Lesson 4: Basic Algorithm
Tools
If, While Do For Loop, Else,
Switch Case
Today's Lesson
Basic Algorithm Tools
What's an algorithm?
What good are they?
Simple Functions
Logic Operators
Loops (while, for)
Logic (if/then, switch/case)
Okay, so what's an Algorithm?
Informally: A set of steps to get you from State A to
State B.
Formally: A type of effective method in which a list of
well-defined instructions for completing a task will, when
given an initial state, proceed through a well-defined series
of successive states, eventually terminating in an end-state.
Informal Example: Read in my Data, Load the middle 10
columns of data, Save those columns to a file (.csv,
.tsv,.txt)
So what Good is an Algorithm?
All programs are Algorithms.
Sketching out an informal Algorithm will help you write
actual programs.
Breaking your thinking into steps can assist in checking
your program for errors.
What do I need to start?
A good mental image of what you want to accomplish.
What is the input? (i.e. your data)
What output do you want? (i.e. a subset of your data)
Some basic tools:
Very simple Matlab formulas
Conditional Operators
Loops
Conditional statements
Playing With Conditional Operators
In your command window, type:
A = 34
B = 76
C = ‘hello’
D = ‘Hello’
Now, try these examples:
A<B
A>B
A == B
A ~= B
isequal(C,D)
~false
true & false
The Situation
• Just finished an experiment
– Subject walked a line while responding to stimuli
– Known data, Age, Accuracy, RT, On Balance
• Need for all subjects prior to losing their balance.
– Their grouping, based on Reaction Time
– Their grade, based on their accuracy
Loops
Why use loops?
Automatically repeat instructions for a large data set
While Loops:
Repeat instructions while a particular condition is true.
For Loops:
Repeat instructions for a particular number of
times.
For i=1:length(infiles)
Is i < length(infiles)?
Execute Loop Code
i=i+1
Exit Loop
Move on
in code.
For Loop
while(condition)
Is counter < length & person on balance?
Execute Loop Instructions
Counter = counter + 1
Exit Loop
Move on in
code
While Loops
If (condition) Else
If age < 65
ageClass = 'Young'
ageClass = 'Elder'
If (condition) Else If(condition) Else
If RTAvg <= 350
Group = 1
Else if RTAvg <= 600
Group = 2
Group = 3
If-Then-Else Block
If(condition) elseif(condition)...else
If Accuracy < 50
Grade = 0
ElseIf Accuracy < 60
Grade = 1
ElseIf Accuracy < 70
Grade = 2
Grade = 3
If – ElseIf Block
Switch and Case
%Set an arm length value dependent on subject age
switch subjectAge
case 10
armLength = 30;
case 15
armLength = 35;
case 20
armLength = 40;
otherwise
armLength = 42;
end
Switch (variable) case...otherwise
switch Age
case 10
armLength = 30
case 15
armLength = 35
case 20
armLength = 40
otherwise
armLength = 42