1. More Loop Examples Functions and Parameters

2. Example: min difference
Given two list of numbers, compute the minimum difference among any pair of numbers, one from each list.
E.g., list1 = [1, 2, 3, 4], list2 = [-2, 10, 5, 0, 6], the function minDiff would return 1, which occurs twice, {1,0}, {4,5}.
Remember the findMin function?
def findMin( aList ):
min = aList[0]
for x in aList:
if min > x:
min = x
return min
We can use similar ideas.

3. Example: min difference
def minDiff( list1, list2 ):
’’’
The parameters list1, list2 are two int lists.
Find minimum difference among any pairs.
minDiffSoFar = # some outrageously big value
for x in list1:
for y in list2:
diff = abs(x – y)
if diff < minDiffSoFar:
minDiffSoFar = diff
return minDiffSoFar

4. Example: min difference
Alternatively, you can use Python’s maximum value as the prime.
import sys
def minDiff( list1, list2 ):
’’’
The parameters list1, list2 are two int lists.
Find minimum difference among any pairs.
minDiffSoFar = sys.maxsize # largest Python value
for x in list1:
for y in list2:
diff = abs(x – y)
if diff < minDiffSoFar:
minDiffSoFar = diff
return minDiffSoFar

5. Functions and Parameters
We’ve learned how to develop functions
def findMax( aList ):
max = aList[0]
for i in range( len(aList) ):
if aList[i] > max:
max = aList[i]
return max
def sumList( aList ):
sum = 0
for i in range( len(aList) ):
sum += aList[i]
return sum
In both cases, ‘aList’ is called a parameter for the function
A function can have multiple parameters
Two types of parameters, mutable and immutable
Let’s try out the examples (25_passing.py)

6. Pass By Value PASS BY VALUE def main() """ calls conform """
print(" Welcome to Conformity, Inc. ")
fav = 7
conform(fav)
print(" My favorite number is", fav )
def conform(fav)
""" sets input to 42 """
fav = 42
return fav 7
fav
type: int
PASS BY
VALUE 7 42
fav
type: int
“Pass by value" means that the data's value is copied when sent to a function...

7. Passing list content by reference …
def main()
""" calls conform2 """
print " Welcome to Conformity, Inc. "
fav = [ 7, 11 ]
conform2(fav)
print(" My favorite numbers are", fav )
def conform2(fav)
""" sets all of fav to 42 """
fav[0] = 42
fav[1] = 42 7 11
fav
fav[0]
fav[1]
type: list
fav
type: list
What gets passed by value here?

8. Passing list content by reference…
def main()
""" calls conform2 """
print " Welcome to Conformity, Inc. "
fav = [ 7, 11 ]
conform2(fav)
print(" My favorite numbers are", fav )
def conform2(fav)
""" sets all of fav to 42 """
fav[0] = 42
fav[1] = 42 42 42 7 11
fav
fav[0]
fav[1]
The reference is copied, i.e., passed by value, but the contents arn’t !
fav
and it can change data elsewhere!

9. Watch out!
You can change the contents of lists in functions that take those lists as input.
(actually, lists or any mutable objects)
Those changes will be visible everywhere.
(immutable objects are safe, however)

10. But lists are passing by value!!!
def main()
""" calls conform3 """
print " Welcome to Conformity, Inc. "
fav = [ 7, 11 ]
conform3(fav)
print(" My favorite numbers are", fav )
def conform3(fav)
""" creates a new fav!!! """
fav = [ 42, 42 ] 7 11
fav
fav[0]
fav[1]
type: list
fav
type: list

11. But lists are passing by value!!!
def main()
""" calls conform3 """
print " Welcome to Conformity, Inc. "
fav = [ 7, 11 ]
conform3(fav)
print(" My favorite numbers are", fav )
def conform3(fav)
""" creates a new fav!!! """
fav = [ 42, 42 ] 7 11
fav[0]
fav[1]
fav 42
fav[0]
fav[1]
fav

12. Mutable vs. Immutable Pass by REFERENCE Pass by VALUE tuple dictionary
float
string
list
bool
int
x = [5,42,'hi']
s = 'hi'
Reference
Pointer id
'hi' 5 42
'hi' x s
x[0]
x[1]
x[2]
Lists and dictionaries are handled by reference (the variables really hold a memory address)
Other types of data, incl. strings, are handled by value: they hold the actual data

13. Lists’ flexibility
Lists can hold ANY type of data
x = [ 42, 75, 70 ] 42 75 70
list
int
int
int x
We can equally well imagine them as vertical structures. 42
list
int x 75
int 70
int

14. Lists’ flexibility Lists can hold ANY type of data 42.0 75.0 70.0 42 7
double
double
double x 42 7
-11
list
int
int
int x
“go”
“red”
“sox!”
list
String
String
String x

15. 2d lists or arrays
Lists can hold ANY type of data -- including lists !
x = [ [1,2,3,4], [5,6], [7,8,9,10,11] ]
list x

16. 2d arrays Lists can hold ANY type of data -- including lists !
x = [ [1,2,3,4], [5,6], [7,8,9,10,11] ]
list
list
x[0] x
list
x[1]
list
x[2]

17. Jagged arrays Lists can hold ANY type of data -- including lists !
x = [ [1,2,3,4], [5,6], [7,8,9,10,11] ]
list
list
x[0] x
list
x[1]
list
x[2]
Rows within 2d arrays need not be the same length

18. Rectangular arrays What does x[1] refer to?
list
list
x[0] x
x[0][0]
list
x[1]
list
x[2]
x[2][3]
What does x[1] refer to?
What value is changed with x[1][2]=42 ?
How many rows does x have, in general ?
How many columns does x have, in general ?