lect3-sums-and-loops - Lecture Notes CMSC 251 high school...

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Lecture Notes CMSC 251 high school algebra. If c is a constant (does not depend on the summation index i ) then n X i =1 ca i = c n X i =1 a i and n X i =1 ( a i + b i )= n X i =1 a i + n X i =1 b i . There are some particularly important summations, which you should probably commit to memory (or at least remember their asymptotic growth rates). If you want some practice with induction, the first two are easy to prove by induction. Arithmetic Series: For n 0 , n X i =1 i =1+2+ ··· + n = n ( n +1) 2 =Θ( n 2 ) . Geometric Series: Let x 6 =1 be any constant (independent of i ), then for n 0 , n X i =0 x i =1+ x + x 2 + + x n = x n +1 - 1 x - 1 . If 0 <x< 1 then this is Θ(1) , and if x> 1 , then this is Θ( x n ) . Harmonic Series: This arises often in probabilistic analyses of algorithms. For n 0 , H n = n X i =1 1 i 1 2 + 1 3 + + 1 n ln n = Θ(ln n ) . Lecture 3: Summations and Analyzing Programs with Loops (Tuesday, Feb 3, 1998) Read: Chapt. 3 in CLR. Recap: Last time we presented an algorithm for the 2-dimensional maxima problem. Recall that the algo- rithm consisted of two nested loops. It looked something like this: Brute Force Maxima Maxima(int n, Point P[1. .n]) { f o ri=1t on{ ... for j = 1 to n { ... ... } } We were interested in measuring the worst-case running time of this algorithm as a function of the input size, n . The stuff in the “. ..” has been omitted because it is unimportant for the analysis. Last time we counted the number of times that the algorithm accessed a coordinate of any point. (This was only one of many things that we could have chosen to count.) We showed that as a function of n in the worst case this quantity was T ( n )=4 n 2 +2 n. 7
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Lecture Notes CMSC 251 We were most interested in the growth rate for large values of n (since almost all algorithms run fast for small values of n ), so we were most interested in the 4 n 2 term, which determines how the function grows asymptotically for large n . Also, we do not care about constant factors (because we wanted simplicity and machine independence, and figured that the constant factors were better measured by implementing the algorithm). So we can ignored the factor 4 and simply say that the algorithm’s worst-case running time grows asymptotically as n 2 , which we wrote as Θ( n 2 ) . In this and the next lecture we will consider the questions of (1) how is it that one goes about analyzing the running time of an algorithm as function such as T ( n ) above, and (2) how does one arrive at a simple asymptotic expression for that running time. A Harder Example: Let’s consider another example. Again, we will ignore stuff that takes constant time (expressed as “. ..” inthe code below). A Not-So-Simple Example: for i = 1 to n { // assume that n is input size ... for j = 1 to 2*i { ... k=j ; while (k >= 0) { ... k=k-1 ; } } } How do we analyze the running time of an algorithm that has many complex nested loops? The answer is that we write out the loops as summations, and then try to solve the summations. Let I () , M () , T () be the running times for (one full execution of) the inner loop, middle loop, and the entire program. To convert the loops into summations, we work from the inside-out. Let’s consider one pass through the innermost loop. The number of passes through the loop depends on
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lect3-sums-and-loops - Lecture Notes CMSC 251 high school...

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