Advanced C 1992

Pointers and Indirection	C C C
	C3C
	C C C
	C

To make up for the shortcomings in C’s character handling, a large number of string functions are in the standard library (see Chapter 14, “ANSI C Library Functions”). Because the particular functionality your application requires might not be present in one of the C library functions, you can write a function to do whatever you want.

This section doesn’t show you how to count words in a string (the demo program does that), but it does show you how easy it is to work with strings and manipulate pointers to strings.

By now, you should not still be writing programs that compare strings using logical operators, as in the following example:

char szFirst[] = {“This is a string”}; char szNext[] = {“Before this one”);

if (szFirst > szNext)

{

/* the test was meaningless! */

}

This comparison simply evaluates the addresses of the two strings, not their contents. The result of the test is undefined because you cannot predict where in memory the strings will be located, nor are their contents related to their memory address.

The correct way to compare two strings is to call the library function strcmp(), which returns a value based on the logical relationship between the two strings:

char szFirst[] = {“This is a string”}; char szNext[] = {“Before this one”);

if (strcmp(szFirst, szNext) > 0)

{

/* szFirst is before szNext! */

}

This relationship is much more useful to your programs than are the string’s addresses. NUMWORD.C counts the number of words in a sentence that are entered from the keyboard (see Listing 3.4).

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Part I • Honing Your C Skills

Listing 3.4. NUMWORD.C.

/* NUMWORD, written 20 May 1992 by Peter D. Hipson */ /* Program to count words in sentences. */

#include <stdio.h> // Make includes first part of file #include <string.h> // For string functions

#define	TRUE	1
#define	FALSE	0

int main(void); // Define main() and the fact that this program doesn’t // use any passed parameters.

int	NumberWords(char		* pString);
#define	BIGEST_LINE	256	/* The biggest line readable from keyboard */

/* Though these variables are defined as external, they can be

*defined inside the function or be allocated dynamically,

*depending on the program’s needs and the amount of memory available */

char szInput[BIGEST_LINE];

int main()

{

int i;

printf(

“Enter lines, when last one is entered\n”

“provide a End-Of-File (ctrl-Z on most systems)\n” “to end the program.\n\n”);

while (gets(szInput))

{

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Pointers and Indirection

printf(“Words = %2d ‘%.50s’\n”,

	NumberWords(szInput),
	szInput);
}
printf(“\n”);
return (0);
}
int	NumberWords(
char	szString[])
{
int	i;
int	nBlank = TRUE;
int	nCount = 0;

for (i = 0; szString[i]; i++)

{

if (szString[i] != ‘ ‘)

{

if (nBlank)

{

++nCount;

}

nBlank = FALSE;

}

else

{

nBlank = TRUE;

}

}

return(nCount);

}

C C C

C3C C

C C C

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Part I • Honing Your C Skills

NUMWORD has a very simple loop that calls gets() until the end-of-file is reached. After gets() returns, the loop itself calls printf(), which has as one of its parameters a call to the NumberWords() function.

printf(“Words = %2d ‘%.50s’\n”, NumberWords(szInput), szInput);

C first calls NumberWords() and then passes to printf() the returned value, along with the other parameters.

for (i = 0; szString[i]; i++)

{

if (szString[i] != ‘ ‘)

{

if (nBlank)

{

++nCount;

}

nBlank = FALSE;

}

else

{

nBlank = TRUE;

}

}

NumberWords() has a loop that looks at the passed string and parses out the words. The format for this loop is a for() loop; while() can be used, however. This loop moves through the character string and increments an index to the passed string. When the loop starts, it is assumed that a blank has been encountered already. This assumption is made by setting the blank flag (nBlank) on so that you can count the first word regardless of whether it’s preceded by blanks. Also, the word count (nCount) is set to zero, which indicates that no words have been counted.

When the first nonblank character is found, the word counter is incremented (a word has been found), and the blank flag is turned off. The loop continues searching for the next blank; when it is found, the blank flag is set to on and the process continues until the end of the string is found.

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Pointers and Indirection	C C C
	C3C
	C C C
	C

Indirection to Access Character Strings

To change NUMWORD to use indirection to access the string, the loop in NumberWords() must change slightly (see Listing 3.5).

Listing 3.5. NUMWORD1.C.

/* NUMWORD1, written 21 May 1992 by Peter D. Hipson */ /* Program to count words in sentences. */

#include <stdio.h> // Make includes first part of file #include <string.h> // For string functions

#define	TRUE	1
#define	FALSE	0

int main(void); // Define main() and the fact that this program doesn’t // use any passed parameters.

int	NumberWords(char		* pString);
#define	BIGEST_LINE	256	/* The biggest line readable from keyboard */

/* Although these variables are defined as external, they can be

*defined inside the function or be allocated dynamically,

*depending on the program’s needs and memory available. */

char szInput[BIGEST_LINE];

int main()

{

int i;

printf(

continues

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Part I • Honing Your C Skills

Listing 3.5. continued

“Enter lines, when last one is entered\n”

“provide a End-Of-File (ctrl-Z on most systems)\n” “to end the program.\n\n”);

while (gets(szInput))

{

printf(“Words = %2d ‘%.50s’\n”,

	NumberWords(szInput),
	szInput);
}
printf(“\n”);
return (0);
}
int	NumberWords(
char	* pString)
{
int	nBlank = TRUE;
int	nCount = 0;
do
{
	if (*(pString) && *(pString) != ‘ ‘)
	{
	if (nBlank)
	{
	++nCount;
	}

nBlank = FALSE;

}

else

{

nBlank = TRUE;

}

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} while(*(pString++));

return(nCount);

}

NumberWords() again has a loop that looks at the passed string and parses out the words. The format for this loop is do()...while(). A straight while() or even a for() loop, however, can be used:

do

{

if (*(pString) && *(pString) != ‘ ‘)

{

if (nBlank)

{

++nCount;

}

nBlank = FALSE;

}

else

{

nBlank = TRUE;

}

} while(*(pString++));

You no longer need to use an index variable, because you are using the pointer that was passed to keep track of where you are in the string. One possible advantage to this method is that by incrementing the pointer rather than an index to a string, the function generally is both faster and smaller.

