How to manually (bitwise) perform (float)x? (homework)
- by Silver
Now, here is the function header of the function I'm supposed to implement:
/*
* float_from_int - Return bit-level equivalent of expression (float) x
* Result is returned as unsigned int, but
* it is to be interpreted as the bit-level representation of a
* single-precision floating point values.
* Legal ops: Any integer/unsigned operations incl. ||, &&. also if, while
* Max ops: 30
* Rating: 4
*/
unsigned float_from_int(int x) {
...
}
We aren't allowed to do float operations, or any kind of casting.
Now I tried to implement the first algorithm given at this site: http://locklessinc.com/articles/i2f/
Here's my code:
unsigned float_from_int(int x) {
// grab sign bit
int xIsNegative = 0;
int absValOfX = x;
if(x < 0){
xIsNegative = 1;
absValOfX = -x;
}
// zero case
if(x == 0){
return 0;
}
//int shiftsNeeded = 0;
/*while(){
shiftsNeeded++;
}*/
unsigned I2F_MAX_BITS = 15;
unsigned I2F_MAX_INPUT = ((1 << I2F_MAX_BITS) - 1);
unsigned I2F_SHIFT = (24 - I2F_MAX_BITS);
unsigned result, i, exponent, fraction;
if ((absValOfX & I2F_MAX_INPUT) == 0)
result = 0;
else {
exponent = 126 + I2F_MAX_BITS;
fraction = (absValOfX & I2F_MAX_INPUT) << I2F_SHIFT;
i = 0;
while(i < I2F_MAX_BITS) {
if (fraction & 0x800000)
break;
else {
fraction = fraction << 1;
exponent = exponent - 1;
}
i++;
}
result = (xIsNegative << 31) | exponent << 23 | (fraction & 0x7fffff);
}
return result;
}
But it didn't work (see test error below):
Test float_from_int(-2147483648[0x80000000]) failed...
...Gives 0[0x0]. Should be -822083584[0xcf000000]
4 4 0 float_times_four
I don't know where to go from here. How should I go about parsing the float from this int?
