implement atof()

It is a little bit different from Java's parseFloat:

The function first discards as many whitespace characters (as in isspace) as necessary until the first non-whitespace character is found. Then, starting from this character, takes as many characters as possible that are valid following a syntax resembling that of floating point literals (see below), and interprets them as a numerical value. The rest of the string after the last valid character is ignored and has no effect on the behavior of this function.

If you are not sure about if a number is valid, open your Microsoft Excel and try it. :)

https://github.com/shirleyyoung0812/Facebook/blob/master/src/facebookCoding/Atof.java

public class Atof {
 public static float atof(String in) throws NumberFormatException{
  if (in == null)
   throw new NumberFormatException("Null String!");
  boolean isNegative = false;
  in = in.trim();
  if (in.length() == 0)
   throw new NumberFormatException("Empty string!");
 
  if (in.charAt(0) == '+' || in.charAt(0) == '-') {
   if (in.charAt(0) == '-')
    isNegative = true;
   in = in.substring(1);
  }
  //case that input is "NaN"
  if (in.charAt(0) == 'N') {
   char[] nan = {'N', 'a', 'N'};
   if (in.length() != nan.length)
    return 0.0f;
   for (int i = 0; i < nan.length; i++) {
    if (in.charAt(i) != nan[i])
     return 0.0f;
   }
   return (float)(Double.NaN);
  }
  //case that input is "infinity"
  if (in.charAt(0) == 'I') {
   char[] infinity = {'I', 'n', 'f', 'i', 'n', 'i', 't', 'y' };
   if (in.length() != infinity.length)
    return 0.0f;
   for (int i = 0; i < infinity.length; i++) {
    if (in.charAt(i) != infinity[i])
     return 0.0f;
   }
   return isNegative ? (float)(Double.NEGATIVE_INFINITY) : (float)(Double.POSITIVE_INFINITY);
  }
 
  //Discard leading zeros
  int indexZ = 0;
  while (indexZ < in.length() && in.charAt(indexZ) == '0') {
   indexZ++;
  }
  //System.out.println(indexZ);
  in = in.substring(indexZ);
  if (in.length() == 0)
   return 0.0f;
  //discard trailing non-digit characters, invalid if 'e' is the last character
  int notDigit = in.length() - 1;
  while (notDigit > 0 && in.charAt(notDigit)  != '.' && !Character.isDigit(in.charAt(notDigit))) {
   notDigit--;
  }
  if (notDigit == 0)
   return 0.0f;
  in = in.substring(0, notDigit + 1);
  //System.out.println("Before parsing: " + in);
  boolean dot = false;
  boolean exp = false;
  int expSignPos = -1;
  boolean isExpNegative = false;
  int dotPos = 0;
  int expPos = 0;
  //check if the string is valid, if any invalid character occurs, parse the valid prefix string
  for (int i = 0; i < in.length(); i++) {
   //System.out.println(in.charAt(i));
   if (Character.isDigit(in.charAt(i)))
    continue;
   //. must be before e
   else if (in.charAt(i) == '.') {
    if (exp || dot) {
     if (in.charAt(i - 1) == 'e' || in.charAt(i - 1) == 'E') {
      exp = false;
      in = in.substring(0, i - 1);
     }
     else
      in = in.substring(0, i);
     break;
    }
    dot = true;
    dotPos = i;
    //System.out.println("dotPos: " + dotPos);
   }
   else if (in.charAt(i) == 'e' || in.charAt(i) == 'E') {
    //e cannot be at the first position
    if (i == 0)
     return 0.0f;
    if (exp) {
     in = in.substring(0, i);
     break;
    }
    expPos = i;
    exp = true;
   }
   else if (in.charAt(i) == '+' || in.charAt(i) == '-') {
    if (i == 0 || (in.charAt(i - 1) != 'e' && in.charAt(i - 1) != 'E')) {
     in = in.substring(0, i);
     break;
    }
    if (in.charAt(i) == '-')
     isExpNegative = true;
    expSignPos = i;
   }
   else {
    in = in.substring(0, i);
    break;
   }
  }
  if (in.length() == 0 || in.equals("."))
   return 0.0f;
  float rst = 0.0f;
  if (dot) {
   if (!exp) {
    rst = parseDot(in, dotPos);
   }
   else {
    rst = parseDot(in.substring(0, expPos), dotPos);
    rst = parseExp(rst, in, expPos, expSignPos, isExpNegative);
   }
  }
  else if (exp) {
   float b4exp = (float)Integer.parseInt(in.substring(0, expPos));
   rst = parseExp(b4exp, in, expPos, expSignPos, isExpNegative);
  }
  else {
   rst = (float)Integer.parseInt(in);
  }
  return isNegative ? -rst : rst;
  
 }
 private static float parseDot(String in, int dotPos) {
  float rst;
  int b4dot = 0;
  if (dotPos != 0)
   b4dot = Integer.parseInt(in.substring(0, dotPos));
  if (dotPos == in.length() - 1)
   return (float)b4dot;
  float aftdot = (float)Integer.parseInt(in.substring(dotPos + 1));
  int pos = in.length() - dotPos - 1;
  while (pos > 0) {
   aftdot /= 10;
   pos--;
  }
  rst = (float)b4dot + aftdot;
  return rst;
  }
 private static float parseExp(float rst, String in, int expPos, int expSignPos, boolean isExpNegative) {
  //System.out.println(expPos);
  int pos = 0;
  if (expSignPos != -1) {
   pos = isExpNegative ? (-Integer.parseInt(in.substring(expSignPos + 1))) :
    (Integer.parseInt(in.substring(expSignPos + 1)));
  }
  else 
   pos = Integer.parseInt(in.substring(expPos + 1));
  if (pos < 0) {
   while (pos < 0) {
    rst /= 10;
    pos++;
   }
  }
  else if (pos > 0) {
   while (pos > 0) {
    rst *= 10;
    pos--;
    if ((long)rst >= (Long.MAX_VALUE / 10))
     return (float)Double.POSITIVE_INFINITY;
   }
  }
  return rst;
 }
 public static void main(String[] args) {
  System.out.println(atof("3.14f"));
  System.out.println(atof("3.14e+03"));
  System.out.println(atof("000314.5e-02dfgf"));
  System.out.println(atof("45346gfbfd.4"));
  System.out.println(atof("5643..23"));
  System.out.println(atof("5643.e03"));
  System.out.println(atof("5241e.03"));
  System.out.println(atof("09590004e.05"));
  System.out.println(atof("--4343"));
  System.out.println(atof("6v4v"));
  System.out.println(atof("-v6"));
  System.out.println(atof("+3.5e-2"));
  System.out.println(atof(".03e1"));
  System.out.println(atof("1e1"));
  System.out.println(atof("0.0"));
  System.out.println(atof("10E5"));
  

 }

}