This loop moves through the character string and increments the passed pointer. Remember that this passed pointer is a private copy for this function and can be modified. It is assumed that a blank has been encountered already, by setting the blank flag on so that you can count the first word regardless of whether it is preceded by blanks. Also, the word count is set to zero so that no words are counted. When the first nonblank character is found, the word counter is incremented (a word has been found) and the blank flag is turned off. The loop continues searching for the next blank; when it is found, the blank flag is set to on and the process continues until the end of the string is found.

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Part I • Honing Your C Skills

Listing 3.6 shows the assembly listing for the version of NumberWords() that uses pointer indexing. The compiler produces this machine code, commented with the original source lines, when the function is compiled.

Listing 3.6. NUMWORD3.COD, the assembly listing for the pointer version of NumberWords().

;Edited for size.

;Static Name Aliases

TITLE numword3.c NAME numword3

.8087
_TEXT	SEGMENT		WORD PUBLIC ‘CODE’
_TEXT	ENDS
_DATA	SEGMENT		WORD PUBLIC ‘DATA’
_DATA	ENDS
CONST	SEGMENT		WORD PUBLIC ‘CONST’
CONST	ENDS
_BSS	SEGMENT		WORD PUBLIC ‘BSS’
_BSS	ENDS
DGROUP		GROUP	CONST, _BSS, _DATA
ASSUME		CS: _TEXT, DS: DGROUP, SS: DGROUP
EXTRN		--acrtused:ABS
EXTRN		--chkstk:NEAR
_TEXT		SEGMENT
ASSUME		CS: _TEXT

;|*** /* NUMWORD3, written 21 May 1992 by Peter D. Hipson */

;|***
;|*** #include <stdio.h>		// Make includes first part of file
;|*** #include <string.h> // For string functions
;|***
;|*** #define	TRUE	1
;|*** #define	FALSE	0
;|***
;|***
;|*** int	NumberWords(char		* pString);
;|***
;|*** int	NumberWords(

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						Pointers and Indirection	C C C
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;|***	char	* pString)					C
;|***
;|*** {
; Line 15
PUBLIC		_NumberWords
_NumberWords		PROC	NEAR
*** 000000		55			push	bp
*** 000001		8b	ec		mov	bp,sp
*** 000003		b8	06	00	mov	ax,6
*** 000006		e8	00	00	call	__ chkstk

;pString = 4

;nBlank = -2

;nCount = -4

;|***

;|*** int		nBlank	= TRUE;
; Line 17
*** 000009		c7	46	fe 01 00		mov	WORD PTR [bp-2],1
;nBlank
;|*** int		nCount	= 0;
; Line 18
*** 00000e		c7	46	fc 00 00		mov	WORD PTR [bp-4],0
;nCount
;|***
;|***	do
; Line 20
			$D239:
;|***	{
; Line 21
;|***		if (*(pString) && *(pString) != ‘ ‘)
; Line 22
*** 000013		8b	5e	04	mov	bx,WORD PTR [bp+4]		;pString
*** 000016		8a	07		mov	al,BYTE PTR [bx]
*** 000018		88	46	fa	mov	BYTE PTR [bp-6],al
*** 00001b		0a	c0		or	al,al
*** 00001d		74	15		je	$I242
*** 00001f		3c	20		cmp	al,32
*** 000021		74	11		je	$I242
;|***		{
; Line 23
;|***		if	(nBlank)

continues

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Part I • Honing Your C Skills

Listing 3.6. continued

; Line 24
*** 000023		83	7e fe 00	cmp	WORD PTR [bp-2],0			;nBlank
*** 000027		74	03	je	$I243
;|***		{
; Line 25
;|***			++nCount;
; Line 26
*** 000029		ff 46 fc		inc	WORD PTR [bp-4]		;nCount
;|***		}
; Line 27
;|***
;|***		nBlank = FALSE;
; Line 29
			$I243:
*** 00002c		c7	46 fe 00 00		mov	WORD PTR [bp-2],0
;nBlank
;|***		}
; Line 30
;|***		else
; Line 31
*** 000031		eb 06		jmp	SHORT $I244
*** 000033		90		nop
			$I242:
;|***		{
; Line 32
;|***		nBlank = TRUE;
; Line 33
*** 000034		c7	46 fe 01 00		mov	WORD PTR [bp-2],1
;nBlank
;|***		}
; Line 34
			$I244:
;|***
;|***	} while(*(pString++));
; Line 36
*** 000039		8b	5e 04	mov	bx,WORD PTR [bp+4]		;pString
*** 00003c		ff 46 04		inc	WORD PTR [bp+4]		;pString
*** 00003f		80	3f 00	cmp	BYTE PTR [bx],0
*** 000042		75	cf	jne	$D239

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