UTK - Math 171 - Alexiades

Floating Point Numbers - Roundoff

Machine numbers

• There are finitely many floating point numbers on any computer.
• They are NOT evenly spaced (same # between consecutive powers of 2) so spread out further and further apart.
• Any given real number x must be approximated by a nearest machine number, fl(x), resulting in roundoff error.
Important consequencies:
1. There is an inherent error in representing real numbers, called round-off.
2. Machine arithmetic is not necessarily commutative, i.e. the order of operations may affect the result!
3. Loss of significant digits in arithmetic may occur:
• unbalanced addition: when adding a very small to a very large real number
• subtractive cancellation: when subtracting nearly equal reals
• unbalanced multiplication: when nultiplying by a very small number (or dividing by a very large number).
4. Number of decimal digits that can be trusted:   up to 7 in single precision, up to 15 in double precision.
5. Should NEVER ask for equality of computed real numbers, use if( abs(a − b) ≤ TOL ) , for some TOLerance
( with TOL no smaller than 1.e-15, for double precision )

Machine epsilon ε
• εmach is the smallest positive number such that 1+εmach > 1
• εmach = 2−p , p=precision = # of bits in mantissa.
• In double precision, p=53, so   εmach = 2−53 ≈ 10−15
• For any real x,   fl(x) = x(1 + ε), so   ε = (fl(x)−x)/x = relative error in representing x by fl(x), with ε ≤ εmach
• Thus, the absolute error in representing x by fl(x) is   roundoff = fl(x)−x = εx , so it is proportional to x .
So, the larger the number the larger the roundoff error!
• In Matlab: eps(x) gives the machine number nearest to x for that computer.
eps(1)   or just eps gives εmach itself.
• For example: eps(1) = 2.2204e-16 , eps(100) = 1.4211e-14 , eps(1000) = 1.1369e-13 , eps(1.e6) = 1.1642e-10 , ...
losing more and more accuracy...
• For example: 1+eps == 1 gives false,   1+eps/2 == 1 gives true
So, any (positive) number less than ε is effectively zero in computation!

Some suggestions
• Guard against loss of digits, (subtraction of nearly equal numbers, multiplying by very small number, etc)
Try to re-write the expression somehow...
• The quadratic formula is subject to loss of significant digits when   0 < 4ac << b2 (much less than)
A safe way to find roots of a quadratic:   q = −( b + sign(b)√(b2−4ac) ) / 2 ,   then x1 = q/a ,   x2 = c / q
• Always evaluate polynomials in nested form: saves operations and may reduce roundoff.
e.g. ax2 + bx + c = c + x*(b+a*x) ,   e.g. ax3 + bx2 + cx + d = d + x*(c+x*(b+a*x)) , etc
• Enter very big and very small numbers in scientific notation: e.g. 3.14e6 , 3.14e-6
• Print very big and very small numbers with '%e' formating.