asked on

Use C to parse data from a file ?

In tmp.txt, I have :

1756,"this, that",01/01/01,A
745,there,,B
75,hello,01/01/01,C
....

How do I write a C prog that will read each line from the tmp.txt and seperate each field into
a variable ?

For above example:

id = 1756
str = "this,that" -- this is compilcated, beacuse I have a comma.
date = 01/01/01
chr = A

id = 745
str = there
date =
chr = B

id = 75
str = hello
date = 01/01/01
chr = C

ASKER CERTIFIED SOLUTION

fcavalier

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tcy08

ASKER

what if I don't want this str = "this,that", take out
the ", so I have str = this,that. What should I change ?

fcavalier

Just a personal preference, but I would do that outside of parseline, with a separate function. Here's a function and updated main().

The parseline() function stays unchanged.

What you are asking for is often part of parsing a database export. In that case, you will also want to convert occurrences of "" inside a string to a single ", because that is how most databsae programs export fields containing ". (Annoying to process.) See the comment inside FixDoubleQuote()

/* EXAMPLE USE of parseline() */

void FixDoubleQuote(char *psz)
{ /* If str starts with a '\"' then strip a leading and
trailing double quote
*/
char *ptr = psz;
const char *ptr2;

if (*ptr != '\"') { /* Not starting with double quote. Don't change */
return;
}
ptr2 = ptr+1; /* ptr2 starts ahead of ptr, so in
essence we are copying the string
down, one character at a time.
*/

while(*ptr2) {

/* NOTE: To convert "" to " within the string, (common in database export
processing, then uncomment the next line.
if ((*ptr2 == '\"')&&(ptr[1]=='"')) ptr2++;
*/
*ptr++ = *ptr2++;
}
*ptr = '\0';

/* Remove trailing quote */
if (*(ptr-1) == '\"') {
*(ptr-1) = '\0';
}
} /* FixDoubleQuote */

#include <stdio.h>
int main(int argc,char **argv)
{
char buf[1024];

char *pField[4];
char *id;
char *str;
char *date;
char *chr;

while(fgets(buf,sizeof(buf)-1,stdin)) {
/* fgets forces a line length limitation! This is just an example!

Might want to use something like afgets() in production, see:
http://www.mibsoftware.com/libmib/astring/

*/

if (parseline(buf,pField,4) == 4) {
id = pField[0];
str = pField[1];
FixDoubleQuote(str);
date = pField[2];
chr = pField[3];
printf("id = %s\n",id);
printf("str = %s\n",str);
printf("date = %s\n",date);
printf("chr = %s\n",chr);
} else {
/* Unexpected number of fields on line! */
}

}

}

tcy08

ASKER

Can you comment these code ?

if (iField < cMaxField) {
ppField[iField] = ptr;
iField++;
}

while(*ptr) {
if (*ptr == ',') {
*ptr++ = '\0';
if (*ptr && (iField < cMaxField)) {
ppField[iField] = ptr;
}
iField++;

} else if (*ptr == '\"') {
ptr++;
while(*ptr && (*ptr != '\"')) {
ptr++;
}
if (*ptr) {
ptr++;
}
} else {
ptr++;
}
}

I am not good with C

tcy08

ASKER

I have + another 20 points for you

fcavalier

The comments at the top are adequate for most moderately experienced C programmers.

If you aren't good with C, then it can be improved to make it easier to understand. I added some explanations.
I also split some of the autoincrement operators, and compound tests to make it easier to follow, but the code is functionally the same.

Note also that in replying I made two improvements:

Now when call with cMaxField==0, you can get an
accurate count. (I don't know how useful this is,
since line gets changed no matter what.)

Correct handling when the last field on the line is
empty. (Not a test case, but it might happen.)

<PRE>
------------------------------------------
/* Always test that we aren't making an assignment
after the end the array ppField[]
*/
if (iField < cMaxField) {
ppField[iField] = ptr;
}
iField++;

while(*ptr) {
/* This loop has three cases to handle,
and it does it with if-else tests.
ptr at a ,
ptr at a "
ptr at anything else
*/

if (*ptr == ',') {
/* We got a field separator. */
/* End the previous field */
*ptr = '\0';
ptr++;

/* Start the next field */
if (iField < cMaxField) {
ppField[iField] = ptr;
}
iField++;

} else if (*ptr == '\"') {
/* Process all characters up to
next '\"' (or end of string.)
*/
ptr++;
while(*ptr) {
if (*ptr == '\"') { /* End of string */
ptr++;
break;
}
ptr++;
}
} else {
ptr++;
}
}
return iField;
</PRE>

tcy08

ASKER

How does this :

if (iField < cMaxField) {
ppField[iField] = ptr;
}

assign the first field to the 1st variable ?
How does it knows where to end the first field ?

tcy08

ASKER

Why strtok will not work correctly ?

For example :

745,there,,B

strtok will assign 745 to 1st variable, there to 2nd and
B to third ?

But if I put a space like 745,there, ,B
the strtok will work correctly. Why ?

ozo

strtok(char *s1,const char *s2) searches the string pointed to by s1 for the first character that is not contained in the current separator string pointed to by s2
strtok considers the string s1 to consist of a sequence of zero or more text tokens separated by spans of one or more characters from the separator string s2.

tcy08

ASKER

ozo,

I have just posted a question "Is there a split function in C ?"
Can you tell me how to fix that strtok ?

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klecki_2000

Hi,

About the first code example by fcavalier.
I think the int iField = 0; should be a static value.
like static int iField = 0;
if not we loose the field number each time we leave the funcion parsing.