Computer Science 1000

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


From the term1.ppt
slides.

 CPU

– Basic Operation

 read

an instruction
 execute that instruction
 repeat for next instruction

CPU

Memory
1

17

2

29

3

46

4

56

5

2576

Computer Program
0000100100010001
0000101000011101
0001001100101000
0000110000111000
0010010101110000

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


 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)




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

#include <iostream>
using namespace std;
int main() {
int length;
cout << "Please enter the rectangle's length: ";
cin >> length;

From the term4.ppt
slides.
This is an example of a
computer program

written in C++.

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;
return 0;
}




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 highlevel code

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;

Compiler

0000100100010001
0000101000011101
0001001100101000
0000110000111000
0010010101110000


 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)



Source code
(source pane)

Available
operations.

Output


 Scratch
 in

“Code”

most programming languages, code is
written as text
 in Scratch, operations are represented as
blocks
 programs are arranged as sequences of
blocks arranged together


Source code
blocks.

 Sprites
 the

operations in Scratch control the
behaviour of the sprite
 different operations available
 moving/turning

sprite
 output (as a text bubble)
 sounds (drums, etc)
 etc …


 Scratch

– First Example

 write

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
 write

– First Example

a program where the sprite says “Hello
World”



 Scratch
 to

– First Example

run the program, double click on the
block
 to reset the program (remove output), click
the stop sign in the top right corner