CS 107 Introduction to Computer Science Lecture 2

CS 107 Introduction to Computer Science Lecture 2

Administrativia • Lab access – Searles 128: daily until 4 pm unless class in progress – Searles 117: 6 -10 pm, Sat-Sun 12 -10 pm • Office hours – M, W 4 -5, Tue after lab – Quick questions: Anytime I am in the office – Email to set up appointment • My office: Searles 219 • Study group – Mondays 8 -10 pm in Searles 224

Last time – What is (not) Computer Science – Algorithms, properties and examples – Expressing algorithms: Pseudocode • Who was Alan Turing and what is he famous for? • Today – More pseudocode elements – Algorithm examples

Expressing algorithms • Is natural language good? – For daily life, yes…but for CS is lacks structure and would be hard to follow – Too rich, ambiguous, depends on context • How about a programming language? – Good, but not when we try to solve a problem. . we want to think at an abstract level – It shifts the emphasis from how to solve the problem to tedious details of syntax and grammar.

Pseudocode • Pseudocode = English but looks like programming • Good compromise – Simple, readable, no rules, don’t worry about punctuation. Lets you think at an abstract level about the problem. – Contains only instructions that have a well-defined structure and resemble programming languages

Pseudocode elements • Basic operations – Read the input from user – Print the output to the user – Cary out basic arithmetical computations • Conditional operations – Execute an operation if a condition is true • Repeat operations – Execute a block of operation multiple times until a certain condition is met

Basic operations – Read the input from user • Get x • Get a, b, c – Print the output to the user • Print x • Print “Your mileage is ” x – Cary out basic arithmetical computations • Set x to 10 • Set y to x*x/3

Conditional statements Specify a statement that may or may not be done: if <condition> then <statement to be done> else <statement to be done otherwise> Example if the value of carry is 0 then set the value of a to 0 else set the vale of a to a+1

Loop statements specify a group of statements that may be done several times (repeated): repeat until <condition> < statements to be repeated > • How does this work? – Condition is evaluated – If it is true than the loop terminates and the next instruction to be executed will be the instruction immediately following the loop – If it is false, then the algorithm executes the <statements to be repeated> in order, one by one

Example Variables: count Step 1: set count to 1 Step 2: repeat step 3 to step 5 until count is > 10 Step 3: set square to count *count Step 4: print value of square and value of count Step 5: add 1 to count Step 6: end • What does this algorithm do? • Note: indentation – Not necessary, but makes reading/understanding algorithms easier

Pseudocode examples Equivalent: – Set the value of a to 1 – Set a to 1 – a=1 Equivalent – – Add 1 to count Set count to count + 1 Increment the value of count by 1 count = count + 1 Writing in pseudocode gives you the freedom to choose any of these!

• Incorrect – Set 1 to a – Add a + b (what do you do with the result? ) – Set a+b +3 • Note: not the same – set a to b – set b to a • Example: what is the output of the following algorithms? set a to 2 set b to 4 set a to b set b to a print a, b

Problem • Adding two n-digit numbers 7597831 + 1287525 ---------8885356 How would you write an algorithm to solve this problem? Assume the basic operation is adding one-digit numbers.

List variables • How to represent (in pseudocode) inputs of arbitrary size? • Suppose that we need to read 100 numbers from the user, or 1000, or. . – we could give each variable a different name…tedious!! • Use a list variable: – Variable: list a of size n – This means that a is a list of n elements: a 1, a 2, a 3, …, an – To read the list from the user use • Get n and a 1, a 2, a 3, …, an – To print the list use • Print a 1, a 2, a 3, …, an – We can treat each element in the list as a variable • Set a 3 to 5 • Set a 4 to a 3 +2

Algorithm for adding two m-digit numbers (Fig 1. 2) Given: m ≥ 1 and two positive numbers a and b, each containing m digits, compute the sum c = a + b. Variables: m, list a 0. . am-1, list b 0 …. bm-1 , list c 0 …cm-1 cm, carry, i 0 Get values for m, am-1 … a 0 and bm-1 … b 0 1 Set the value of carry to 0. 2 Set the value of i to 0. 3 Repeat steps 4 -6 until i > m-1 4 Set the value of ci to ai + bi + carry 5 if ci ≥ 10 then subtract 10 from ci and set the value of carry to 1 else set the value of carry to 0 6 Add 1 to i 7 Set the value of cm to carry 8 Print value of c = cm cm-1 cm-2 … c 0

