Computer Science 1000

Algorithms

Programs we now have some idea on how computers execute

an instruction a system of gates is connected to process a series of bits

in some fashion (e.g. adding binary numbers)

your processor has a list of instructions that it can perform

arithmetic: adding, subtracting, etc … memory operations: loading data from memory, saving

data to memory and many others

Programs the language of the processor is binary

that is, each instruction is a sequence of binary digits

e.g. consider our example from term1.ppt

CPU – Basic Operation read an instructionexecute that instruction repeat for next instruction

00001001000100010000101000011101000100110010100000001100001110000010010101110000

place value "17" in memory location 1

place value "29" in memory location 2

add values in loc. 1 and 2, place in loc. 3

place value "56" in memory location 4

multiply values in loc. 3 and 4, place in loc. 5

1

2

3

4

5

MemoryCPU

17

29

46

56

2576

00001001000100010000101000011101000100110010100000001100001110000010010101110000

00001001000100010000101000011101000100110010100000001100001110000010010101110000

00001001000100010000101000011101000100110010100000001100001110000010010101110000

00001001000100010000101000011101000100110010100000001100001110000010010101110000

00001001000100010000101000011101000100110010100000001100001110000010010101110000

Computer Program

From the term1.ppt slides.

Programming programming in binary is considerably difficult fortunately, most software development today is

done using a high level programming language C/C++, Java, Python, Visual Basic, etc.

a programming language represents a compromise between natural language (human) and binary code (computer)

#include <iostream> using namespace std;

int main() {

return 0;}

Write program to compute the perimeter and area of a rectangle. The size of the rectangle will be specified by the user.

int length;cout << "Please enter the rectangle's length: ";cin >> length;

int width;cout << "Please enter the rectangle's width: ";cin >> width;

int perimeter = 2 * (length + width);

int area = length * width;

cout << "The perimeter of the rectangle is " << perimeter << endl;cout << "The area of the rectangle is " << area << endl;

From the term4.ppt slides.

This is an example of a computer program written in C++.

Algorithms how does a program written in a programming language

become binary code that a processor can understand? Answer: a compiler

a compiler is a program that creates other programs from high-level code

00001001000100010000101000011101000100110010100000001100001110000010010101110000

Compiler

int length;cout << "Please enter the rectangle's

length: ";cin >> length;

int width;cout << "Please enter the rectangle's

width: ";cin >> width;

int perimeter = 2 * (length + width);

int area = length * width;

cout << "The perimeter of the rectangle is " << perimeter << endl;

cout << "The area of the rectangle is " << area << endl;

Programming programs written in a programming language are

typically made up of a set of statements roughly speaking, each statement defines an

operation that you would like the computer to perform

output something to screen perform a mathematical operation save information to a file send a request for a webpage to a server etc …

Programming Language in many respects, programming languages are

similar to a natural language only accepts certain keywords

for example, the word while in C++ creates a loop, the word kev has no meaning to C++

statements have a particular syntax that must be followed blocks of code must be enclosed in { } statements must end with a semicolon

Programming given a problem, a programmer’s task is to:

devise an algorithm for solving that problem translate that algorithm into source code compile the source code into a program that the computer

can understand

our topic for this week is to consider this task in other words, a light introduction to programming our programming language: Scratch

Scratch a programming language and

environment written by MIT Labs originally intended to introduce

children to programming, Scratch has become a popular choice for introducing programming in other settings (e.g. universities)

freely available (GPL)

Available operations.

Output

Source code (source pane)

Scratch “Code” in most programming languages, code is

written as text in Scratch, operations are represented as

blocksprograms are arranged as sequences of

blocks arranged together

Source code blocks.

Sprites the operations in Scratch control the

behaviour of the spritedifferent operations available

moving/turning spriteoutput (as a text bubble)sounds (drums, etc)etc …

Scratch – First Examplewrite a program where the sprite says

“Hello!”solution:

find the following block under the Looks category

drag this block onto the source code pane

Scratch – First Examplewrite a program where the sprite says “Hello

World”

Scratch – First Example to run the program, double click on the

block to reset the program (remove output), click

the stop sign in the top right corner

Outputprevious was an example of program outputeach programming language has its own

version of an output statement:C: printf(“Hello!”);Java: System.out.println(“Hello!”);Python: print “Hello!”

Changing Outputwe can change the output by modifying the

text in the white box for example, modify previous example so

that output becomes “Hello World!”

