Basic calculator using Lex and Yacc

Basic calculator using Lex and Yacc

Problem Statement

Develop a simple calculator with the following functionalities:

• Addition
• Subtraction
• Multiplication
• Division
• Prefix notation
• Postfix notation
• Variables

Code walk-through

• The code is divided into 2 parts
  • Lex file
  • Yacc file
• Lex file contains the regular expressions for the tokens
• Yacc file contains the grammar rules and the actions to be performed on the tokens
• Sample regex in Lex file

  # to identify input as a variable
  t_ID = r'[a-zA-Z_][a-zA-Z0-9_]*'
  # identify decimal form string and convert the input to float
  def t_FLOAT(t):
      r'\d+\.\d+'
      t.value = float(t.value)
  return t

• Sample grammar rule in Yacc file

  # E -> E + T ==> Recursion at E
  def p_expression_plus(p):
      '''expression : expression PLUS term'''
      p[0] = p[1] + p[3]

How to run

py yacc2.py

Sample output

Input> 3+2
5
Input> prefix(9+8-9)
+9-89
Input> postfix(9+8-9)
98+9-
Input> 25/5
5.0
Input> 8*9
72
Input> 5*(9+8)
85
Input> x=5
Input> y=6
Input> x+y
11
Input> x+y+z
invalid input 'z'
Input>