CMU 15-112: Fundamentals of Programming and Computer Science
Class Notes: 2d Lists


  1. Creating 2d Lists
  2. Getting 2d List Dimensions
  3. Copying and Aliasing 2d Lists
  4. Printing 2d Lists
  5. Nested Looping over 2d Lists
  6. Accessing 2d Lists by Row or Column
  7. Non-Rectangular ("Ragged") 2d Lists
  8. 3d Lists

  1. Creating 2d Lists
    • Static Allocation
      # create a 2d list with fixed values (static allocation) a = [ [ 2, 3, 4 ] , [ 5, 6, 7 ] ] print(a)

    • Dynamic (Variable-Length) Allocation
      • Wrong: Cannot use * (Shallow Copy)
        # Try, and FAIL, to create a variable-sized 2d list rows = 3 cols = 2 a = [ [0] * cols ] * rows # Error: creates shallow copy # Creates one unique row, the rest are aliases! print("This SEEMS ok. At first:") print(" a =", a) a[0][0] = 42 print("But see what happens after a[0][0]=42") print(" a =", a)

      • Right: Append Each Row
        # Create a variable-sized 2d list rows = 3 cols = 2 a = [] for row in range(rows): a += [[0]*cols] print("This IS ok. First:") print(" a =", a) a[0][0] = 42 print("And now see what happens after a[0][0]=42") print(" a =", a)

      • Another good option: use a list comprehension
        rows = 3 cols = 2 #This is what's called a "list comprehension" a = [ ([0] * cols) for row in range(rows) ] print("This IS ok. First:") print(" a =", a) a[0][0] = 42 print("And now see what happens after a[0][0]=42") print(" a =", a)

      • Best option: make2dList()
        def make2dList(rows, cols): return [ ([0] * cols) for row in range(rows) ] rows = 3 cols = 2 a = make2dList(rows, cols) print("This IS ok. At first:") print(" a =", a) a[0][0] = 42 print("And now see what happens after a[0][0]=42") print(" a =", a)

  2. Getting 2d List Dimensions
    # Create an "arbitrary" 2d List a = [ [ 2, 3, 5] , [ 1, 4, 7 ] ] print("a = ", a) # Now find its dimensions rows = len(a) cols = len(a[0]) print("rows =", rows) print("cols =", cols)

  3. Copying and Aliasing 2d Lists
    • Wrong: Cannot use copy.copy (shallow copy)
      import copy # Create a 2d list a = [ [ 1, 2, 3 ] , [ 4, 5, 6 ] ] # Try to copy it b = copy.copy(a) # Error: creates shallow copy # At first, things seem ok print("At first...") print(" a =", a) print(" b =", b) # Now modify a[0][0] a[0][0] = 9 print("But after a[0][0] = 9") print(" a =", a) print(" b =", b)

    • Right: use copy.deepcopy
      import copy # Create a 2d list a = [ [ 1, 2, 3 ] , [ 4, 5, 6 ] ] # Try to copy it b = copy.deepcopy(a) # Correct! # At first, things seem ok print("At first...") print(" a =", a) print(" b =", b) # Now modify a[0][0] a[0][0] = 9 print("And after a[0][0] = 9") print(" a =", a) print(" b =", b)

    • Limitations of copy.deepcopy
      a = [[0]*2]*3 # makes 3 shallow copies of (aliases of) the same row a[0][0] = 42 # appears to modify all 3 rows print(a) # prints [[42, 0], [42, 0], [42, 0]] # now do it again with a deepcopy import copy a = [[0]*2]*3 # makes 3 shallow copies of the same row a = copy.deepcopy(a) # meant to make each row distinct a[0][0] = 42 # so we hope this only modifies first row print(a) # STILL prints [[42, 0], [42, 0], [42, 0]] # deepcopy preserves any already-existing aliases perfectly! # best answer: don't create aliases in the first place, unless you want them.

