1. CIT 383: Administrative Scripting
Flow Control and Arrays
CIT 383: Administrative Scripting

2. CIT 383: Administrative Scripting
Topics
Boolean Expressions
Conditionals
While Loops
Creating Arrays
Indexing Arrays
Array Methods
Iterators
CIT 383: Administrative Scripting

3. CIT 383: Administrative Scripting
What are true and false?
false is
false, the one member of the FalseClass
nil, the one member of NilClass
true is
true, the one member of the TrueClass
anything that’s not false, including 0, “”, NaN,...
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4. CIT 383: Administrative Scripting
Comparison Operators
Binary operators that return true or false.
== equals
!= not equals
> greater than
>= greater than or equal to
< less than
<= less than or equal to
CIT 383: Administrative Scripting

5. CIT 383: Administrative Scripting
Boolean Operators
Operate on Boolean values.
! not
not not (lower precedence)
&& and
and and (lower precedence)
|| or
or or (lower precedence)
CIT 383: Administrative Scripting

6. CIT 383: Administrative Scripting
Boolean Expressions
x == 0
y > 1
z <= 5
x == 0 && y > 1
x == 0 && y > 1 || z <= 5
is_x_zero = x == 0
big_y = y > 2**16
is_x_zero && big_y
CIT 383: Administrative Scripting

7. CIT 383: Administrative Scripting
Conditionals
if statement
if statement with else clause
elsif
if expressions
unless statement
post-conditionals
case statement
CIT 383: Administrative Scripting

8. CIT 383: Administrative Scripting
if statement
expr
code
expression
true
false
if expression
code
end
expr
color = hilight
x%2 == 0
true
false
if x % 2 == 0
color = hilight
end
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9. CIT 383: Administrative Scripting
Differences from Java
Parentheses not required around expression
expression begins after if and space
expression ends with newline
If statement must be terminated by an end
There is no punctuation like Java’s braces.
CIT 383: Administrative Scripting

10. CIT 383: Administrative Scripting
if statement with else
if expression
code1
else
code2
end
true
false
expression
code1
code2
CIT 383: Administrative Scripting

11. Multiple conditionals with elsif
if expression1
code1
elsif expression2
code2 .
elsif expressionN
codeN
else
code
end
if x == 1
name = “one”
elsif x == 2
name = “two”
elsif x == 3
name = “three”
elsif x == 4
name = “four”
else
name = “lots”
end
CIT 383: Administrative Scripting

12. CIT 383: Administrative Scripting
if expressions
Ruby has no statements, only expressions.
Everything returns a value, even if it’s nil.
if returns the value of the last expression evaluated.
Example:
name = if x == 1 then “one”
elsif x == 2 then “two”
elsif x == 3 then “three”
elsif x == 4 then “four”
else “many”
end
CIT 383: Administrative Scripting

13. CIT 383: Administrative Scripting
unless statement
unless expression
code
end
true
expression
false
code
expr
unless expression
code1
else
code2
end
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14. CIT 383: Administrative Scripting
Post-conditionals
Pre-conditional form
if expression then code end
Equivalent post-conditional form
code if expression
Examples
puts message if debug > 1
puts message if !quiet && verbose
CIT 383: Administrative Scripting

15. Equivalent if statement
Case statement
Case statement
Equivalent if statement
name = case
when x == 1 then “one”
else “many”
end
name = if x == 1 then “one”
elsif x == 2 then “two”
elsif x == 3 then “three”
elsif x == 4 then “four”
else “many”
end
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16. Alternate case statement format
name = case x
when 1 then “one”
when 2 then “two”
when 3 then “three”
when 4 then “four”
else “many”
end
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17. CIT 383: Administrative Scripting
while loop
while expression
code
end
false
expression
true
code
expr
x = 0
while x < 10
puts x
x = x + 1
end
expr
puts x
x = x + 1
x < 10
true
false
x = 0
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18. CIT 383: Administrative Scripting
until loop
until expression
code
end
true
expression
false
code
expr
x = 0
until x > 10
puts x
x = x + 1
end
x = 0
true
x > 10
false
expr
puts x
x = x + 1
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19. CIT 383: Administrative Scripting
Arrays in Ruby
a = [10, 20, 30, 40, 50, 60]
a[0] 10 20
a[1] 30
a[2]
a[3] 40
a[4] 50
a[5] 60
a[-1]
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20. CIT 383: Administrative Scripting
Arrays of Strings
Define the same way as arrays of integers
a = [‘spam’, ‘and’, ‘eggs’, ‘for’, ‘breakfast’]
Alternative syntax for strings w/o spaces:
a = %w[spam and eggs for breakfast]
a = %w|spam and eggs for breakfast|
%W works like double quoted strings
a = [“\t”, “\n”, “\r”]
a = %W(\t \n \r)
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21. CIT 383: Administrative Scripting
Creating Empty Arrays
Use the [] notation
a = []
Create like any other object
a = Array.new
Similar to
str = “”
str = String.new
CIT 383: Administrative Scripting

