Thread: About malloc,realloc,pointer and array

i have an expression in an array, i want the expression to omit space and tab characters and put it into an buffer,but i dont know how to go on to do it.

Code:

#include <string.h>

char * InitExpression(char *pExpression, char *pBuf)
{
	char *p = NULL;
	int i = 0;

	pExpression = (char *)malloc(1);
	p = pExpression;

	while ('\0' != *pBuf)
	{
		if (32 != *pBuf && 9 != *p)
		{
			*p = *pBuf;
			p = (char *)realloc(pExpression, 1);
			++p;
		}
		++pBuf;
	}
	return pExpression;
}

int main()
{
	struct Polyn *pUnitaryPolynomial = NULL;
	char cExpression[] = {"	95 x ^  2	-	4  x +	2	"};
	char *szExpression = NULL;

	szExpression = InitExpression(szExpression, cExpression);

	return 0;
}

Originally Posted by bernt

The second argument to realloc is the size of the new block of memory, not how much to increase it by.
Calling realloc inside of a loop seems like a bad idea to me anyway - you'll want to consider ways of allocating enough space for the new string on the first malloc.

Actually using realloc on a byte by byte basis is a real bad idea (Read: Bottleneck). Probably our friend would be better watching the buffer pointer as he adds stuff and reallocating in at least 64 byte blocks.

It is better practice to check against ' ' and '\t' (space and tab) rather than the ASCII numeric equivalents. Easier to read, more portable if there was ever any question about which character mapping is in use, etc.

u can malloc a buffer first, and test if the buffer is full, if it is full, then u can relloc the buffer

now, i updated the code:

Code:

char * InitExpression(char *pExpression, char *pBuf)
{
	char *p = NULL;
	int i = 0;

	pExpression = (char *)malloc(1);

	while ('\0' != *pBuf)
	{
		p = pExpression;
		if (' ' != *pBuf &&	'\t' != *pBuf)
		{
			*(p+i) = *pBuf;
			pExpression = (char *)realloc(pExpression, 1);
			++i;
		}
		++pBuf;
	}
	return pExpression;
}

and i wanna put the pBuf into pExpression without any space and tab, but the content of the pExpression is always wrong, i dont know how to modify it!help

You don't even need to realloc() really.
pExpression cannot take more space than strlen(pBuf) + 1.
Just allocate that amount and if you want, realloc() /shrink to actual size.

Code:

char *ret = malloc( strlen(pExpression) + 1 );
// check malloc return...
while( *pBuf != '\0') {
   if(*pBuf != ' ' && *pBuf != '\t')
      *ret++ = *pBuf;

   pBuf++;
}
*ret = '\0';   // don't forget!

thanks Bayint Naung, i now know how to code ^_^

Code:

char * InitExpression(char *pExpression, char *pBuf)
{
	char *p = NULL;

	pExpression = (char *)malloc(strlen(pBuf) + 1);
	p = pExpression;

	while ('\0' != *pBuf)
	{
		if (' ' != *pBuf &&	'\t' != *pBuf)
		{
			*p++ = *pBuf;
		}
		++pBuf;
	}

	*p ='\0';

	return pExpression;
}

Since you are returning dynamic memory allocation.
1st parameter of your function is redundant.
Question 4.8

Code:

char * InitExpression(char *pBuf)
{
	char *p = NULL;
        char *pExpression, 
	pExpression = (char *)malloc(strlen(pBuf) + 1);
	p = pExpression;

	while ('\0' != *pBuf)
	{
		if (' ' != *pBuf &&	'\t' != *pBuf)
		{
			*p++ = *pBuf;
		}
		++pBuf;
	}

	*p ='\0';

	return pExpression;
}

And you need not cast malloc return value. Question 7.7

pBuf includes the content of my expression, and in my main function i code:

Code:

int main()
{
    char cExpression[] = {"-2+4x^2-3"};
    char *szExpression = NULL;

    szExpression = InitExpression(szExpression, cExpression);

    free(szExpression);
    szExpression = NULL;

    return 0;
}