String concatenation without libraries in C

Question

I am having trouble concatenating strings in C without library function. I tried the following:

#include <stdio.h>

struct word {
  char *str;
  int wordSize;
};

void concat(struct word words[], int arraySize, int maxSize) { // word array, its size, and max size given
  char result[maxSize];
  int resultSize = 0;
  struct word tmp;

  for (int i = 0; i < arraySize; i++) {
    tmp = words[i];
    for (int j = 0; j < words[i].wordSize; j++,  resultSize++) {
      result[resultSize + j] = tmp.str[j];
    }
  }

  puts(result);
}

For example, if the struct array words contain [{"he", 2}, {"ll", 2}, {"o", 1}], the result should be hello. However, this code prints h�l�o where the second and fourth letters are questionmark. Can anyone help me debug this?

Answer 1

Keep it simple and the bugs will fix themselves. Don't mix up the position in the result buffer with the loop iterators. No need for temporary variables.

#include <stdio.h>

typedef struct {
  char *str;
  int wordSize;
} word;

void concat(word words[], int arraySize, int maxSize) { 
  char result[maxSize];
  int count=0;
  
  for(int i=0; i<arraySize; i++)
  {
    for(int j=0; j<words[i].wordSize; j++)
    {
      result[count]= words[i].str[j];
      count++;
    }
  }
  result[count] = '\0';
  
  puts(result);
}

int main()
{
  word w[3] = { {"he", 2}, {"ll", 2}, {"o", 1} };
  concat(w, 3, 128);
}