Added additional error checking, to check for invalid expressions

calculator.py

@@ -1,3 +1,41 @@
+"""
+===============================================================================
+ENGR 13300 Fall 2023
+
+Program Description
+	An algebraic expression calculator. This program can evaluate expressions, assign values to variables, and use variables in additional expressions.
+
+	The program consists of three main parts:
+	1. Parsing the expression. This is accomplished using the 'parse' function, defined in parse.py. The expression is parsed, and a list of 'tokens' is produced, with each token bein a part of the expression. For example '5 + 7 * 3' is parsed into '['5', '+', '7', '*', '3'].
+	2. Find the inner-most expression. This only applies to expressions that have parantheses. The 'find_inner' function is called recursively, until the inner expression is found. For example, given the expression '3 + (7 * (4 + 8))', the 'find_inner' function is called once, to produce '7 * (4 + 8)', and then called again, to produce '4 + 8'.
+	3. Evaluate the expression. The 'evaluate' function searches for operators, and then evaluates the operator with the preceding and succeeding token (if one of the tokens is a variable, the value of the variable replaces the variable). The result of the evaluation replaces the expression. Example: Given the expression '3 + (7 * (4 + 8))', the inner expression is evaluated first. This produces '3 + (7 * 12)'. The inner expression is evaluated again, producing '3 + 84'. Finally, the rest of the expression is evaluated, to produce 87.
+	4. If the expression consists of an assignment, the evaluated expression is stored into the variable on the left. The list of variables is maintained as a dictionary.
+
+Assignment Information
+    Assignment:     Individual Project
+    Author:         Aadhavan Srinivasan, srini193@purdue.edu
+    Team ID:        LC3 - 19
+
+
+Contributor:    Name, login@purdue [repeat for each]
+    My contributor(s) helped me:
+    [ ] understand the assignment expectations without
+        telling me how they will approach it.
+    [ ] understand different ways to think about a solution
+        without helping me plan my solution.
+    [ ] think through the meaning of a specific error or
+        bug present in my code without looking at my code.
+    Note that if you helped somebody else with their code, you
+    have to list that person as a contributor here as well.
+    
+ACADEMIC INTEGRITY STATEMENT
+I have not used source code obtained from any other unauthorized
+source, either modified or unmodified. Neither have I provided
+access to my code to another. The project I am submitting
+is my own original work.
+===============================================================================
+"""
+
 import re
 from parse import *
 
@@ -28,8 +66,14 @@ def print_error(error_code):
 			print("One of your values is improperly formatted.")
 		case 5:
 			print("Uninitialized variable.")
+		case 6:
+			print("Invalid expression.")
+
+	return
 	
 def check_errors(expr):
+
+# Check for errors before parsing the expression
 	expr_small = expr.replace(" ", "") # Remove spaces from the string, to make it easier to parse
 
 	# Check if number of opening parantheses is equal to number of closing parantheses
@@ -56,12 +100,18 @@ def check_errors(expr):
 
 
 
+# Check for errors after parsing the expression
 	expr = parse(expr)
+
+	if expr[0].isalpha() and (len(expr) == 1 or expr[1] != '='): # If you just have an expression with a letter eg. 'x', or you use a variable without an assignment (e.g. 'x 5')
+		return 6
+
 	for val in expr:
 		if val.count('.') > 1:  # A value can have at most 1 period
 			return 4
 		if val.isalpha() and not val in variables and val != expr[0]: # If the token is a string, and isn't in the dictionary, and isn't the variable at index 0 (e.g. 'x' in 'x = 4')
 			return 5
+
 	return 0