Download
1 / 29
290 likes | 431 Vues
ALGORITHMS THIRD YEAR. BANHA UNIVERSITY FACULTY OF COMPUTERS AND INFORMATIC. Lecture four. Dr. Hamdy M. Mousa. Growth of Functions. overview. We know that: The order of growth of the running time of an algorithm.
E N D
ALGORITHMSTHIRD YEAR BANHA UNIVERSITY FACULTY OF COMPUTERS AND INFORMATIC Lecture four Dr. Hamdy M. Mousa
overview We know that: • The order of growth of the running time of an algorithm. • The input size nbecomes large enough, merge sort, with its (n lg n)worst-case running time, beats insertion sort, whose worst-case running time is (n2). • At input sizes large enough to make only the order of growth of the running time relevant, we are studying the asymptotic efficiency of algorithms.
overview • A way to describe behavior of functions in the limit. We’re studying asymptotic efficiency. • Describe growth of functions. • Focus on what’s important by abstracting away low-order terms and constant factors. • How we indicate running times of algorithms. • A way to compare “sizes” of functions: ≈ ≤ ≈ ≥ ≈ = o ≈ < ω ≈ >
Asymptotic notation The notations we use to describe the asymptotic running time of an algorithm are defined in terms of functions whose domains are the set of natural numbers N = {0, 1, 2, . . .}. Such notations are convenient for describing the worst-case running-time function T (n).
-notation • The worst-case running time of insertion sort is T (n) = (n2). • Let us define what this notation means. For a given function g(n), • we denote by (g(n))the set of functions (g(n))= { f (n) : there exist positive constants c1, c2, and n0 such that 0 ≤ c1g(n) ≤ f (n) ≤ c2g(n) for all n ≥ n0} .
O-notation O(g(n)) = {f (n) : there exist positive constants c and n0 such that 0 ≤ f (n) ≤ cg(n) for all n ≥ n0} • g(n) is an asymptotic upper bound for f (n). • If f (n) O(g(n)), we write f (n) = O(g(n)) (will precisely explain this soon).
O-notation Example : 2n2 = O(n3), with c = 1 and n0 = 2. Examples of functions in O(n2): n2 n2 + n n2 + 1000n 1000n2 + 1000n Also, n n/1000 n1.99999 n2/ lg lg lg n
-notation • (g(n)) = { f (n) : there exist positive constants c and n0 such that 0 ≤ cg(n) ≤ f (n) for all n ≥ n0} . • (g(n)) = { f (n) : there exist positive constants c and n0 such that 0 ≤ cg(n) ≤ f (n) for all n ≥ n0} .
-notation (g(n)) = { f (n) : there exist positive constants c1, c2, and n0 such that 0 ≤ c1g(n) ≤ f (n) ≤ c2g(n) for all n ≥ n0} . g(n) is an asymptotically tight bound for f (n). Example: n2/2 − 2n = (n2), with c1 = 1/4, c2 = 1/2, and n0 = 8.
Theorem f (n) = (g(n)) if and only if f = O(g(n)) and f = (g(n)) . Leading constants and low-order terms don’t matter.
Asymptotic notation in equations • When on right-hand side: O(n2) stands for some anonymous function in the set O(n2). • 2n2+3n+1 = 2n2+(n) means 2n2+3n+1 = 2n2+ f (n) for some f (n) (n). • In particular, f (n) = 3n + 1.
By the way, we interpret # of anonymous functions as = # of times the asymptotic notation appears: • O(i ) OK: 1 anonymous function • O(1) + O(2)+…..+ O(n) not OK: n hidden constants no clean interpretation
When on left-hand side: No matter how the anonymous functions are chosen on the left-hand side, there is a way to choose the anonymous functions on the righthand side to make the equation valid. Interpret 2n2 + (n) = (n2) as meaning for all functions f (n) (n), There exists a function g(n) (n2) such that 2n2 + f (n) = g(n). Can chain together: 2n2 + 3n + 1 = 2n2 + (n) = (n2) .
Interpretation: • First equation: There exists f (n) (n) such that 2n2+3n+1 = 2n2+ f (n). • Second equation: For all g(n) (n) (such as the f (n) used to make the first equation hold), there exists h(n) (n2) such that 2n2 + g(n) = h(n).
o-notation o(g(n)) = { f (n) : for all constants c > 0, there exists a constant n0> 0 such that 0 ≤ f (n) < cg(n) for all n ≥ n0} .
ω-notation ω(g(n)) = { f (n) : for all constants c > 0, there exists a constant n0> 0 such that 0 ≤ cg(n) < f (n) for all n ≥ n0}
Comparisons of functions Relational properties: Transitivity: f (n) = (g(n)) and g(n) = (h(n)) f (n) = (h(n)). Same for O,, o, and ω. Reflexivity: f (n) = ( f (n)). Same for O and . Symmetry: f (n) = (g(n)) if and only if g(n) = ( f (n)). Transpose symmetry: f (n) = O(g(n)) if and only if g(n) = ( f (n)). f (n) = o(g(n)) if and only if g(n) = ω( f (n)).
Comparisons of functions Comparisons: • f (n) is asymptotically smaller than g(n) if f (n) = o(g(n)). • f (n) is asymptotically larger than g(n) if f (n) = ω(g(n)). No trichotomy. Although intuitively, we can liken O to ≤ , , to ≥, etc., unlike real numbers, where a < b, a = b, or a > b, we might not be able to compare functions. Example:n1+sinnand n, since 1 + sin n oscillates between 0 and 2.
Standard notations and common functions Monotonicity • f (n) is monotonically increasing if m ≤ n f (m) ≤ f (n). • f (n) is monotonically decreasing if m ≥ n f (m) ≥ f (n). • f (n) is strictly increasing if m < n f (m) < f (n). • f (n) is strictly decreasing if m > n f (m) > f (n).
Exponentials A suprisingly useful inequality: for all real x, ex≥ 1 + x . As x gets closer to 0, exgets closer to 1 + x.
Logarithms Notations: lg n = log2n (binary logarithm) , ln n = loge n (natural logarithm) , lgk n = (lg n)k(exponentiation) , lg lg n = lg(lg n) (composition) . Logarithm functions apply only to the next term in the formula, so that lg n + kmeans (lg n) + k, and not lg(n + k).
In the expression logb a: • If we hold b constant, then the expression is strictly increasing as a increases. • If we hold a constant, then the expression is strictly decreasing as b increases.
Useful identities for all real a > 0, b > 0, c > 0, and n, and where logarithm bases are not 1:
Changing the base of a logarithm from one constant to another only changes the value by a constant factor, so we usually don’t worry about logarithm bases in asymptotic notation. Convention is to use lg within asymptotic notation, unless the base actually matters.
