Really weird bug

Forum 2005-2010 (read only) Software Bugs & Suggestions
1

I'm trying to write some code to convert 4bytes to single float IEEE 754. The code looks like this:

float bytesToFloat(int a,int b,int c,int d) {
  long exponent;
  float mantissa;
  float result;
  float e23=8388608;
  exponent = (0x7F&a)<<1 | (0x80&b)>>7;
  mantissa = (0x7F&b)*pow(2,16) + c*pow(2,8) + d;
  result = (1+mantissa/e23)*pow(2,exponent-127)   
  return result;
}

The weird thing is that when I call this function I get ("the right result" -1) but if I write the calculation inside the loop() function the result is perfect!!! :o

For example the following bytes: 41 - 60 - 00 - 00 should give as result 14 but I get 13 calling the function and 14 if I write it inside the function.

Here is a complete sample code for testing purposes:

/*
 * Weird error
 * by Jose A. Jimenez Berni
 *
 * Sample routine to convert from bytes to IEEE754 single float
 * The results are different if the routine is inside the loop
 * or it is called as a function...
 * 
 */
 
 #include <math.h>
int ledPin = 13;                // LED connected to digital pin 13

void setup()                    // run once, when the sketch starts
{
  Serial.begin(9600);
  pinMode(ledPin, OUTPUT);      // sets the digital pin as output
}

void loop()                     // run over and over again
{
  digitalWrite(ledPin, HIGH);   // sets the LED on
  delay(1000);                  // waits for a second
  digitalWrite(ledPin, LOW);    // sets the LED off
  delay(1000);                  // waits for a second
  int a,b,c,d;
  long exponent;
  float mantissa;
  float result;
  float e23=8388608;
  a=0x41;
  b=0x60;
  c=0x00;
  d=0x00;
  exponent = (0x7F&a)<<1 | (0x80&b)>>7;
  Serial.print("Inside the Loop -> Exponent: ");
  Serial.print(exponent,DEC);  
  mantissa = (0x7F&b)*pow(2,16) + c*pow(2,8) + d;
  Serial.print(" / Mantissa: ");
  Serial.print(mantissa,DEC);  
  result = (1-2*(0x80&a))*(1+mantissa/e23)*pow(2,exponent-127);
  Serial.print(" / Result: ");
  Serial.println(result,DEC);
  result = bytesToFloat(a,b,c,d);

}

float bytesToFloat(int a,int b,int c,int d) {
  long exponent;
  float mantissa;
  float result;
  float e23=8388608;
  exponent = (0x7F&a)<<1 | (0x80&b)>>7;
  Serial.print("Inside the Fuction -> Exponent: ");
  Serial.print(exponent,DEC);  
  mantissa = (0x7F&b)*pow(2,16) + c*pow(2,8) + d;
  Serial.print(" / Mantissa: ");
  Serial.print(mantissa,DEC);  
  result = (1-2*(0x80&a))*(1+mantissa/e23)*pow(2,exponent-127);
  Serial.print(" / Result: ");
  Serial.println(result,DEC);    
  return result;
}

Here is the output:

Inside the Loop -> Exponent: 130 / Mantissa: 6291456 / Result: 14

Inside the Fuction -> Exponent: 130 / Mantissa: 6291456 / Result: 13

Any ideas?