Strings in c, how to get subString

Question

I am pretty new to C.

I have a string:

char * someString;

If I want the first 5 letters of this string and want to set it to otherString, how would I do it?

Answer 1

char* someString = "abcdedgh";
char* otherString = 0;

otherString = (char*)malloc(5+1);
memcpy(otherString,someString,5);
otherString[5] = 0;

UPDATE:
Tip: A good way to understand definitions is called the right-left rule (some links at the end):

Start reading from identifier and say aloud => "someString is..."
Now go to right of someString (statement has ended with a semicolon, nothing to say).
Now go left of identifier (* is encountered) => so say "...a pointer to...".
Now go to left of "*" (the keyword char is found) => say "..char".
Done!

So char* someString; => "someString is a pointer to char".

Since a pointer simply points to a certain memory address, it can also be used as the "starting point" for an "array" of characters.

That works with anything .. give it a go:

char* s[2]; //=> s is an array of pointers to char
char** someThing; //=> someThing is a pointer to a pointer to char.
//Note: We look in the brackets first, and then move outward
char (* s)[2]; //=> s is a pointer to an array of char

Some links: How to interpret complex C/C++ declarations and How To Read C Declarations

Answer 2

strcpy, strdup, otherString[5] = '\0'. Some combo of the preceeding.

Answer 3

#include <string.h>
...
char otherString[6]; // note 6, not 5, there's one there for the null terminator
...
strncpy(otherString, someString, 5);
otherString[5] = 0;

Answer 4

strncpy(otherString, someString, 5);

Don't forget to allocate memory for otherString.

Answer 5

You'll need to allocate memory for the new string otherString. In general for a substring of length n, something like this may work for you (don't forget to do bounds checking...)

char *subString(char *someString, int n) 
{
   char *new = malloc(sizeof(char)*n+1);
   strncpy(new, someString, n);
   new[n] = '\0';
   return new;
}

This will return a substring of the first n characters of someString. Make sure you free the memory when you are done with it using free().

Answer 6

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

int main ()
{
        char someString[]="abcdedgh";
        char otherString[]="00000";
        memcpy (otherString, someString, 5);
        printf ("someString: %s\notherString: %s\n", someString, otherString);
        return 0;
}

You will not need stdio.h if you don't use the printf statement and putting constants in all but the smallest programs is bad form and should be avoided.

Answer 7

Generalized:

char* subString (const char* input, int offset, int len, char* dest)
{
  int input_len = strlen (input);

  if (offset + len > input_len)
  {
     return NULL;
  }

  strncpy (dest, input + offset, len);
  return dest;
}

char dest[80];
const char* source = "hello world";

if (subString (source, 0, 5, dest))
{
  printf ("%s\n", dest);
}

Answer 8

Doing it all in two fell swoops:

char *otherString = strncpy((char*)malloc(6), someString);
otherString[5] = 0;