how to type cast string into float

you can just read them in as floats, via >> operations

infile >> floatvar;

or you can convert them with atof (ascii to float)

open a file. use the >> operator, its exactly like "cin" or, call the function "atof" on a string. Im not sure what else to say about either one? Both are c++ language features.

ifstream inf;
inf.open("file.txt");
float f;
inf >> f;

or
char ch[1000];
inf>> ch;
f = atof(ch);
...
of course you can use string class or whatever as well...