So, how does this work? ? ? For example, the input is m = 4, a = 3276, and b = 7345. After step 0, the variables m, a, and b have those values: m 4 3 2 a 3 a 2 7 6 a 1 a 0 7 3 b 2 4 5 b 1 b 0 After steps 1 and 2, the variables i and carry are initialized. i 0 carry 0 Next, steps 4 -6 are repeated until the value of i > 3. Each repetition computes a single digit of c. c 4 c 3 c 2 c 1 c 0

A model for visualizing an algorithm’s behavior Computer Input (keyboard) Algorithm Variables Output (screen)

E. g. , Visualizing Fig 1. 2 Computer Input (keyboard) 4 3276 7345 0 Get values for … … 8 Print value of … m 4 i 0 3 2 a 3 a 2 7 3 b 2 c 4 c 3 c 2 carry 7 6 a 1 a 0 4 5 b 1 b 0 c 1 c 0 Output (screen) 0

Algorithm for computing MPG (Fig 2. 5) Write a pseudocode algorithm to compute the distance traveled and the average miles per gallon on a trip when given as input the number of gallons used and the starting and ending mileage readings on the odometer. Variables: response, gallons, start, end, distance, mpg 0 Set response to “Yes” 1 Repeat steps 2 -8 until response = “No” 2 Get gallons, start, end 3 Set distance to end - start 4 Set mpg to distance ÷ gallons 5 Print mpg 6 if mpg > 25. 0 then print “You are getting good gas mileage” else print “You are NOT getting good gas mileage” 7 Print “Do you want to do this again, Yes or No? ” 8 Get response 9 Stop

So, how does this work? ? ? For example, suppose we use 25 gallons, beginning at 12000 and ending at 13000 on the odometer. Then, after step 2, some variables have the following values: Yes 25 12000 13000 response gallons start end After step 4, the variables distance and mpg are computed. distance 1000 mpg 40 Steps 5 -9 displays these results on the output screen: 40 You are getting good gas mileage Do you want to do this again, Yes or No?

Visualizing Fig 2. 5 Computer Input (keyboard) 25 12000 13000 0 Set response … … 11 Stop response Yes gallons start distance end mpg Output (screen)

Designing Algorithms: A Methodology 1. Read the problem, identifying the input and the output. 2. What variables are needed? 3. What computations are required to achieve the output? 4. Usually, the first steps in your algorithm bring input values to the variables. 5. Usually, the last steps display the output 6. So, the middle steps will do the computation. 7. If the process is to be repeated, add loops.

How was the MPG program (Fig 2. 5) designed? Problem statement: Write a pseudocode algorithm to compute the distance traveled and the average miles per gallon on a trip when given as input the number of gallons used and the starting and ending mileage readings on the odometer. Input: number of gallons, starting mileage, ending mileage Output: distance traveled, average miles per gallon Variables: gallons, start, end, distance, mpg Calculate: distance = end - start mpg = distance / gallons Put the steps in order: input, calculate, output (steps 2 -8) Determine if a loop is needed (steps 0, 1, 9, 10)

A Search Algorithm Problem statement: Write a pseudocode algorithm to find the location of a target value in a list of values. Input: a list of values and the target value Output: the location of the target value, or else a message that the value does not appear in the list. Variables:

The (sequential) search algorithm (Fig 2. 9) Variables: target, n, list of n values Get the value of target, n, and the list of n values Set index to 1 Set found to false Repeat until found = true or index > n If the value of listindex = target then Output the index Set found to true else Increment the index by 1 If not found then Output a message that target was not found Stop

Over the weekend… Continue reading chapter 2. 2 and start on 2. 3 Discuss and solve the problems in the Lab 1 – – – Lab 1 is due on Tuesday in class/lab. If you have (brief) questions we’ll discuss them in class on Monday Study group is on Monday evening.