Python Refresher

Question 1

How are statements printed in python?

How are results stored in python?

Answer 1

print ("hello world")
myresult = 5*6

Question 2

Let’s say that we have a variable x = 10
Write code to detect whether x is postive, 0 or negative.

Answer 2

x = 10
if x > 0:
    print("Positive")
elif x == 0:
    print("Zero")
else:
    print("Negative")

Question 3

What is the format for a for loop?

What is the format for a while loop?

Answer 3

Example of a for loop
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

Example of a while loop
count = 0
while count < 5:
    print(count)
    count += 1

Question 4

In python how can we initialize variables with the basic data types of integers, floats and strings.

How about composite data types? How do we access numbers in an array? How do we access members of a dictionary?

Answer 4

Integers: myVar = 5;
Floats: myVar = 6.3;
Strings: myVar = "Hello World"

many_numbers = [5, 42, 1, 10]
many_numbers[1] //will return 42

nums = {"best number":4, "still great": 8}
nums["best number"] // will return 4

Note that in python we use 0-based indexing

Question 5

What is JSON?

Answer 5

JSON is a simple textual format derived from Javascript syntax

Files in JSON format can be read/written by Python code and can also be edited by jand if necessary.

{
    "name": "Lorenzo"
    "location": "Calgary"
    "numbers": [0,1,1,2,3,5]
}

Question 6

Assume that the contents of data.json is
{ "name" : "Lorenzo", "location" : "Calgary", "numbers" : [1,2,3,42] }

Write code so that we will print out name, and the number 42.

Answer 6

# First things first - import the json module
import json

Load the file
with open("data.json", "r") as inF:
    content = json.load(inF)

Print the content
print(content["name"]) #will print out Lorenzo
nums = content["numbers"] #assigns nums to the array
print(nums[3]) #prints out 42

Question 7

Write code so that mydict = { "Temperature" : -36, "Location" : "Calgary", "Feeling" : "miserable", "numbers" : [0, 1, 1, 2, 3, 5, 8, 13] } is written to a file called output.json

Answer 7

mydict = {
    "Temperature" : -36,
    "Location" : "Calgary",
    "Feeling" : "miserable",
    "numbers" : [0, 1, 1, 2, 3, 5, 8, 13]
}

with open("output.json", "w") as fifi:
    json.dump(mydict, fifi)

Question 8

What does the following print:
~~~
#Example of List Comprehension without filters
numbers = [1, 2, 3, 4]

numsq = [n*n for n in numbers]

print(numsq)
~~~

Answer 8

#Example of List Comprehension without filters
numbers = [1, 2, 3, 4]

numsq = [n*n for n in numbers]

print(numsq) #will print [1, 4, 9, 16]

Question 9

What will the following code do?
~~~
numbers2 = [1, 2, 3, 4, 5]

even_numbers = [x for x in numbers2 if x % 2 == 0]
print(even_numbers)

~~~

Answer 9

[2, 4]

Question 10

What is the format for list comprehension with conditionals?

Answer 10

dst = [comp1(e) if cond(e) else comp2(e) for e in numbers]

numdst = [n*n if n<5 else n/2 for n in numbers]

Question 11

Write a factorial function using recursion.

Answer 11

def factorial(n):
    if n == 1:
        return 1
    else:
        return n*factorial(n-1)

Question 12

What do the following matplotlin functions do?
~~~
plt.plot(x, y)
plt.bar(x, y)
plt.hist(x)
~~~

Answer 12

plt.plot(x, y) #plots x, y using lines
plt.bar(x, y) #plots x, y data using bars
plt.hist(x) # plots the distribution (histogram) of data in x 

Question 13

What does the following code do?
~~~
plt.savefig(“filename.png”)
~~~

Answer 13

Calling this after generating a plot will save the plot to a file.