divide string into small strings

[ripley]

Hello,

I have a problem to divide the string into smaller strings, which have 3 char form the main string. I want to put the small strings into the field of pointers. I am doing something like this, but it is completly wrong:

char *str = "hello world";

	int i = strlen(str);
	char **str2 ;
	str2 = new char*[i];

	if(i > 3)
	{
		for(int a = 0, b = 0; a<i; b++ ) {
		strncpy(str2[b], str+a, 3);	
		a =a+3;
		}
 	}

Could anyone explain me, how to do that properly, and why this doesnt work? Thank you.

[tummychow]

Switch over to std::string and you can use substr() to get the job done.
http://www.cplusplus.com/reference/string/string/substr/

[ripley]

Yes, I know, that its possible through std::string, but how it is possible if I have a char* ? Thank you.

[tummychow]

If you must use char[]... try creating a function to get the job done. Give it a starting and ending position (left-inclusive) and, starting from that position and cycling in a for loop as long as your counter is less than the end position, copy the characters into a return array of a length equal to the difference between those two, plus one for the null.
Got that? (I mean this last line humorously because I know how long that is and how difficult it may be to read.)

[ripley]

No, I dont need to use char[], I just wanted to know, how is that posible. Thank you fro your explanation.

[wasabi]

I think the problem with your code is simply that you wrote the strcpy() wrong. Well, that and the fact that you didn't allocate any memory for str2[b].

Your strcpy_s() should be strcpy_s(str2[b],&str[a],4); (str+a == str[a], which is of type char, while strcpy_s (or strncpy) requires a second argument of type char*). Actually, I can't rememeber if the size_value is the second or third argument... anyways. And the size_value has to be equal to 4 because you want three characters + '\0' at the end of your c-string.

And allocate memory fo str2[b]. That always helps, too.

[blackcoder41]

do you mean this

#include <iostream>
using namespace std;

int main () {
  string name = "hello";
  char* c = new char;
  *c = name[3];
  cout << c << '\n';
  return 0;
}

why are you using char**? any good reason?

[ripley]

No Blackoder, what you wrote, is only returning one symbol, and I wanted to return 3 symbols. I used char** because its a pointer to field of pointers, isn it? And its content should by like this:

hel  //str2[0]
lo     //str2[1]
wor //str2[2]
ld      //str2[3] 

And Wasabi, didnt I allocate memory for str2[b] if I allocated the memory for str2? Or was it only for that pointer, which points to another pointers?

[blackcoder41]

#include <iostream>
#include <conio.h>
using namespace std;

int main () {
  char *str = "i love this forum!";
  int i = strlen(str);
  char **str2 = new char*[i/3];

  for(int x=0; x*3<i; x++) {
    str2[x] = new char[3];
    strncpy(str2[x], str+x*3, 3);
    cout << str2[x] << '\n';
  }
  getch();
  return 0;
}

[ripley]

I figured that out by myself. Thank you.