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 <<
is
cout <<
<<
"The
" <<
"The
area
perimeter of the rectangle
perimeter << endl;
area of the rectangle is "
<< 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 “Code”
in 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 – First Example

write 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


Output
previous was an example of program output
 each programming language has its own
version of an output statement:

C:
 Java:
 Python:

printf(“Hello!”);
System.out.println(“Hello!”);
print “Hello!”

Changing Output
we 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 sprite

double click:

Programs
our 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

Programs

note that the blocks execute from top to
bottom, one after the other

we 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


Timing

how 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

Timing

how 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. Executable

when a programmer creates a program,
rarely do we ever see the source code


we only see, and run, the executable program
Scratch also allows us to run a program
without seeing the source code
presentation mode
 click the
symbol in top-right corner


Arithmetic Operations

every major computer language permits at
least some mathematical operations
simple: +, -, *, /
 not so simple: sin, cos, log …


Operators

four basic arithmetic operators
Operator

Symbol
Addition
+
Subtraction
-
Multiplication
*
Division
/
these operators behave in the same way as they do in Excel

Arithmetic Expression
to use an arithmetic operator, use one of the
following blocks (from the operators group):
 in each white space, you put:

a number
 another arithmetic expression


for example, use Scratch to compute the
value 86 + 43

The block:
in Scratch creates an expression
more 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 Expressions
suppose I wanted to compute 1314 + 829 +
74 using Scratch
 how 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 Expressions

write 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
expression

output should come from the sprite

A more useful example:


write a program that converts 20 degrees
Fahrenheit to Celsius
Formula:
5
C  ( F  32)
9

A more useful example:


write a program that converts 20 degrees
Fahrenheit to Celsius
Formula:
5
C  ( F  32)
9

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

Input
Scratch allows us to ask the user for a value
 once the user has entered a value, it can be
used in the program, like any other value
 use 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

Input
our program is reading a value from the
user
 where 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:
5
C  ( F  32)
9