string function: insert - self implemented via a copy function - c

Question

i got a new task from my teach to implement some basic string operations like append, remove, substr and insert.
while i was thinking at how should i approach this problem i thought i could just write a function which does the copying and ...
int copy(char* buffer,char * string,int begin,int end)
{
if(end == 0)
end = length(string);
//Copy from begin to end and save result into buffer
for(int i = 0; i < end;i++)
buffer[i] = *(string+begin+i);
return end;
}
with that implementation, so my idea i could just implement all the other functions asked by my teacher like this :
void insert(char* buffer,char * string, char * toInsert, int begin,int end)
{
//Copy till the position of the original string
begin = copy(buffer,
string,0,begin);
//Insert
//copy from the last change of the original string
begin = copy(buffer+begin,
toInsert,0,end);
//Copy whats left
copy(buffer+begin,
string);
}
So if i now try to insert something with this function i get some weird output:
int main() {
char * Hallo = "Hello World how are things?";
char * appendix = "Halt die schnauze!";
char buffer[128];
for (int i = 0; i < 128;i++)
buffer[i] = -0;
insert(buffer,Hallo,appendix,5,0);
printf("%s\n",buffer);
return 0;
}
output: HelloHalt die schnHello World how are things?
I simply cannot get my head around why the output does look like this. Do i have a logical mistake there?

Answer 1

fix like this:
#include <stdio.h>
size_t length(const char *s){
size_t len = 0;
while(*s++){
++len;
}
return len;
}
int copy(char *buffer, const char *string, int begin, int end){
int len = 0;//copy length
if(end == 0)
end = length(string);
for(int i = begin; i < end; i++)//End position is not included
buffer[len++] = string[i];
return len;
}
void insert(char *buffer, const char *string, const char *toInsert, int begin, int end){
int len;
len = copy(buffer, string, 0, begin);
len += copy(buffer + len, toInsert, 0, end);
len += copy(buffer + len, string, begin, end);
buffer[len] = 0;
}
int main(void) {
char * Hallo = "Hello World how are things?";
char * appendix = "Halt die schnauze!";
char buffer[128] = {0};
insert(buffer, Hallo, appendix, 5, 0);
printf("%s\n",buffer);
return 0;
}