Computer Science 15-110, Spring 2011
Class Notes:  Two-Dimensional Lists

1. Creating 2d Lists
   
2. Getting 2d List Dimensions
   
3. Nested Looping over 2d Lists
   
4. Printing 2d Lists
   

1. Creating 2d Lists
   
   
1. Static Allocation
   
   # create a 2d list with fixed values (static allocation)
   a = [ [ 2, 3, 4 ] , [ 5, 6, 7 ] ]
   print a
   
   
2. Dynamic (Variable-Length) Allocation
   
   
1. 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
   
   print "This SEEMS ok.  At first:"
   print "   a =", a
   
   a[0][0] = 42
   print "But see what happens after a[0]=42"
   print "   a =", a
   
   
2. 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.  At first:"
   print "   a =", a
   
   a[0][0] = 42
   print "And now see what happens after a[0]=42"
   print "   a =", a
   
   
3. Even Better:  make2dList() 
   
   def make2dList(rows, cols):
       a=[]
       for row in range(rows): a += [[0]*cols]
       return a
   
   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. 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
   
   
4. Printing 2d Lists
   
   # Helper function for print2dList.
   # This finds the maximum length of the strings
   # in each column of a 2d list
   def getFieldWidths(a):
       rows = len(a)
       cols = len(a[0])
       fieldWidths = [0] * cols
       for row in range(rows):
           for col in range(cols):
               field = str(a[row][col])
               fieldWidths[col] = max(fieldWidths[col], len(field))
       return fieldWidths
   
   # 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])
       fieldWidths = getFieldWidths(a)
       for row in range(rows):
           if (row == 0):
               line = "[ ["
           else:
               line = "  ["
           for col in range(cols):
               if (col > 0): line += ", "
               field = str(a[row][col])
               for space in range(fieldWidths[col] - len(field)):
                   line += " "
               line += field
           line += "]"
           if (row == (rows-1)): line += " ]"
           print line
   
   # Let's give the new function a try!
   a = [ [ 1, 2, 3.4 ] , [ "wahoo", 55, 6 ] ]
   print2dList(a)
   

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