explain with the code, thank you! initial code has been provided

(25 points) Fill in the implementation of the evaluateExpressionO function in Calcul ator .h in a way that it can receive any expression in prex. inr, or postr and evaluates and returns its result. The function should recognize and use the +, -, *, / and " operators. as well as an average operator @ which receives three arguments and returns their average (take average as the lowest in priority). and evaluate the expression when = is encountered. It should require spaces between all operators and =. You can safely assume that all operands are single digit numbers from [7) to 9. For simplicity we assume the appearance of the average operator (G!) is invalid in inx expressions. You may add as many as functions to Calculator . h as you need to support an easier implementation for evaluateExpressionO. as well, you may implement validateExpressionO to be able to skip the invalid expressions and not going through with the processing of the invalid expressions