Question About Reading Lines from stdin

[Zildjian]

Hello,

I was just wondering if it's possible to do the following. Is it possible to use fgets to read in lines from stdin more than once? For example, could you go:

 while (fgets(buf,BUFSIZ,stdin) != NULL)
   // do something

while ( fgets(buf2,BUFSIZ,stdin) != NULL)
    //do something else

Basically, I want to know if it's possible to read in lines from stdin once, and then read in lines again from the beginning? I thought of this because I was thinking of a way to calculate the size of input given from stdin, and then doing something else with the lines from stdin later on. Does this even make sense? Thanks for the help.

[WaltP]

Once you've read the line, the line has been read and can't be read again. Since you read it into a buffer (I assume) simply use the buffer again.

[Zildjian]

Could the sizeof function be used on stdin to calculate the size of the input? For instance:

 int size = sizeof(stdin);

Is this valid? Or is there some other way to determine the size of the input?

[WaltP]

No.

That will return the size of the stdin pointer.

[Zildjian]

Ok. Now how about something completely different?

If I read in a line from stdin, use strtok to tokenize it, and then use atoi to change the string to an int, if the tokenized string has a \n character on the end of it, will that mess up atoi? Say the string is 10\n and then I used atoi on it. Normally I would just tokenize and use a \n as a delimiter, but for the problem I'm about to work on, I won't know how many tokens will be on each line. Sorry for all the questions, but I'm trying to get things sorted out in my head before I dive into this problem. Thanks.

[quzah]

Originally posted by Zildjian
Ok. Now how about something completely different?

If I read in a line from stdin, use strtok to tokenize it, and then use atoi to change the string to an int, if the tokenized string has a \n character on the end of it, will that mess up atoi? Say the string is 10\n and then I used atoi on it.

Why don't you just try it and find out?

void foo( void )
{
     printf("%d\n", atoi( "10\n" ) );
}

Originally posted by Zildjian
Normally I would just tokenize and use a \n as a delimiter, but for the problem I'm about to work on, I won't know how many tokens will be on each line. Sorry for all the questions, but I'm trying to get things sorted out in my head before I dive into this problem. Thanks.

Why don't you just remove the \n first?

if( (ptr=strchr( buffer, '\n' ) != NULL )
    *ptr = '\0';

Quzah.

[jim mcnamara]

A. getc() moves forward in a stream and ungetc() moves backward.

So it is possible, although not necessarily desirable, to read the same line (or a group of characters) from stdin - twice

B. length of input line - use strlen() in string.h

char tmp[256];
memset(tmp,0x00,sizeof(tmp));
if(fgets(tmp,255,stdin)!=NULL) printf("length=%u\d\n",strlen(tmp));

C. atoi(), atof(), strtod() all ignore control characters and spaces. They stop reading the string when they encounter one.

You should get a copy of Herbert Schild's book 'C The complete reference' or something like it. It's organized so that if you look in the File I/O section for a few minutes you could have found ungetc()

[quzah]

Originally posted by jim mcnamara
You should get a copy of Herbert Schild's book 'C The complete reference' or something like it.

*reels in horror* Nooooooooooooooooo!

Quzah.