Lecture 8 Notes

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Unformatted text preview: largest floaJng point number? x = (1.5)*2^1 => (1.5+eps)*2^1 = x + eps*2^1 = 3 + 2*eps is next largest floaJng point number 8 2/20/14 eps •  x = (1+f)2^e •  For x = 9, what is the next largest floaJng point number? x = (1.125)*2^3 => (1.125+eps)*2^3 = x + eps*2^3 = 9 + 8*eps is next largest floaJng point number floatgui.m •  Numbers within [2^e, 2^(e+1)] are equally spaced •  One and eps and are in red •  t = number bits used to store f (52 in IEEE 754) –  2^t*f is an integer, 0<=2^t*f<=2^t •  emin < e < emax (- 1022 <= e < 1- 23 in IEEE 754) •  logscale shows distribuJon of numbers within each binary interval is the same •  What is the pa|ern? –  For x=(1+f)2^e –  The difference between x and the next largest floaJng point numbers is (2^e)*eps! •  What does this mean in pracJce? –  Big numbers are “further apart” than small numbers –  We can represent small values very precisely, as values increase, the absolute precision drops –  But relaJve precision remains the same (this is great) Regarding the QuesJon… The loop terminates, final value of k is 53, i.e. 1==1+eps/2! ! k = 0; while 1 + 1/2^k > 1 k = k+1; end k Insight Through CompuJng The Moral The 1991 Patriot Missile Disaster Elementary misperceptions about the finiteness of computer arithmetic. 30+ died. To produce reliable numerical results you must appreciate floaJng point arithmeJc. Insight Through CompuJng Insight Through CompuJng 9 2/20/14 The Sevng External clock counts Jme in tenths of seconds. TargeJng soXware needs Jme to compute trajectories. The method: Time = (# external clock Jcks) x (1/10) The problem is here Insight Through CompuJng One- Tenth in Binary Exact: .00011001100110011001100110011… Patriot System used: .00011001100110011001100110011… Error = .000000095sec every clock Jck Insight Through CompuJng Error Time = (# external clock Jcks) x (1/10) Error = (# external clock Jcks) x (.000000095) Insight...
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