Question

I have the following code in C:

    char *str = "Hello World";
    char *s = malloc(strlen(str));
    int i =0;
    for(;i<strlen(str)-5;i++)
    {
        s += *(str+i);
    }
    printf(s);

It shows nothing. What I want is to get the substring of str stored in s.

In Java I would do the following:

    String str = "Hello World";
    String s="";

    for(int i=0;i<str.length()-5; i++)
        s+=str[i];

    System.out.println(s);

Or instead use the substring method. As String s = str.substring(1,2); for example.

How can I achieve it?

Answer 1

Use the strcpy function.

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

int main(int argc, char *argv[] ) 
{
  char *str = "Hello World";
  size_t length = strlen(str);

  char *s = (char*)malloc(sizeof(char) * (length + 1));

  strcpy(s, str);

  s[length] = '\0'; // makes sure it's NUL terminated

  printf("%s", s);

  free(s);

  return 0;
}

When allocating the destination buffer, pay attention to the fact that strings are terminated by the NUL character.

To only copy a substring, use strncpy:

strncpy(s, str + 6, strlen(str) - 6);

will just copy “World” into s.

In any case, make sure your C strings are NUL terminated before using functions like printf.

See also strcat and strncat. And well, familiarize yourself with C arrays and pointers.