CS 325 Homework 1


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CS 325 
HW 1 – 30 points
1) (3 pts) Describe a (n lgn) time algorithm that, given a set S of n integers and another integer x,
determines whether or not there exist two elements in S whose sum is exactly x. Explain why
the running time is (n lgn).
2) (3 pts) For each of the following pairs of functions, either f(n) is O(g(n)), f(n) is Ω(g(n)), or f(n) =
Θ(g(n)). Determine which relationship is correct and explain.
a. f(n) = n0.25
; g(n) = n0.5
b. f(n) = log n2
; g(n) = ln n
c. f(n) = nlog n; g(n) =n√?
d. f(n) = 4
; g(n) = 3
e. f(n) = 2n
; g(n) = 2n+1
f. f(n) = 2n
; g(n) = n!
3) (4 pts) Let f1 and f2 be asymptotically positive non-decreasing functions. Prove or disprove each
of the following conjectures. To disprove give a counter example.
a. If f1(n) = O(g(n)) and f2(n) = O(g(n)) then f1(n)+ f2(n) = O(g(n)).
b. If f(n) = O(g1(n)) and f(n) = O(g2(n)) then g1(n) = (g2(n) )
4) (10 pts) Merge Sort and Insertion Sort Programs
Implement merge sort and insertion sort to sort an array/vector of integers. You may
implement the algorithms in the language of your choice, name one program “mergesort” and
the other “insertsort”. Your programs should be able to read inputs from a file called “data.txt”
where the first value of each line is the number of integers that need to be sorted, followed by
the integers.
Example values for data.txt:
4 19 2 5 11
8 1 2 3 4 5 6 1 2
The output will be written to files called “merge.out” and “insert.out”.
For the above example the output would be:
2 5 11 19
1 1 2 2 3 4 5 6
Submit a copy of all your code files and a README file that explains how to compile and run
your code in a ZIP file to TEACH. We will test execution with an input file named data.txt.
CS 325 Winter 2018
HW 1 – 30 points
5) (10 pts) Merge Sort vs Insertion Sort Running time analysis
The goal of this problem is to compare the experimental running times of the two sorting
a) Now that you have proven that your code runs correctly using the data.txt input file, you can
modify the code to collect running time data. Instead of reading arrays from a file to sort, you
will now generate arrays of size n containing random integer values from 0 to 10,000 and then
time how long it takes to sort the arrays. We will not be executing the code that generates the
running time data so it does not have to be submitted to TEACH or even execute on flip. Include
a “text” copy of the modified code in the written HW submitted in Canvas.
b) Use the system clock to record the running times of each algorithm for n = 1000, 2000, 5000,
10,000, …. You may need to modify the values of n if an algorithm runs too fast or too slow to
collect the running time data. If you program in C your algorithm will run faster than if you use
python. You will need at least seven values of t (time) greater than 0. If there is variability in
the times between runs of the same algorithm you may want to take the average time of several
runs for each value of n.
c) For each algorithm plot the running time data you collected on an individual graph with n on
the x-axis and time on the y-axis. You may use Excel, Matlab, R or any other software. Also plot
the data from both algorithms together on a combined graph. Which graphs represent the data
d) What type of curve best fits each data set? Again you can use Excel, Matlab, any software or
a graphing calculator to calculate a regression equation. Give the equation of the curve that
best “fits” the data and draw that curve on the graphs of created in part c).
e) How do your experimental running times compare to the theoretical running times of the
algorithms? Remember, the experimental running times were “average case” since the input
arrays contained random integers.
EXTRA CREDIT: It was the best of times, it was the worst of times…
 Generate best case and worst case input for both algorithms and repeat the analysis in parts b)
to d) above. Discuss your results and submit your code to TEACH.
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