Flow Control

1
Flow Control
So far, our programs have been simple...
function area circle_area( radius ) function
area circle_area( radius ) area pi radius2
Are functions just shorthand?
No! They can be much more powerful... with Flow
Control.
2
Flow Control?
'Flow control' can sound a little odd at first....
But it makes sense we want to control the flow
of evaluation. Which means...

• Making decisions

• Repeating things

inputs
function
outputs
3
Decisions
Execute one piece of code instead of another if
some thing is true
Example How to make your experiment work
if isGoodForMe( subject.data ) keepInStudy(
subject ) else throwOutSubject( subject ) end
Now the program branches... more than one
possible execution path.
if-statements are called conditionals.
4
Repetitions
We may want to do something more than once.
So we use loops...
While Loop
For Loop
for indexbeginincrend expressions end
while condition expressions end
5
While
While loops evaluate statements while something
is true.
Count 'e' in a string str 'Sleeplessness' num
_e 0 index 1 while index lt
length(str) if str(index) 'e' num_e
num_e 1 end index index 1 end
6
For
For loops iterate though values of an index
Count 'e' in a string str 'Sleeplessness' num
_e 0 for index1length(str) if str(index)
'e' num_e num_e 1 end end
7
More Control
We are able to control how loops execute

• continue skip the rest and start a new loop.

• break stop executing the loop and go to the end.

n 0 x 0 while n lt 10 n n 1 if n lt
5 continue end break end
8
Things we know

• Code can branch using if statements.
• Code can repeat using loops
• while loops execute while a condition is true.
• for loops iterate over a set of values.
• continue starts over.
• break ends.