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Introduction to Python

Data Structures (cont.)

Lists or Arrays

Additional information available here and also here

Lists are a “little” similar to Strings in the sense that there is some information stored at some index

Input

brics = ["Brazil", "Russia", "India", "China", "South Africa"]
index content
0 "Brazil"
1 "Russia"
2 "India"
3 "China"
4 "South Africa"

Input

brics[0] # get the 1st element of the list

Input

brics[4] # get last element

Input

brics[-1] # also get last element

Input

brics[5]

Output

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

Input

# append adds an element to the list
brics.append('Philippines')

Input

# finds the position of an element in the list
brics.index('Philippines')

Input

# deletes an element from the list
# bye Brazil
del brics[0]

Loops

Input

for country in brics:
    print("This is a loop. I am iterating over the list. variable country = ", country)
  • After the for statement, read from right to left as in:
    • for the elements in list brics we have a variable country that will store one of the values in the list (starting with Brazil)
    • For that value, we execute everything in the indentation block
    • After we are done. We repeat and country is Russia then India and so forth

Output

('This is a loop. I am iterating over the list. variable country = ', 'Brazil')
('This is a loop. I am iterating over the list. variable country = ', 'Russia')
('This is a loop. I am iterating over the list. variable country = ', 'India')
('This is a loop. I am iterating over the list. variable country = ', 'China')
('This is a loop. I am iterating over the list. variable country = ', 'South Africa')

Back to our practical problem

Suppose we want to find all the sentences of a text file that explicitly mention global and warming

Input

with open('G20-2019.txt', 'r') as f:
    data = f.read().split('\n')
    print(data)

We need to add a loop iterating over the data. We also need a condition to identify a sentence that explicitly mentions global and warming.

Conditionals

Let’s start small. We wan sentences that just mention global

Input

with open('G20-2019.txt', 'r') as f:
    data = f.read().split('\n')
    for line in data:
        if 'global' in line:
            print(line)
            print()

Output

We have global warming, but we have also global political warming, and this ....

And it is also a moment in which there are uncertainties about the global economy ...

... we need to make sure that we do not reach more than 1.5 degrees Celsius of global warming at the end of the century.

More than one conditional

Input

with open('G20-2019.txt', 'r') as f:
    data = f.read().split('\n')
    for line in data:
        if 'global' in line and 'warming' in line:
            print(line)
            print()

Input

with open('G20-2019.txt', 'r') as f:
    data = f.read().split('\n')
    for line in data:
        if 'global' in line or 'warming' in line:
            print(line)
            print()

If we are on track, try to:

1. create two numeric variables cnt_2016 and cnt_2019

2. Open G20-2019.txt

3. Iterate the lines in the file counting every line that mentions global warming

increment cnt_2019 for every line that mentions global warming

4. Open G20-2016.txt

5. Iterate the lines in the file counting every line that mentions global warming

increment cnt_2016 for every line that mentions global warming

6. print the two numeric variables


Don’t spoil the fun. The stick figure is watching you