C++ program for expression conversion as infix to postfix

Problem Statement:-

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Implement C++ program for expression conversion as infix to postfix and its evaluation using stack based on given conditions i. Operands and operator,both must be single character. ii. Input Postfix expression must be in a desired format. iii. Only '+', '-', '*' and '/ ' operators are expected.

Code:-

	#include<iostream>
	#include<cctype>
	#include<stack>
	using namespace std;
	// returns the value when a specific operator
	// operates on two operands
	int eval(int op1, int op2, char operate) {
	switch (operate) {
	case '*': return op2 * op1;
	case '/': return op2 / op1;
	case '+': return op2 + op1;
	case '-': return op2 - op1;
	default : return 0;
	}
	}
	int getWeight(char ch) {
	switch (ch) {
	case '/':
	case '*': return 2;
	case '+':
	case '-': return 1;
	default : return 0;
	}
	}
	// evaluates the postfix operation
	// this module neither supports multiple digit integers
	// nor looks for valid expression
	// However it can be easily modified and some additional
	// code can be added to overcome the above mentioned limitations
	// it's a simple function which implements the basic logic to
	// evaluate postfix operations using stack
	int evalPostfix(char postfix[], int size) {
	stack<int> s;
	int i = 0;
	char ch;
	int val;
	while (i < size) {
	ch = postfix[i];
	if (isdigit(ch)) {
	// we saw an operand
	// push the digit onto stack
	s.push(ch-'0');
	}
	else {
	// we saw an operator
	// pop off the top two operands from the
	// stack and evalute them using the current
	// operator
	int op1 = s.top();
	s.pop();
	int op2 = s.top();
	s.pop();
	val = eval(op1, op2, ch);
	// push the value obtained after evaluating
	// onto the stack
	s.push(val);
	}
	i++;
	}
	return val;
	}
	// main
	int main() {
	char postfix[] = {'a','b','c','+','*','d','/'};
	int i=0;
	// cout<<"Postfix 0"<<postfix[i];
	while(postfix[i]!='\0')
	{
	if(getWeight(postfix[i])==0)
	{
	cout<<" Enter value for "<<postfix[i]<<" = ";
	cin>>postfix[i];
	}
	i++;
	}
	int size = sizeof(postfix);
	int val = evalPostfix(postfix, size);
	cout<<"\nExpression evaluates to "<<val;
	cout<<endl;
	return 0;
	}

view raw Evaluation.cpp hosted with ❤ by GitHub

	#include<iostream>
	#include<cstring>
	#include<stack>
	using namespace std;
	// get weight of operators as per precedence
	// higher weight given to operators with higher precedence
	// for non operators, return 0
	int getWeight(char ch) {
	switch (ch) {
	case '/':
	case '*': return 2;
	case '+':
	case '-': return 1;
	default : return 0;
	}
	}
	// convert infix expression to postfix using a stack
	void infix2postfix(char infix[], char postfix[], int size) {
	stack<char> s;
	int weight;
	int i = 0;
	int k = 0;
	char ch;
	// iterate over the infix expression
	while (i < size) {
	ch = infix[i];
	if (ch == '(') {
	// simply push the opening parenthesis
	s.push(ch);
	i++;
	continue;
	}
	if (ch == ')') {
	// if we see a closing parenthesis,
	// pop of all the elements and append it to
	// the postfix expression till we encounter
	// a opening parenthesis
	while (!s.empty() && s.top() != '(') {
	postfix[k++] = s.top();
	s.pop();
	}
	// pop off the opening parenthesis also
	if (!s.empty()) {
	s.pop();
	}
	i++;
	continue;
	}
	weight = getWeight(ch);
	if (weight == 0) {
	// we saw an operand
	// simply append it to postfix expression
	postfix[k++] = ch;
	}
	else {
	// we saw an operator
	if (s.empty()) {
	// simply push the operator onto stack if
	// stack is empty
	s.push(ch);
	}
	else {
	// pop of all the operators from the stack and
	// append it to the postfix expression till we
	// see an operator with a lower precedence that
	// the current operator
	while (!s.empty() && s.top() != '(' &&
	weight <= getWeight(s.top())) {
	postfix[k++] = s.top();
	s.pop();
	}
	// push the current operator onto stack
	s.push(ch);
	}
	}
	i++;
	}
	// pop of the remaining operators present in the stack
	// and append it to postfix expression
	while (!s.empty()) {
	postfix[k++] = s.top();
	s.pop();
	}
	postfix[k] = 0; // null terminate the postfix expression
	}
	// main
	int main() {
	char infix[100];//"A*(B+C)/D";
	cout<<"\nENter Infix Operation:";
	cin>>infix;
	int size = strlen(infix);
	char postfix[size];
	infix2postfix(infix,postfix,size);
	cout<<"\nInfix Expression :: "<<infix;
	cout<<"\nPostfix Expression :: "<<postfix;
	cout<<endl;
	return 0;
	}

view raw InfixToPostfix.cpp hosted with ❤ by GitHub

Output:-