Substring in C Programming

Question

I just want to extract the particular word from the string.
My program is:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BUFFER_SIZE 100

int main() {

    FILE *f;
    char buffer[100];
    char buf[100];
    int count=0;
    char res[100];


  f=fopen("1JAC.pdb","rb");

while(fgets(buffer,BUFFER_SIZE,f))
{

if(strncmp(buffer,"ATOM",4)==0 && strncmp(buffer+13,"CA",2)==0 && strncmp(buffer+21,"A",1)==0)
{
strcpy(buf,buffer);

}

printf (buf);

Output of the program is ATOM 1033 CA LEU A 133 33.480 94.428 72.166 1.00 16.93 C

I just want to extract the word "LEU" using substring. I tried something like this:

Substring(17,3,buf);

But it doesn't work...
Could someone please tell about the substring in C.

Do not use string-reading function like fgets for binary files. Either the file is binary, in which case you should use the proper functions to read it, or the file is a text-file in which case you should open it as such. It's doubly problematic here since one of the things that differs between a text-file and a binary file is newline handling, and fgets depends on newlines being handled correctly. — Some programmer dude
– Some programmer dude, Commented Jul 9, 2015 at 10:37
Regarding your question, when you say "it doesn't work", can you please elaborate on that? What do you mean by "it doesn't work"? And what is the Substring function you reference? — Some programmer dude
– Some programmer dude, Commented Jul 9, 2015 at 10:39
Oh, and don't use input from anywhere as a format string to printf. Think about what would happen if there was printf formatting codes in the input you read? — Some programmer dude
– Some programmer dude, Commented Jul 9, 2015 at 10:40

Community · Accepted Answer · 2017-05-23 12:23:26Z

1

Memcpy seems to be best way to do this ...

memcpy( destBuff, sourceBuff + 17, 3 );
destBuff[ 3 ] = '\0';

Please remember to add the null terminators if needed (as I have done in the example).

Also this has been answered before, several times on Stack-overflow

(Get a substring of a char*)

edited May 23, 2017 at 12:23

CommunityBot

11 silver badge

answered Jul 9, 2015 at 10:41

Neil

1,0669 silver badges19 bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

Shlomi Agiv Over a year ago

memcpy should be used only if you're dealing with binary data.

Neil Over a year ago

What are we dealing with here? It's all binary data when copying a part of string to another. I disagree with your comment Shlomi. As long as the coder takes into account the size of the chars.

Shlomi Agiv Over a year ago

since he copied the text from the file to the buffer using strcpy, it has to be a text buffer

Neil Over a year ago

We are dealing with C here - what is the difference with a TEXT buffer and a BINARY buffer?? Nothing - they are all bytes, bit patterns, binary. I really do not get what you are attempting to put across here Shlomi.

Shlomi Agiv Over a year ago

I consider it bad practice to mix string functions with binary functions unless required. You're right that in the current situation There is no difference

|

Rohit Daundkar · Accepted Answer · 2015-07-09 10:52:02Z

1

//Use the following substring function,it will help you.
int main(int argc, char *argv[])
{

FILE *filepointer;
char string[1700];


filepointer=fopen("agg.txt", "r");

if (filepointer==NULL) 
{
        printf("Could not open data.txt!\n");
    return 1; 
}

while (fgets(string, sizeof(string), filepointer) != NULL) 
{   
    char* temp=substring(string,17,3);/*here 17 is the start position and 3 is the length of the string to be extracted*/ 
}

return 0;   
}




char *substring(char *string, int position, int length) 
{
char *pointer;
int c;
pointer = (char*) malloc(length+1);

if (pointer == NULL)
{
   printf("Unable to allocate memory.\n");
   exit(1);
}

 for (c = 0 ; c < length ; c++)
{
   *(pointer+c) = *(string+position-1);      
    string++;   
}

*(pointer+c) = '\0'; 
return pointer;
}

answered Jul 9, 2015 at 10:52

Rohit Daundkar

172 bronze badges

4 Comments

Neil Over a year ago

Good example Rohit, however wouldn't it be better to return NULL on a malloc error and let the user of the function handle the error? I realize this is an example of what to use that you have supplied, just thought I'd mention the error handling, thanks

Rohit Daundkar Over a year ago

thanks Neil, yes you are correct if the argument passed is NULL then then there will be the malloc error.It is users responsibility to check for NULL values.

Lundin Over a year ago

Funny how people insist on using dynamic memory "to save space", and then in the next breath allocate 1700 bytes on the stack without concern...

Rohit Daundkar Over a year ago

Lundin it was a good joke but before commenting please ask why have i used 1770 bytes?? thats bcoz the file which i am reading contains FIXED LENGTH 1700 bytes of data on a single line.

Shlomi Agiv · Accepted Answer · 2015-07-09 10:48:01Z

0

char out[4] = {0};
strncpy(out, buf+17, 3);

edited Jul 9, 2015 at 10:48

answered Jul 9, 2015 at 10:39

Shlomi Agiv

1,1988 silver badges18 bronze badges

3 Comments

Some programmer dude Over a year ago

Note that strncpy might not terminate the destination string, and in this case it won't.

Lundin Over a year ago

Don't use strncpy(), it is a dangerous function, because of this very null termination issue. It is safer (and faster) practice to use memcpy() and manually terminate the string.

Shlomi Agiv Over a year ago

You have a point, yet once you've started using string functions, you should stick with it. mixing memcpy with strncmp/fgets can cause a whole new set of problems.

Collectives™ on Stack Overflow

Substring in C Programming

3 Answers 3

8 Comments

4 Comments

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

8 Comments

4 Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related