Lecture 3
Loops, dictionaries
Programming in geoinformatics
Autumn 2017
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Loops
Dictionaries, sets
Homework
For loops
While loop
Exercise 1
Exercise 2
For loops
When we need to repeat a code for n elements
alphabet = 'abcdefghijklmnopqrstuvwxyz'
for letter in alphabet:
print letter
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
For loops
While loop
Exercise 1
Exercise 2
For loops
When we need to repeat a code for n elements
alphabet = 'abcdefghijklmnopqrstuvwxyz'
for letter in alphabet:
print letter
We can use range() to create a list of numbers:
range(5) # → [0, 1, 2, 3, 4]
range(2,8) # → [2, 3, 4, 5, 6, 7]
range(len(alphabet)) # → [0, 1, 2, … 24, 25]
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
For loops
While loop
Exercise 1
Exercise 2
For loops
When we need to repeat a code for n elements
alphabet = 'abcdefghijklmnopqrstuvwxyz'
for letter in alphabet:
print letter
We can use range() to create a list of numbers:
range(5) # → [0, 1, 2, 3, 4]
range(2,8) # → [2, 3, 4, 5, 6, 7]
range(len(alphabet)) # → [0, 1, 2, … 24, 25]
That way we can use the item’s index too!
for position in range(len(cities)):
print "City no.", position + 1, cities[position]
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
For loops
While loop
Exercise 1
Exercise 2
Conditionals in loops
The last homework in a loop:
polygons = [
[[1,7], [1,3], [2,3], [2,7]], # polygon 1
[[1,1], [1,5], [3,3]], # polygon 2
[[0,0], [0,5], [2,10], [4,7], [3,10], [8,2]] # polygon 3
]
for polygon in polygons:
if len(polygon) < 3:
print "This is not a polygon!"
elif len(polygon) == 3:
print "This is a triangle"
elif len(polygon) == 4:
print "This has four sides (might be a square?)."
else:
print "This is a more complex polygon."
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
For loops
While loop
Exercise 1
Exercise 2
Line with random coordinates
Some useful stuff. Need a random line for testing?
import random
length = 10
line = []
for i in range(length):
coordinates = [random.random() * 1000, random.random() * 1000]
line.append(coordinates)
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
For loops
While loop
Exercise 1
Exercise 2
While loop
While loop runs until a condition is no longer true:
# Blackjack! Dealer draws until he has 17 points.
total = 0
while total < 17:
total += 2 * random.randint(1,5) # blackjack cards values are
2, 4, 6, 8, 10 (also 1 and 11, whatever)→
if total > 21:
print "The dealer lost!"
else:
print "The dealer has " + str(total) + " points."
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
For loops
While loop
Exercise 1
Exercise 2
While loop
Beware infinite loops!
i = 5
while i < 6:
print i
This will never end!
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
For loops
While loop
Exercise 1
Exercise 2
Exercise 1
Two lists are defined:
hats = ["red", "black", "blue", "yellow"]
ids = [2, 7, 15, 22, 25, 34]
1 Create a new list idsNew, where
you add 1 to even numbers
you subtract 1 from odd numbers
e.g. 3 → 2; 8 → 9
2 Print pairs in idsNew and hats, such as: "person with id 5 has a red hat"
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
For loops
While loop
Exercise 1
Exercise 2
Exercise 2
Cards in a card game have set values:
points are set:
7, 8, 9, 10 cards → 7, 8, 9, 10 points
J, Q, K → 10 points
A → 15 points
suite of the card multiplies its points:
hearts → 4×
diamonds → 3×
spades → 2×
clubs → 1×
1 Print each card combination
2 Calculate the value of every card and if value > 30 print it to console
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
Dictionaries
Sets
Exercise 3
Dictionaries
Defined as: dict = {}
Has its own keys compared to list’s indexes:
# great way to store coordinates
coords = { "lat": 49.1876, "lon": 16.3273, "elev": 420.3 }
# or attributes
row = { "id": 0, "city": True, "name": "Brno", "population":
377440, "ranking": 2, "latitude": 49.2, "longitude": 16.6,
"universities": ["MUNI", "VUT", "MENDELU", "VFU", "JAMU",
"UNOB"] }
→
→
→
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
Dictionaries
Sets
Exercise 3
Dictionaries
We can access dictionary values similarly to lists:
print coords["lat"]
print row["population"]
# if we don't have numbers as keys, this will not work:
print row[0]
# but this will
dict = {1: "a", 2: "b"}
print dict[1]
# again, this won't
print dict[0]
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
Dictionaries
Sets
Exercise 3
Dictionaries
Some methods:
a_dict = {'a': 156, 'b': 89, 'c': 41, 'd': 547}
print a_dict.items() # [('b', 89), ('a', 156), ('d', 547), ('c',
41)]→
print a_dict.keys() # ['b', 'a', 'd', 'c']
print a_dict.values() # [89, 156, 547, 41]
print len(a_dict) # 4
# get() returns None if key is not defined and not an error
print a_dict.get(4) # None
print a_dict.get('a') # 156
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
Dictionaries
Sets
Exercise 3
Sets
Similar to a dictionary:
points = { 9, 7, 9, 10, 3 } # → set([9, 10, 3, 7]) – no duplicities
points.add(15) # set([15, 9, 10, 3, 7])
Useful for union, intersect, … operations.
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
Dictionaries
Sets
Exercise 3
Exercise 3
A dictionary is defined:
countriesStats = {
'Nigeria' : {'area': 923768, 'population': 182202000,
'languages': ['English']},→
'South Africa' : {'area': 1219912, 'population': 54490000,
'languages': ['Zulu', 'Xhosa', 'Afrikaans', 'English']
},
→
→
'Ethiopia' : { 'area': 1127127, 'population': 99391000,
'languages': ['Amharic']}→
}
1 find your favourite African country and add it to the dictionary (after the dict
definition)
2 calculate the population density for every country
3 list all languages spoken in those countries with no duplicates (use sets)
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
Homework 1
Bonus homework
Homework 1
We have two rivers:
riverA = [[3,7], [3,9], [4,11], [6,12]]
riverB = [[12,4], [10,6], [6,7], [3,9], [2,4]]
1 Calculate their distances using loops
2 Hints
use for i in range(…)
you have to get the value of two points in each cycle, not one
3 Make a list of points the rivers have in common
4 Hint: point in line
Autumn 2017 Lecture 3
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Loops
Dictionaries, sets
Homework
Homework 1
Bonus homework
Bonus homework
2 points
Write a script that will calculate the area of a polygon. Assume it is a valid simple
polygon: has only one part, edges are straight and the edges are not crossing each
other.
How to calculate a polygon area:
http://www.mathopenref.com/coordpolygonarea.html
Autumn 2017 Lecture 3