    • Advanced: alias-breaking deepcopy
      # Advanced: now one more time with a simple deepcopy alternative that does # what we thought deepcopy did... # NOTE: this uses recursion. We'll go over how that works in the future. import copy def myDeepCopy(a): if (isinstance(a, list) or isinstance(a, tuple)): return [myDeepCopy(element) for element in a] else: return copy.copy(a) a = [[0]*2]*3 # makes 3 shallow copies of the same row a = myDeepCopy(a) # once again, meant to make each row distinct a[0][0] = 42 # so we hope this only modifies first row print(a) # finally, prints [[42, 0], [0, 0], [0, 0]] # now all the aliases are gone!

  4. Printing 2d Lists
    # Helper function for print2dList. # This finds the maximum length of the string # representation of any item in the 2d list def maxItemLength(a): maxLen = 0 rows = len(a) cols = len(a[0]) for row in range(rows): for col in range(cols): maxLen = max(maxLen, len(str(a[row][col]))) return maxLen # Because Python prints 2d lists on one row, # we might want to write our own function # that prints 2d lists a bit nicer. def print2dList(a): if (a == []): # So we don't crash accessing a[0] print([]) return rows = len(a) cols = len(a[0]) fieldWidth = maxItemLength(a) print("[ ", end="") for row in range(rows): if (row > 0): print("\n ", end="") print("[ ", end="") for col in range(cols): if (col > 0): print(", ", end="") # The next 2 lines print a[row][col] with the given fieldWidth formatSpec = "%" + str(fieldWidth) + "s" print(formatSpec % str(a[row][col]), end="") print(" ]", end="") print("]") # Let's give the new function a try! a = [ [ 1, 2, 3 ] , [ 4, 5, 67 ] ] print2dList(a)

  5. Nested Looping over 2d Lists
    # Create an "arbitrary" 2d List a = [ [ 2, 3, 5] , [ 1, 4, 7 ] ] print("Before: a =", a) # Now find its dimensions rows = len(a) cols = len(a[0]) # And now loop over every element # Here, we'll add one to each element, # just to make a change we can easily see for row in range(rows): for col in range(cols): # This code will be run rows*cols times, once for each # element in the 2d list a[row][col] += 1 # Finally, print the results print("After: a =", a)

  6. Accessing 2d Lists by Row or Column
    • Accessing a whole row
      # alias (not a copy! no new list created) a = [ [ 1, 2, 3 ] , [ 4, 5, 6 ] ] row = 1 rowList = a[row] print(rowList)

    • Accessing a whole column
      # copy (not an alias! new list created) a = [ [ 1, 2, 3 ] , [ 4, 5, 6 ] ] col = 1 colList = [ ] for i in range(len(a)): colList += [ a[i][col] ] print(colList)

    • Accessing a whole column with a list comprehension
      # still a copy, but cleaner with a list comprehension! a = [ [ 1, 2, 3 ] , [ 4, 5, 6 ] ] col = 1 colList = [ a[i][col] for i in range(len(a)) ] print(colList)

  7. Non-Rectangular ("Ragged") 2d Lists
    # 2d lists do not have to be rectangular a = [ [ 1, 2, 3 ] , [ 4, 5 ], [ 6 ], [ 7, 8, 9, 10 ] ] rows = len(a) for row in range(rows): cols = len(a[row]) # now cols depends on each row print("Row", row, "has", cols, "columns: ", end="") for col in range(cols): print(a[row][col], " ", end="") print()

  8. 3d Lists
    # 2d lists do not really exist in Python. # They are just lists that happen to contain other lists as elements. # And so this can be done for "3d lists", or even "4d" or higher-dimensional lists. # And these can also be non-rectangular, of course! a = [ [ [ 1, 2 ], [ 3, 4 ] ], [ [ 5, 6, 7 ], [ 8, 9 ] ], [ [ 10 ] ] ] for i in range(len(a)): for j in range(len(a[i])): for k in range(len(a[i][j])): print("a[%d][%d][%d] = %d" % (i, j, k, a[i][j][k]))