我自己的类似strcat（）函数仅打印一个字符数组

Question

I've been trying to practice programming so I decided to try to type the strcat() function myself, or a similar one you know. I typed this code in order to proceed it and I don't know where the problem is.

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


void main(){

  int i, j;
  char a[100]; char b[100];
    printf("enter the first string\n");
    scanf("%s", &a);
    printf("enter the second string\n");
    scanf("%s", &b);

    for(i =0; i<100; i++){
      if(a[i] == '\0')
      break;
    }


  //  printf("%d", i);

   for(j = i; j<100; j++){
      a[j+1] = b[j-i];
      if(b[j-i] == '\0')
      break;
  }

    printf("%s", a);

}

there are no syntax errors(I hope) the compiler gives me that result: It doesn't concatenate the strings, nothing happens.

It gives me the same array the same array the user entered, Does anyone has the answer?

PS: I don't know about the pointers yet.

Answer 1

Implementing strcat as a "naive byte-copy loop" is not hard, just do something like this:

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

char* strdog (char* restrict s1, const char* restrict s2)
{
  s1 += strlen(s1); // point at s1 null terminator and write from there
  do
  {
    *s1 = *s2; // copy characters including s2's null terminator
    s1++;
  } while(*s2++ != '\0'); 

  return s1;
}

int main(void)
{
  char dst[100] = "hello ";
  char src[] = "world";

  strdog(dst, src);
  puts(dst);
}

Professional libraries will do the copy on "aligned chunk of data" basis, to get a slight performance boost.