I have read a lot of topics with these subject many of theme where on these forum but what ever i try i cant get this working.

I have tried 4 or 5 function for converting string to float but i always get compiler error left from xxx must be class/struct ....

here is the function

```
void calculate(string& part_){
float number_one=0;
float number_two=0;
for(int i=0; i<4; ++i){
for (int j=0; j<part_.length(); ++j){
if(part_[j]== '/'){
number_one = part_.at(j-1); // need to be converted to float somehow
number_two = part_[j+1]; // these too
number_one /= number_two;
part_[j-1] = number_one;
part_.erase(j, j+2);
}
}
}
}
```