input: Write a class to perform various matrix operations like addition, subtraction, multiplication and transpose.
  class Matrix:
      def __init__(self, m, n, lst):
          self.m = m
          self.n = n
          self.mat = []
  
          c = 0
          for i in range(self.m):
              a =[]
              for j in range(self.n):
                      a.append(lst[c])
                      c += 1
              self.mat.append(a)
  
      def __str__(self):
          output = ""
          for i in self.mat:
              for j in i:
                  output += str(j) + " "
              output += '\n'
          return output
  
      def add(self, mat2):
          for i in range(self.m):
              for j in range(self.n):
                  self.mat[i][j] += mat2.mat[i][j]
  
      def sub(self, mat2):
          for i in range(self.m):
              for j in range(self.n):
                  self.mat[i][j] -= mat2.mat[i][j]
      
      def mul(self, mat2):
          result = []
          for i in range(self.m):
              a =[]
              for j in range(mat2.n):
                  s = 0
                  for k in range(self.n):
                      s += self.mat[i][k] * mat2.mat[k][j]
                  a.append(s)
              result.append(a)
          self.mat = result
          self.m = len(result)
          self.n = len(result[0])
  
      def transpose(self):
          result = [[self.mat[j][i] for j in range(len(self.mat))] for i in range(len(self.mat[0]))]
          self.mat = result
          self.m = len(result)
          self.n = len(result[0])
  
  assert str(Matrix(2, 2, [1, 2, 3, 4])) == "1 2 \n3 4 \n"
  assert str(Matrix(2, 2, [1, 2, 3, 4]).add(Matrix(2, 2, [1, 1, 1, 1]))) == "2 3 \n4 5 \n"
  assert str(Matrix(2, 2, [1, 2, 3, 4]).sub(Matrix(2, 2, [1, 1, 1, 1]))) == "0 1 \n2 3 \n"
  assert str(Matrix(2, 2, [1, 2, 3, 4]).mul(Matrix(2, 2, [1, 1, 1, 1]))) == "3 3 \n7 7 \n"
  assert str(Matrix(2, 2, [1, 2, 3, 4]).transpose()) == "1 3 \n2 4 \n"
output: def __init__(self, m, n, lst):
      self.m = m
      self.n = n
      self.mat = []
  
      c = 0
      for i in range(self.m):
          a =[]
          for j in range(self.n):
                  a.append(lst[c])
                  c += 1
          self.mat.append(a)
  
  def __str__(self):
      output = ""
      for i in self.mat:
          for j in i:
              output += str(j) + " "
          output += '\n'
      return output
  
  def add(self, mat2):
      for i in range(self.m):
          for j in range(self.n):
              self.mat[i][j] += mat2.mat[i][j]
  
  def sub(self, mat2):
      for i in range(self.m):
          for j in range(self.n):
              self.mat[i][j] -= mat2.mat[i][j]
      
  def mul(self, mat2):
      result = []
      for i in range(self.m):
          a =[]
          for j in range(mat2.n):
              s = 0
              for k in range(self.n):
                  s += self.mat[i][k] * mat2.mat[k][j]
              a.append(s)
          result.append(a)
      self.mat = result
      self.m = len(result)
      self.n = len(result[0])
  
  def transpose(self):
      result = [[self.mat[j][i] for j in range(len(self.mat))] for i in range(len(self.mat[0]))]
      self.mat = result
      self.m = len(result)
      self.n = len(result[0])