defining and using arrays in python

Question

hi I wrote a code to take number of students and number of lessons I have a array I want to put grades of lessons and average of them in array but it does not work and code just prints last input instead of all students this is my code:

def find_averag(numberOfLessons, grades, linenumber):
    sum = 0
    for x in range(numberOfLessons):
        sum += grades[linenumber][x + 1]
    return sum / numberOfLessons


val = input("Enter number of students: ")
numberOfStudents = int(val)
val = input("Enter number of lessons : ")
numberOfLessons = int(val)

arr = [[0] * (numberOfLessons + 2)] * numberOfStudents
print(arr)
for x in range(numberOfStudents):
  val = input("Emter student name: ")
  print(val)
  arr[x][0] = val
  print(arr[x][0])
  for y in range(numberOfLessons):
      val = input("Enter grade")
      degree = int(val)
      arr[x][y+1] = degree
arr[x][numberOfLessons + 1] = find_averag(numberOfLessons, arr, x)
for t in range(numberOfStudents):
        print(arr[t])

Tim Roberts · Accepted Answer · 2021-05-03 06:44:15Z

1

This is a key Python concept. Let's say numberOfLessons is 3 a numberOfStudents is 5. Then, this statement:

arr = [[0] * (numberOfLessons + 2)] * numberOfStudents

Does not actually create a 3x5 array. What it creates is a list with 5 pointers to the SAME 3-element list. If you change array[0][0], that's also going to be seen in array[1][0] and array[4][0].

The easy way to handle this is to build up your array from nothing, adding a row at a time:

arr = []
print(arr)
for x in range(numberOfStudents):
  val = input("Emter student name: ")
  print(val)
  row = [val]
  print(row)
  for y in range(numberOfLessons):
      val = input("Enter grade")
      degree = int(val)
      row.append( degree )
  arr.append( row )

answered May 3, 2021 at 6:44

Tim Roberts

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2 Comments

AKX Over a year ago

Or if you want to pre-create a list of lists in one expression, use a list comprehension: arr = [[0] * (numberOfLessons + 2)] for x in range(numberOfStudents)]

Tim Roberts Over a year ago

That's a good point, but in my experience it's usually more readable and more "Pythonic" to build up the list of lists like I have done here. The desire to pre-create 2D arrays usually comes from another language. On the other hand, with numpy, the pre-create option makes good sense.

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