algorithms, algorithmic thinking and structured code
TRANSCRIPT
Algorithms,
Algorithmic Thinking and
Structured Code
Tell me how to make a Jam Sandwich
Tell me how to draw this tree (write the instructions in Simple English)
Tell me how to count the money in my right pocket (write the instructions in Simple English)
Origins of Algorithms
Edsger Wybe Dijkstra
Spaghetti Code mov eax,3
mov ebx,3
L1: decjmpreg eax,L2
decjmpreg ebx,L3
jmp L1
L2: eax = 0
decjmpreg ebx, L4
jmp L5
L3: ebx = 0
inc eax
decjmpreg eax, L4
jmp L5
L4: mov eax,0
hlt
L5: mov eax,1
hlt
“Peeling Potatoes for a Family Supper”
We observe the cook doing this
“He gets the potato basket from the cellar,
gets the pan from the cupboard,
peels the potatoes
returns the basket to the cellar”
Sequence
“Go to the other side of the road”
“Go to Hell”
Meaningful Instructions
Effective
Meaningful Instructions
Unambiguous
A wedding journalist reported that the four bridesmaids wore the same dress.
Instructions
“He gets the potato basket from the cellar,
gets the pan from the cupboard,
peels the potatoes
returns the basket to the cellar”
get the potato basket from the cellar,
get the pan from the cupboard,
peel the potatoes
return the basket to the cellar
Sequence: Correct Order
Jordan does something different
get the potato basket from the cellar,
get the pan from the cupboard,
IF clothes are light colored THEN put on an apron
peel the potatoes
return the basket to the cellar
IF some condition THEN some action
Selection
“peel the potatoes”
Halting
“peel the potatoes”
IF number of peeled potatoes is not sufficient THEN peel a potato
IF number of peeled potatoes is not sufficient THEN peel a potato
IF number of peeled potatoes is not sufficient THEN peel a potato
IF number of peeled potatoes is not sufficient THEN peel a potato
IF number of peeled potatoes is not sufficient THEN peel a potato
“peel the potatoes”
WHILE the number of potatoes is insufficient DO peel a potato
get the potato basket from the cellar,
get the pan from the cupboard,
IF clothes are light colored THEN put on an apron
WHILE the number of potatoes is insufficient DO peel a potato
return the basket to the cellar
WHILE condition is true DO an action
Iteration
“peel the potatoes”
Instruction Blocks
WHILE condition is true DO
action1
action2
action3
END WHILE
Algorithms – a definition
• Sequence of Instructions (ordered)
• Instructions are Effective and Unambigous
• Sequence must Halt in a finite time
• On halting, the sequence must have a Result
• Instructions may be
• Simple Actions
• Selections
• Iterations
action
IF condition THEN action
WHILE condition is true DO action
Pseudo-code for Structured Programming
WHILE condition is true
action
action
END WHILE
DO
action
action
WHILE condition is true
IF condition is true
action
action
END IF
actions
move robot forward by 20 cm
get a coin from your pocket
press a key on your phone
conditions
robot sensor shows obstacle
coin is a 50p
pocket is empty
key pressed is 7
compound conditions
NOT condition1
condition1 AND condition2
condition1 OR condition2
actions
add, subtract, multiply, divide
get, put (set)
conditions
equals
less than, greater than
combi’s of above
Pseudo-code Examples
1. Robot moves forward until senses obstacle
2. Check that you have typed in a password that ends with 5
3. Check that you have entered a 6-digit phone number
Computational Thinking - 1
Computational Thinking – 2
(Eight Coins Puzzle)
Dr.C’s “Atomic Language” Conjecture(weak version)
Any computer program can be expressed using only the following:
•Declaration: as many variable you need
•Just 2 Operations:
•One loop construct:
We do not need:
IF condition THEN action
FOR loop
Operations of multiplication, division and anything else.
WHILE condition action actionEND WHILE
add1 to a variablesubtract 1 from a variable
Music
Algorithmic?
get the potato basket from the cellar,
get the pan from the cupboard,
peel the potatoes
return the basket to the cellar
I
Have
A
Lovely
Bunch
Of
Coconuts
x = 5
Y = 3
Temp = X
X = Y
Y = Temp
Computational Thinking – 3
Eight Coins Puzzle
