.

Upvotes: 0

Ewald
Ewald

Reputation: 5751

Yes, you are correct. It's a way of "peeking" ahead into the "string" and then returning to where you were if the string was at it's end.

Upvotes: 0

Julian
Julian

Reputation: 2061

In the same way as ++ increments the pointer and points to the next character, -- decrements it, pointing to the previous character. In this case, it puts it back to the last real character from the terminating null.

Upvotes: 1

Related Questions