22. Arrays of Different Types
Ruby arrays can have data of different types
a=[1,2.718,‘a string’,“\t”,[3,4]]
Use like any other array
a[0] == 1
a[1] == 2.718
a[2] == ‘a string’
a[3] == “\t”
a[4] == [3, 4]
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23. CIT 383: Administrative Scripting
Indexing Arrays
Single indexes
a[n] is nth element from beginning starting at 0
a[-n] is nth element from end starting at -1
Double indexes (slicing)
a[0,1] == a[0]
a[0,2] == [ a[0], a[1] ]
a[1,3] == [ a[1], a[2], a[3] ]
a[-2, 2] == [ a[-2], a[-1] ]
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24. CIT 383: Administrative Scripting
Index Assignments
Single index
a[n] = 5 sets nth element to be 5
If a[n] isn’t already defined, extends array to be long enough to have an nth element.
Double index
a[0,2] = [10,20] sets first two elements to 10,20
a[0,0] = [1,2,3] inserts 3 elements at beginning
a[0,2] = [] deletes first two elements of array
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25. CIT 383: Administrative Scripting
Array Operations
Concatenation
a = [1,2,3] + [4,5] # [1,2,3,4,5]
a = a + [6,7,8] # [1,2,3,4,5,6,7,8]
Subtraction
[1,2,3,4,5,6] – [2,3,4] # [1,5,6]
Append
a = []
a << # [1]
a << 2 << # [1, 2,3]
a << [4,5,6] # [1,2,3,[4,5,6]]
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26. CIT 383: Administrative Scripting
Array Operations
Union: |
Concatenates then removes duplicates.
Intersection: &
Array of elements in both arrays, no duplicates.
Examples
a = [1,1,2,2,3,3,4]
b = [5,5,4,4,3,3,2]
a | b # [1,2,3,4,5]
b | a # [5,4,3,2,1]
a & b # [2,3,4]
b & a # [4,3,2]
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27. CIT 383: Administrative Scripting
Array Methods
reverse
Reverses order of array elements.
sort
Sorts array elements from lowest to highest
All array elements must be of same type.
sort!
Previous methods return altered array.
Methods ending in ! change array in place.
CIT 383: Administrative Scripting

28. Using Arrays w/o Indexing
We often need just a list of items to:
Add items from the list.
Remove items from the list.
Perform an action for each item of the list.
We don’t need to deal with indices since
The 5th item isn’t special.
We just want to add, remove, or do something for each item of the list.
Leaves only one choice: add and remove from
End of array (push and pop)
Beginning of array (shift and unshift)
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29. CIT 383: Administrative Scripting
Push and pop
push adds an element to end of array. 10
a[0]
a[0] 10 20 20
a.push(40) 30 30
a[-1] 40
a[-1]
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30. CIT 383: Administrative Scripting
Push and pop
pop removes an element from end of array. 10
a[0]
a[0] 10 20 20
x = a.pop 30
a[-1] 30
a[-1] 40
x == 40
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31. CIT 383: Administrative Scripting
Shift and unshift
unshift adds an element to start of array. 5
a[0]
a[0] 10 10 20
a.unshift(5) 20 30
a[-1] 30
a[-1]
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32. CIT 383: Administrative Scripting
Shift and unshift
shift removes an element f/ start of array. 10
a[0]
a[0] 5 20 10
x = a.shift 30
a[-1] 20
a[-1] 30
x == 5
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33. CIT 383: Administrative Scripting
The each iterator
Each executes code for each item of array
Assigns current element to |variable|
Executes block of code.
Sets current element to next element.
Example:
langs = %w(python ruby scheme)
langs.each do |language|
puts “#{language} is good.”
end
CIT 383: Administrative Scripting

34. CIT 383: Administrative Scripting
The each iterator
array.each do |var|
code
end
n = 0
var = array[n]
code
false
is n last element?
n = n + 1
true
CIT 383: Administrative Scripting

35. CIT 383: Administrative Scripting
References
Michael Fitzgerald, Learning Ruby, O’Reilly, 2008.
David Flanagan and Yukihiro Matsumoto, The Ruby Programming Language, O’Reilly, 2008.
Hal Fulton, The Ruby Way, 2nd edition, Addison-Wesley, 2007.
Dave Thomas with Chad Fowler and Andy Hunt, Programming Ruby, 2nd edition, Pragmatic Programmers, 2005.
CIT 383: Administrative Scripting