C Segmentation fault error on print

Question

In the following code with the printf() statement I get a segmentation fault: 11. Without it I do not get any of errors but I want to be able to see the correct values are in the newstring value. How do I go about doing this?

  char* newstring;
  for(int i = 0; i < len; i++)
  {
    printf("value %d\n", tempfullstring[i]);
    if (tempfullstring[i]>=97 && tempfullstring[i] <=122)
    {
      char value = tempfullstring[i];
      newstring += value;
    }
  }
  printf("The new string is %s", newstrng);
  return 0;

Answer 1

I think you have a misunderstanding of how C strings work:

• They do not get initialized on declaration (so char* newstring; must be assigned separately, or you get undefined behavior)
• They cannot be concatenated with a += operator (so newstring += value; is invalid)
• The space for C strings needs to be managed explicitly (so you would either need to allocate your newstring in the automatic storage area, or add a free at the end).

The easiest way of fixing your program is to guess how long the newstring is going to be, and use strcat to append data to it:

char newstring[1000]; // some max length
newstring[0] = '\0';  // make it an empty string
...
strcat(newstring, value); // instead of newstring += value

Answer 2

newstring += value

You are appending to a string in an illegal way, what you are actually doing is changing an uninitialized pointer, so you are moving an changing address to another invalid address instead .

You should, first of all, have some room in which you are going to store the new string such with

char newstring[64];

and then append a character by doing

newstring[j] = tempfullstring[i];

This won't append the NUL terminating character through, you will have to add it to the end manually or use a different approach (such as using strncat and append directly from the original string:

strncat(newstring+j, tempfullstring+i, 1);

Answer 3

this is the code you'll want to make this work.

 char* newstring = malloc (sizeof(char)*len+1);   // this will be worst case (all are >=97 o <=122
    int j=0;
      for(int i = 0; i < len; i++)
      {
        printf("value %d\n", tempfullstring[i]);
        if (tempfullstring[i]>=97 && tempfullstring[i] <=122)
        {
          char value = tempfullstring[i];
          newstring[j]= value;
          j++;
        }
      }
      newstring[j]='\0';
      printf("The new string is %s", newstrng);
      return 0;