Updated the Variable object to reconize 'k' as an installment variable type. Updated the main program's interactive UI.

.gitignore

@@ -1,2 +1,3 @@
 __pycache__
 *.json
+*.txt

expression.py

@@ -19,7 +19,9 @@ class Variable:
 		"i" : "Interest",
 		"I" : "Interest",
 		"n" : "Time",
-		"N" : "Time"
+		"N" : "Time",
+		"k" : "Installment",
+		"K" : "Installment"
 	}
 	
 	def __init__(self, name, type, starting_point = 0, ending_point = 0):

main.py

@@ -8,9 +8,10 @@ import itertools
 import copy
 
 base_list = [1, 2, 4, 6, 12]
-tolerance = .28
+tolerance = .05
 
 variables = {}
+
 def print_vars(vars):
 	i = 0
 	for key in vars.keys():
@@ -74,10 +75,21 @@ def modify_variable(var, vars):
 			
 exp_str = input("Enter the expression the student will use: ")
 wrong_response = input("Enter the wrong response for the expression: ")
+
 try:
 	wrong_response = float(wrong_response)
 except ValueError:
-	print("The anwser must be numeric! Will not be included in output file.")
+	print("The wrong response must be numeric.")
+	exit()
+
+tolerance = input("Enter the tolerance to use: ")
+
+try:
+	tolerance = float(tolerance)
+except ValueError:
+	print("The tolerance must be numeric.")
+	exit()
+
 tokens = tokenizer.get_tokens_from_expression_string(exp_str)
 rev_tokens = copy.deepcopy(tokens)
 #rev_tokens.reverse()
@@ -88,23 +100,25 @@ for t in rev_tokens:
 
 selected_variable = None
 
-while True:
-	print("Variables in expression:")
-	print_vars(variables)
-	response = input("Enter a variable name to modify its properties or continue (default): ")
-	
-	if response == "continue" or response == "":
-		break
+if len(variables) > 0:
+
+	while True:
+		print("Variables in expression:")
+		print_vars(variables)
+		response = input("Enter a variable name to modify its properties or continue (default): ")
 		
-	if response in variables:
-		selected_variable = variables[response]
-	
-	if selected_variable != None:
-		modify_variable(selected_variable, variables)
+		if response == "continue" or response == "":
+			break
+			
+		if response in variables:
+			selected_variable = variables[response]
 		
-	else:
-		print("Invalid selection %s." %(response))
-		continue
+		if selected_variable != None:
+			modify_variable(selected_variable, variables)
+			
+		else:
+			print("Invalid selection %s." %(response))
+			continue
 
 exp = expression.Expression(exp_str, variables, wrong_response)
 #exp.variables