Newbe String Problem

[tgreiner]

I am trying to write a function that reads numbers out of a string of comma separated values. The first function is:

unsigned get_frame(unsigned &index, char input[])
 {
  const comma=',';
  char *letter;
  char *accum="";
  unsigned out;
  while (input[index]!=comma)
   {
    *letter=input[index];
    strcat(accum,letter);
    index++;
   };
   out=atoi(accum);
   index++;
   return(out);
 };

This function reads the string "input" until it finds a comma, then returns the value of the numbers it just read as a unsigned integer. This works fine. However, when I repeat the function to read the next number as a float, the problems occur. Here is the next function:

float get_matrix(unsigned &index, char input[])
 {
  const comma=',';
  char *letter;
  char *accum="";
  float out;
   while (input[index]!=comma)
   {
    *letter=input[index];
    strcat(accum,letter);
    index++;
   };
   out=atof(accum);
   index++;
   return(out);
 };

As you can see, it's the same function, this time using atof and float. But, the second function does not work. The string accumulator "accum" starts with a long string of nonsense characters that do not go away.

I don't know where they come from, or how to deal with them. Why does this function work the first time and not the second?

Any help will be appreciated.

[Narue]

Both functions are broken in quite a few ways

>const comma=',';
C++ doesn't support implicit int.

>char *accum="";
There are two serious problems here. First, you're not allowed to modify the contents of a string literal, but that's precisely what you're trying to do. Second, a pointer doesn't imply infinite memory. If you want to copy characters to accum, you have to allocate enough memory to do so.

>*letter=input[index];
letter doesn't point anywhere meaningful, so dereferencing it is wrong.

>strcat(accum,letter);
This is wrong in so many ways. I listed two of them when looking at your declaration of accum, but the fact that letter is not a string (because it's not terminated with a '\0' character) means that you can't predict how many characters strcat will attempt to copy.

I'm seeing a fairly fundamental misunderstanding about how memory, pointers and C-style strings work. Maybe you should go back to your books and study up on it a bit.