Lecture Notes on 7 Nov 2012

# In all these matrix operations we would like to
# keep the original matrix intact

# reverse the rows of a matrix
def reverse_rows (b):
  c = []
  for i in range (len(b)):
    a = b[i][:]
    a.reverse()
    c.append(a)
  return c

# reverse the columns of a matrix
def reverse_cols (b):
  c = []
  for i in range (len(b) - 1, -1, -1):
    c.append (b[i])
  return c

# sum the rows of a matrix
def sum_rows (b):
  for i in range (len(b)):
    print (sum(b[i]))

# sum the columns of a matrix
def sum_cols (b):
  num_cols = len (b[0])
  for j in range (num_cols):
    sum = 0
    for i in range (len (b)):
      sum = sum + b[i][j]
    print (sum)

# sum the diagonals of a matrix
def sum_diagonals (b):
  # sums diagonal from upper left to lower right
  sum = 0
  for i in range (len(b)):
    sum = sum + b[i][i]
  print (sum)

# switches the rows for the columns in a matrix
def transpose (b):
  c = []
  num_cols = len (b[0])
  for j in range (num_cols):
    a = []
    for i in range (len(b)):
      a.append (b[i][j])
    c.append(a)
  return c

main():
  b = [[8, 2, 3], [5, 6, 1], [7, 4, 9]]
  c = reverse_rows (b)
  print (c)

  c = reverse_cols (b)
  print (c)

main()