Program 8 - Infix to Postfix Conversion

	#include <stdio.h>
	#include <ctype.h>
	char stack[100];
	int top = -1;
	int priority(char ch){
	if(ch == '*' || ch == '/'){
	return 2;
	} else if (ch == '+' || ch == '-'){
	return 1;
	} else {
	//ch = '('
	return 0;
	}
	}
	void infix_to_postfix(char infix[], char postfix[]){
	int i, j = 0;
	char ch;
	for(i = 0; infix[i] != '\0'; i++){
	ch = infix[i];
	//If scanned character is operand put it in postfix
	if(isdigit(ch) || isalpha(ch)){
	postfix[j++] = ch;
	} else if(ch == '('){
	// If scanned char is ( push it in stack
	stack[++top] = ch;
	} else if (ch == ')'){
	// If scanned char is ) pop everthing till we get ( and discard that (
	while(top != -1 && stack[top] != '('){
	postfix[j++] = stack[top--];
	}
	// Now discard the (
	if(top == -1){
	printf("Invalid expression\n");
	exit(0);
	} else {
	ch = stack[top--];
	}
	} else if(ch == '*' || ch == '/' || ch == '+' || ch == '-'){
	/* If scanned char is operator and stack is empty then push into stack
	* unless the char on top has higher or equal priority
	* than the scanned char then go on popping
	* till we get an operator which has less priority than ch
	* then push the scanned operator on the stack
	*/
	if(top == -1 || priority(ch) > priority(stack[top])){
	stack[++top] = ch;
	} else {
	while(priority(stack[top]) >= priority(ch) && top != -1){
	postfix[j++] = stack[top--];
	}
	stack[++top] = ch;
	}
	} else {
	printf("Invalid character\n");
	exit(0);
	}
	}
	//Now that the infix expression is totally scanned we empty the stack
	while(top != -1 && stack[top] != '('){
	postfix[j++] = stack[top--];
	}
	postfix[j++] = '\0';
	}
	int main(){
	char infix[40], postfix[40];
	printf("Enter infix expression: \n");
	gets(infix);
	infix_to_postfix(infix, postfix);
	printf("The postfix equivalent is: \n");
	printf("%s", postfix);
	}