CMU 15-110: Principles of Computing
2d Lists

1. Creating 2d Lists
2. Getting 2d List Dimensions
3. Nested Looping over 2d Lists
4. Printing 2d Lists
5. Copying 2d Lists
6. Reading CSV Files

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 # 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)
      
      
      2. Right: make2dList()
         def make2dList(rows, cols, defaultValue=None): a=[] for row in range(rows): a.append([defaultValue]*cols) return a rows = 3 cols = 2 a = make2dList(rows, cols, 0) print("This still is ok. At first:") print(" a =", a) a[0][0] = 42 print("But now see what happens after a[0][0]=42") print(" a =", a)
   
   
   3. 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)
   
   
   4. 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)
   
   
   5. Printing 2d Lists
      def print2dList(a): # For a nicer but more complex version, see http://goo.gl/eiZ42n print('[') for row in range(len(a)): print(' ', a[row]) print(']') # Let's give the new function a try! a = [ [ 1, 2, 3 ] , [ 4, 5, 67 ] ] print2dList(a)
   
   
   6. Copying 2d Lists
      import copy print('Ways that do not work:') a = [ [1], [2] ] b = a + [ ] c = copy.copy(a) a[0][0] = 42 print(b) # Fail! print(c) # Fail! print() print('Must use copy.deepcopy for 2d lists:') a = [ [1], [2] ] b = copy.deepcopy(a) a[0][0] = 42 print(b) # Success!
   
   
   7. Reading CSV Files
      # Note: As this requires read-write access to your hard drive, # this will not run in the browser in Brython. def readFile(path): # This makes a very modest attempt to deal with unicode if present with open(path, 'rt', encoding='ascii', errors='surrogateescape') as f: return f.read() def readCsvFile(path): # Returns a 2d list with the data in the given csv file result = [ ] for line in readFile(path).splitlines(): result.append(line.split(',')) return result def readSampleCsvFile(): data = readCsvFile('sample.csv') # download sample.csv first! print('Reading sample.csv') rows = len(data) cols = len(data[0]) print('In sample.csv, rows =', rows, ' and cols=', cols) print('2d list of data:', data) readSampleCsvFile()

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