Pass istream& object into function

[chopo]

Hi,

I have a program that imports data from a file input.txt. The file is very long and not all the data is needed.

I what to write a function getData() that inputs a string from the file, evaluates whether the string is numeric and returns it if it is, or moves onto the next string if not:

main(){
  //Some double variables are defined

  ifstream in;
  in.open("input.txt"); // It is important that this is in the main program and not in the function

  floatVariable0= getData(in);
  floatVariable1= getData(in);
  floatVariable2= getData(in);
  //...

  in.close()
  //...
}

float getData(ifstream &in){
  string str;
  float rtnVal;
  int isNum= 0;
  int isDot= 0;

  in >> str;

  for( i=0; i<strlen(str)-1; i++) {
    for(int j=0; j<=9; j++) {
      if(atoi(&str[i]) != j || &str[i] != ".") {
        isNum++;
      }
      if(&str[i] == "."){
        isDot++
      }
    }
  }

  if(isDot<2 && isNum==0){
    rtnVal = atof(str);
    return rtnVal;
  }else{
    getData(in);
  }
}

Not 100% sure the code within the getData() function works but for now just treat it as an example of what I want to do, My main concern is how to pass the in ifstream into the function. I've used an & but I know that doesn't work.
Any suggestions?

Thanks

Chopo

[Bazzy]

I think is better if you first check if a character in the stream is =='.' or if it isDigit, then you could directly get rtnVal from the stream

[jsmith]

You are correct in passing the ifstream by non-const reference. Why do you think it is wrong?

[chopo]

J Smith: In testing I've written a more simple version of the above function that simply prints to screen the string it inputs from the file and all it returns is a white space. I've got to go now but I'll post that tonight, maybe I've made a mess with the type of variables I used, or I should be using char[] rather than arrays. Seeing as you think I'm on the right track I'll check to make sure I'ts not something silly, thanks.

Bazzy: Not sure I understand what you mean:

if(atoi(&str[i]) != j || &str[i] != ".") {
  isNum++;
}
if(&str[i] == "."){
  isDot++
}

and

if(&str[i] == "."){
  isDot++
}
if(atoi(&str[i]) != j || &str[i] != ".") {
  isNum++;
}

are equivalent as far as I can see.

Thanks for pointing out the isDigit function, didn't know it existed.

[Bazzy]

I meant:

char nextchar;
in.peek(nextchar);
if( isDigit(nextchar) || nextchar=='.' )
 in >> rtnVal;

[chopo]

I do apologies it turns out that it is possible to pass an ifstream into a function by using by non-const reference (ie &). The reason my test code did not work was that I carelessly put the print statement before the line that read from the input file. I was so convinced that my approach to solving this problem was wrong that I failed to thoroughly check my test code before posting this question.

Sorry Bazzy and Jsmith, I hope this topic will turn out to be useful to someone in the future.