stacks listing function for validating +source file implementation of the algorithm for validating balanced brackets in +source file from lliststack import stack def isvalidsourcesrcfile ) stack(for line in srcfile for token in line if token in "{[( pushtoken elif token in "}])if isempty(return false else left pop(if (token ="}and left !"{"or (token ="]and left !"["or (token =")and left !"("return false return isempty(the characters that correspond to one of the three types of delimiter pairs all other characters can be ignored when an opening delimiter is encounteredwe push it onto the stack when closing delimiter occurswe first check to make sure the stack is not empty if it is emptythen the delimiters are not properly paired and balanced and no further processing is needed we terminate the function and return false when the stack is not emptythe top item is popped and compared to the closing delimiter the two delimiters do match corresponding opening and closing delimiterswe again terminate the function and return false finallyafter the entire file is processedthe stack should be empty when the delimiters are properly paired and balanced for the final testwe check to make sure the stack is empty and return either true or falseaccordingly evaluating postfix expressions we work with mathematical expressions on regular basis and they are rather easy for humans to evaluate but the task is more difficult in computer program when an expression is represented as string given the expression we know will be performed firstfollowed by the division and concluding with addition when evaluating this expression stored as string and scanning one character at time from left to righthow do we know the addition has to wait until after the divisionyour first response is probably that we know the order of the precedence for the operators but how do we represent that in our string