[ask]read file and get substr

[zvtral]

hlow all ,

iam begin in c programming so i need your advice

i have sample.txt

308201a1a0108009090318035

i want to read the file and get substr start 1 to 3 character from that file

this my code but this still error

 pFile = fopen(szFileName, "r");    char ch ;
    while ( 1 )
    {
       ch = fgetc ( pFile ) ;
       if ( ch == EOF )
       break ;
       printf ( "%c", ch ) ;
       char *to = (char*) malloc(6);
       strncpy(to, from+0, 3);
       printf("\nopening file: %s !", to);

       
    }
      fclose ( pFile ) ;

i want get output like this

308

any one can help me for fix this code

thx for your advice and help

br\\zvtral

[dmh2000]

1. what does 'from' contain? nothing as far as your code shows
2. you only read one character from the file and then did not use it.
3. your 'to' was malloc'ed for 6 characters but malloc doesn't set the data to 0 so it may not be 0's
4. strncpy won't put the 0 delimiter at the end of 'to' if the character count limit (3) is reached. so you need to be sure you add a zero delimiter

use fgets with 'from' as the destination buffer.
do the strncpy
add a 0 delimiter at to[3]

[zvtral]

@dmh2000
thx for your advice

my code i get from internet so this code not correct
can you explain about your advice
i have fileinput example.txt

308201a1a0108009090318035

so i want output can display only select substr

output

820

if i using shell script like this

export i=`cat example.txt`
echo $i|awk '{print substr("'"$i"'",3,3)}'

can you help me how do that with c code

1. what does 'from' contain? nothing as far as your code shows
2. you only read one character from the file and then did not use it.
3. your 'to' was malloc'ed for 6 characters but malloc doesn't set the data to 0 so it may not be 0's
4. strncpy won't put the 0 delimiter at the end of 'to' if the character count limit (3) is reached. so you need to be sure you add a zero delimiter

use fgets with 'from' as the destination buffer.
do the strncpy
add a 0 delimiter at to[3]

[Matticus]

iam begin in c programming so i need your advice

my code i get from internet so this code not correct

If you want to learn C, you'd best start at the beginning and slowly work your way through the basics.

The chances of "finding" good code on the internet that does exactly what you want are slim - knowledge of programming will help you understand/modify such code. However, the best thing you can do for yourself while learning is *not* simply taking code from elsewhere, but writing it yourself.

can you help me how do that with c code

You probably won't receive a complete program here, because that's not what we do. We assist people who have specific problems with their code.

If you have some knowledge of C, then the following links can help guide you to a solution:

C File I/O Tutorial - Cprogramming.com

C Strings - Cprogramming.com

It's usually best to determine the algorithm you need to solve your problem by hand on paper first. Start at the beginning, and take it step by step, noting everything you do to arrive at a solution. These notes can help you generate the code you are looking for.

When you start to write code, just write little bits at a time, compile, and test. See this link:

A development process

When you get stuck on a particular problem, share your code here (be sure to use code tags) and ask specific questions, and you'll likely get good answers to guide you.

[c99tutorial]

echo $i|awk '{print substr("'"$i"'",3,3)}'

can you help me how do that with c code

To replicate this in C, just read from stdin via getchar() and don't worry about fopen, etc.

int pos = 0;
int c = EOF;
while ((c = getchar()) != EOF) {
  printf("pos: %d, char: '%c'\n", pos, c);
  pos++;
}

Start with this basic program and modify it to do what the "awk" part of your pipeline does.