discovered that isvalidstr was not working properly when i entered 3? which should end with a print error. corrected it now
added minus sign to isvalidstr so minus numbers can be entered without having an error
initial value in calculator wasn't restarting when an error was occuring, have now corrected this
FractionCalculator completed and seems to be working well in all inputs i have given. have made it so operator is not restarted for each new line so user can input 4 newline + newline 3 and receive 7