Movement in addition to text output, we can control the

position and direction of the sprite simple movement operations:

move in the direction that the sprite is facing turn clockwise turn counterclockwise

example: construct a program that moves the sprite 25 steps

Example #2: write a program that turns the sprite 15 degrees clockwise

Movement to reset the position of the sprite

double click:you may need to put zeroes in the boxes

to reset the direction of the spritedouble click:

Programsour programs so far have been composed

of single operations the real power of programming comes in

combining statements in a typical programming language, we

simply write more than one line of code in Scratch, we place more than one block in

our source pane, and attach them

Example: write a program in which the sprite moves 10 steps, and then says “Finished!”

Example: write a program that moves 100 steps, and then turns 45 degrees clockwise

Programsnote that the blocks execute from top to

bottom, one after the otherwe call this sequential execution

the sequence of our blocks can affect the final outcome of the program

for example, what if we reverse the order of our previous blocks?

Example #2: Write a program that writes “Hello …”, and then “… world!”

Timing in previous example, when we double click

on the code blocks, we only see “… world!”why?

the code blocks execute one after the other they execute so quickly, that we only see the

results of the second

Timinghow can we address this issue?

Solution #1 (text only): there is a block called Say Hello for 2 seconds we can set the message that we want to display, and

the amount of time that it should be displayed before executing the next block

Timinghow can we address this issue?

Solution #2 (general): there is a block in the control group called Wait 1 sec.

This block does not change the output, it simply waits the specified number of seconds before executing the next block.

note that this can be applied before any block

Example: write a program that moves the sprite around an imaginary square of size 100x100. The sprite should end up in its original position, facing the same direction remember to include some kind of delay,

otherwise, the sprite will appear not to move (because it occurs so quickly)

Entry Point most programs (such as those you are used to)

have an entry point tells the operating system where to start executing

the program in Scratch, we can designate the entry point to a

program using the When <green flag> clicked block program begins when green flag

clicked

Source vs. Executablewhen a programmer creates a program,

rarely do we ever see the source codewe only see, and run, the executable program

Scratch also allows us to run a program without seeing the source code

presentation modeclick the symbol in top-right corner

Arithmetic Operationsevery major computer language permits at

least some mathematical operationssimple: +, -, *, /not so simple: sin, cos, log …

Operators four basic arithmetic operators

these operators behave in the same way as they do in Excel

Operator Symbol

Addition +

Subtraction -

Multiplication *

Division /

Arithmetic Expression to use an arithmetic operator, use one of the

following blocks (from the operators group): in each white space, you put:

a numberanother arithmetic expression

for example, use Scratch to compute the value 86 + 43

The block:

in Scratch creates an expressionmore specifically, an arithmetic expression the value of the expression is:

whatever the formula evaluates to in the above example, the value of the

expression is 129

Where are arithmetic expressions used? anywhere an expression is valid in Scratch, this is usually in the white box

(input) of another block for example, we can have our sprite output

the value of our arithmetic expression place the expression block in the white box of

the Say block

Example 2: Write a program that moves the sprite 117+43 steps, and turns 46-19 degrees

instead of doing the math yourself, let Scratch do it

Math Expressions what will the result of running the following

program be?

Math expressions are only evaluated in their corresponding operator blocks.

Math Expressionssuppose I wanted to compute 1314 + 829 +

74 using Scratchhow would I do it?

1314 + 829 + 74 = (1314 + 829) + 74 in other words, there are two plus operators the value of the first plus operator becomes the

left input to the second operator

Math Expressionswrite a program in which the sprite tells us

the value of (22+13) / 72

Note that Scratch rounded our answer to 2 decimal places. The actual answer is 0.486111…

Joins an operator that allows us to concatenate two

values

the result of this operator is the left text, followed by the right text

Example: Write a program that prints 34x78 = , followed by the value of that expressionoutput should come from the sprite

A more useful example: write a program that converts 20 degrees

Fahrenheit to Celsius

Formula: )32(9

5 FC

A more useful example: write a program that converts 20 degrees

Fahrenheit to Celsius

Formula: )32(9

5 FC

What do you notice about the previous program?what happens if the temperature is 15

degrees? the source code itself would have to be edited

that is, all of our programs so far, produce exactly the same output unless we change the source code

InputScratch allows us to ask the user for a valueonce the user has entered a value, it can be

used in the program, like any other valueuse the following block:

Sprite will ask this question, and then wait for the user to input a value.

Input note that you can ask any question that you like

Inputour program is reading a value from the

userwhere does it go?

into a block called answer this block can be added to the input of another

block, and will be replaced with the user’s input for example: write a program that asks for the

user’s name, and then prints it on the screen.

Example: Convert the previous program, so that the output becomes Hello followed by the user’s name

Example: Convert the previous program, so that the output becomes Hello followed by the user’s name

Back to our temperature example: convert this program to:

ask the user for a temperature, in Fahrenheit print the corresponding value, in Celsius

Formula: )32(9

5 FC