more on for loops (class slides)

CSc 110 - more on for loops lists

lists within lists

We’ve worked with lists of lists, but here’s a review:

lists = [[9,  8,  7,  8],
         [10, 20, 30, 4],
         [5,  50, 55, 4]]
lists[0]

[9, 8, 7, 8]

lists[0][0]

9

2D lists

• The “first dimension” is the outer list
• The “second dimension” are the inner lists

lists = [[9,  8,  7,  8], # index 0 of outer list
         [10, 20, 30, 4], # index 1 of outer list
         [5,  50, 55, 4]] # index 2 of outer list

2D lists

• When we draw pictures of 2D lists, the first dimension is the vertical axis, the second is the horizontal

lists = [[9,  8,  7,  8],
         [10, 20, 30, 4],
         [5,  50, 55, 4]]

Evaluate the Expressions

numbers = [[4, 2, 3],
           [0, 2, 5]]
numbers[1][2]   

items = [[9, 7, 8],
         [1, 2, 3, 4, 5, 6],
         [5],
         [10, 20, 30]]
items[1][5]
items[2][5]
items[3][2]

Evaluate the Expressions

numbers = [[4, 2, 3],
           [0, 2, 5]]
numbers[1][2]

5

items = [[9, 7, 8],
         [1, 2, 3, 4, 5, 6],
         [5],
         [10, 20, 30]]
items[1][5]

6

items[2][5] # will throw error

items[3][2]

30

for loops with lists of lists

items = [[9, 7, 8],
         [5],
         [10, 20, 30]]
         
for i in range(len(items)):
  for j in range(len(items[i])):
    print( items[i][j] )

9
7
8
5
10
20
30

Write a function

1. Its name is flatten
2. It takes one parameter, which is a list of lists of integers: numbers
3. It returns a single list that flattens that list of lists into a single list that contains exactly the same values
4. Do not use any list methods besides .append()

assert flatten([]) == []
assert flatten([[1, 2, 3], [4, 5, 6]]) == [1, 2, 3, 4, 5, 6]
assert flatten([[1, 2, 3], [4]]) == [1, 2, 3, 4]

Write a function – solution

def flatten(numbers):
  new_list = []
  for i in range(len(numbers)):
    for j in range(len(numbers[i])):
      new_list.append(numbers[i][j])
  return new_list

def main():
  assert flatten([]) == []
  assert flatten([[1, 2, 3], [4, 5, 6]]) == [1, 2, 3, 4, 5, 6]
  assert flatten([[1, 2, 3], [4]]) == [1, 2, 3, 4]

main()

Write a function

1. Its name is get_highest_average
2. It takes an argument, lists, which is a 2D list of numbers (floats or integers)
3. It calculates the mean of each sublist, and returns the highest average

assert get_highest_average([[1, 1, 1], [2, 2]]) == 2

Write a function – solution

def get_highest_average(numbers):
  means = []
  for i in range(len(numbers)):
    if len(numbers[i]) > 0:
      total_sublist = 0
      for j in range(len(numbers[i])):
        total_sublist += numbers[i][j]
      mean_sublist = total_sublist / len(numbers[i])
      means.append(mean_sublist)
  
  if len(means) > 0:
    highest_mean = means[0]
    for i in range(len(means)):
      if means[i] > highest_mean:
        highest_mean = means[i]
        
    return highest_mean

def main():
  assert get_highest_average([[1, 1, 1], [2, 2]]) == 2
  assert get_highest_average([[1, 1, 1], []]) == 1
  assert get_highest_average([[], []]) == None
  assert get_highest_average([]) == None
  
  weeks = [ [100, 100, 105, 105, 102, 102, 100],
            [70, 71, 75, 72, 81, 80, 74],
            [78, 72, 74, 71, 70, 70, 70],
            [30, 32, 40, 38, 31, 32, 30] ]
  average = get_highest_average(weeks)
  assert average == 102
  
main()

Submit attendance

Name your solution two_d_highest_average.py and submit it to gradescope