5.8. Lists

5.8.1. Accessing elements, lists and for loops

The built-in function range can be used to create a long list of integers as in the following program:

a = list(range(1,500))
print(a)

Create a program (save it as li_intro.py) that:

• creates a list (using the range(1,500) statement),

• uses a while loop traversing the list, printing each element, and its square root,

• uses the same while loop to calculate the sum of the square roots of the elements, and prints the sum.

It is recommended to use a shorter list first (when developing the program), e.g. range(1,5). But note that the question below refers to the list created with range(1,500).

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Question What is the sum of the square roots of the elements?

1. 9201

2. 8723

3. 6310

4. 7442

Correct answers: d.

Feedback:

import math

a = list(range(1,500))
print(a)

sum = 0.0
i = 0
while i < len(a):
  v = a[i]
  vSqrt = math.sqrt(v) 
  print(v, vSqrt)
  sum = sum + vSqrt 
  i = i + 1
print("The sum of the square roots of the values is", sum)

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5.8.2. List deletion

Create a program (name it li_delete1.py) that creates a list with 5 elements of type floating-point. Now, extend the program such that all elements smaller than a threshold value are removed from the list.

This can be solved by traversing the list using a while statement, but this may cause problems. As you will know, deleting an element in a list can be done using del. This results in removal of the element and consequently, the list becomes one element shorter. This will cause a problem: it is not possible to go through a list using a for loop and delete elements from the same list simultaneously because in that way the for loop will not reach all elements. A similar kind of problem happens when you try to add lists elements while traversing the same list. The solution is to create an empty list, and copy the elements that you want to keep to that clone list. Approach the problem in a stepwise manner as follows:

1. use a while loop to print only the list elements smaller than or equal to the threshold value, test the program,

2. above the while loop, create an empty list (e.g. newList),

3. in the while loop, append the list elements smaller than or equal to the threshold value to this empty list. For each element, append it to list. This can be done with newList.append(a), where a is the value that needs to be appended.

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Question How can you create an emtpy list?

1. Using []

2. Using a.emptyList()

3. Using range(0)

Correct answers: a.

Feedback: FeedbackText

a = [100.9,34.0,34.2,7.8,9.0]
print(a)

# empty list 
aNew = [] 

i = 0
while i < len(a):
    if a[i] >= 10:
        aNew.append(a[i])
    i = i+1

print(aNew)

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5.8.3. Matrices and gridded maps

Write a program matrix.py that prints the nested list:

matrix = [[1,2,3],[4,5,6],[7,8,9]]

formatted like this:

1 2 3
4 5 6
7 8 9

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Question An element from this nested list matrix can be accessed through:

1. matrix[1,2]

2. matrix{1}{2}

3. matrix[1][2]

Correct answers: c.

Feedback:

matrix = [[1,2,3],[4,5,6],[7,8,9]] 

i = 0
while i < len(matrix):
    j = 0
    while j < 3:
        print(matrix[i][j], end=' ')
        j = j + 1
    i = i + 1
    print()

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5.8.4. Strings and lists

Write a program that asks the user to enter two numbers separated by a comma, printing the product (the number resulting from multiplication) of the two numbers. Save it as multi.py.

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Question You can use a string method (from str). Which one?

1. split

2. replace

3. lower

4. count

Correct answers: a.

Feedback:

numbers = input("Enter two numbers separated by a comma ")

# split the string using ',' as a separator, resulting in a list
numbersList = str.split(numbers,",")

firstNumber = float(numbersList[0])
secondNumber = float(numbersList[1])
product = firstNumber * secondNumber
print("The product of", firstNumber, "and", secondNumber, "is", product)

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