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General Computer Science for Engineers CISC 106 Midterm 2 Review James Atlas Computer and Information Sciences 11/06/2009

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Important Notes on Exam Write code Study labs + project Study Midterm review

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Switch construct color = ‘yellow’; switch (color) case ‘red’ disp(‘Stop now!’); case ‘green’ disp(‘Proceed through intersection.’); case ‘yellow’ disp(‘Prepare to stop.’); otherwise disp(‘Illegal color encountered.’); end

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Logical Operators &, && - AND |, || - OR ~ - NOT

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a = [ ] b = [1; 2; 3; 4] c = [1 2; 3] (error) d = [1 2; 3 4] f = d(1,2) g(4,5) = 7 a(3:end) a(1:2:end) d’ Array commands

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For Loops Used when you know how many times code is to be executed. Syntax for = : : Variable is initially the start value At end of iteration variable changes by increment If value is not greater than end the loop runs again.

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Example Problem total = 0; for i = 1:1:1000 loop starts at 1 total = total + i;loop increments by 1 end loop ends at 1000 disp(total);

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A Loop Analogy (for) The runner executes a loop. If they know the distance they want to run For loop for lapCount = start : 1 : end runLap() end

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A Loop Analogy (while) The runner executes a loop. If they don’t know the distance they want to run (run until tired) While loop tired = false; while(~tired) tired = runLap() end

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Linear Search Given [ ] find which position 6 occupies Alternatively, does the array contain the number 6? foundIndex = -1; for index = 1:length(values) if (values(index) == 6) foundIndex = index; end

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Binary Search Now, what if the array is sorted, can we search it faster?

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Binary Search Find N in list Pick a number X halfway between the start and end of the list If X == N we are done else if X < N search top half of list else search bottom half of list

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Algorithm for sorting (Selection Sort) 1. Find the minimum data in the set 2. Swap it with the first element of the set 3. Repeat Steps 1-2 for the remaining elements

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Running Time (Selection Sort) loop progress

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Algorithm for sorting (Quick Sort) 1. Choose a random pivot point 2. Split your data into two sets: 1.Low - the data < pivot 2.High - the data > pivot 3. Repeat Steps 1-2 for both data sets 4. The sorted data is [low pivot high]

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Running Time (Quick Sort) recursion progress If we divide the size of the data, n, in half at each step, how many steps does this take? log n area of table = width * height = n * log n

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Structures in MATLAB

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A Database Application Given: Name: Chris Credits: 27 Graduation: 12/15/2011 Name: Sola Credits: 18 Graduation: 05/17/2011 Name: Roger Credits: 55 Graduation: 06/10/2009 Name: Tom Credits: 15 Graduation: 05/22/2012

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Given: We can implement it with arrays like this: Name Credits Grad Name: Chris Credits: 27 Graduation: 12/15/2011 Name: Sola Credits: 18 Graduation: 05/17/2011 Name: Roger Credits: 55 Graduation: 06/10/2009 Name: Tom Credits: 15 Graduation: 05/22/ Chris12/15/ Sola05/17/ Roger06/10/ Tom05/22/2012 A Database Application

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Given: OR we can do it like this an array with structs:.d Name: Chris Credits: 27 Graduation: 12/15/2011 Name: Sola Credits: 18 Graduation: 05/17/2011 Name: Roger Credits: 55 Graduation: 06/10/2009 Name: Tom Credits: 15 Graduation: 05/22/2012 Students (1). Name: Chris Students (1).Credits: 27 Students (1). Graduation: 12/15/2011 Students (2).Name: Sola Students (2).Credits: 18 Students (2).Graduation: 05/17/2011 Students (3). Name: Roger Students (3). Credits: 55 Students (3). Graduation: 06/10/2009 Students (4). Name: Tom Students (4). Credits: 15 Students (4). Graduation: 05/22/2012 A Database Application

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MATLAB Array Initialization y = []; for i = 1:10 y(i) = i; end; This is an example of “growing” an array

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MATLAB Array Initialization y = zeros(1,10); for i = 1:10 y(i) = i; end; Initializes the array first

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Vectorization Additional examples ◦ x = [ ]; ◦ x < 3 ◦ x(x < 3) ◦ x(x 3) x < 3 produces a mask

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Masking Masking selects only certain elements of an array to perform an operation Masking uses an array of only 0’s and 1’s that is the same size as the argument ◦ y = x < 3 ◦ whos y ◦ y is a mask of x that selects only the elements that are less than 3

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Masking x = [ ]; y = x < 3 x(y) = x(y).* 2;

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MATLAB functions - find find ◦ locates index of all nonzero elements of array z = [ ]; find(z) ◦ [1 2 6]

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MATLAB functions - any/all x = [ ]; any(x < 3) any(x < 0) all(x > 1) all(x > 0)

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MATLAB functions - randi rand() randi(100)

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