Infix to postfix conversion and evaluation including whitespaces and double quotes - c

Question

I have the code below, but I need the code to account for white spaces and double digits, for example if i enter (7-3)/(2+2) it should come out 73-22+/ result: 1. If i enter (7 - 3) / (2 + 2) it should come out 7 3 - 2 2 + / result 1. If I enter (22 - 10)/(2 + 2) it should come out 22 10 - 2 2 + / Result: 3
Here is the code I have:
#include<stdio.h>
char stack[100];
int top = 0;
int eval_top = -1;
int eval_stack[100];
void push(char x) // Push char into stack
{
stack[top++] = x;
}
char pop() // Pop char to top of stack
{
if (top == -1)
return -1;
else
return stack[top--];
}
/* functions for evaluation of postfix expression */
// push function
void eval_push(int x) { // Find push result
eval_stack[++eval_top] = x;
}
// pop function
int eval_pop() { // Find pop result
if (eval_top == -1) {
return -1;
} else {
return eval_stack[eval_top--];
}
}
int priority(char x) // check priority order
{
if (x == '(')
return 0;
if (x == '+' || x == '-')
return 1;
if (x == '*' || x == '/')
return 2;
}
// function to evaluate the postfix expression
void EvalPostfix(char postfix[]) {
int A, B;
int val;
char ch;
int i;
//find postfix
for (i = 0; postfix[i] != ')'; i++) {
ch = postfix[i];
if (isdigit(ch)) {
eval_push(ch - '0');
} else if (ch == '+' || ch == '-' || ch == '*' || ch == '/') {
A = eval_pop();
B = eval_pop();
switch (ch) {
case '*':
val = B * A;
break;
case '/':
val = B / A;
break;
case '+':
val = B + A;
break;
case '-':
val = B - A;
break;
}
eval_push(val); //send value on top of stack
}
}
printf("\n Result: %d \n", eval_pop());
}
main() {
int i = 0;
char * e, x;
char postfix[100]; // store postfix for later evaluation
char exp[100];
printf("Infix expression : ");
scanf("%s", exp); // asking the user to enter the infix expression
printf("Postfix expression: ");
e = exp;
while ( * e != '\0') {
if (isalnum( * e)) { // if character is alphabet or number , it is printed
printf("%c", * e);
postfix[i++] = * e;
} else if ( * e == '(') // if it is open parenthesis, it is pushed into the stack without any priority
push( * e);
else if ( * e == ')') // if it is closed parenthesis , pop the elements in the stack and print them until the we see ( symbol
{
while ((x = pop()) != '(') {
printf("%c", x);
postfix[i++] = x;
}
} else // if character is symbol like +, -, *, / then based on their priority character is pushed if it high priority otherwise high priority symbols are popped and it is pushed
{
while (priority(stack[top]) >= priority( * e)) {
x = pop();
printf("%c", x);
postfix[i++] = x;
}
push( * e);
}
e++;
}
while (top != -1) // printing remaining elements in the stack
{
x = pop();
printf("%c", x);
postfix[i++] = x;
}
postfix[i] = ')'; // this is to add at the end for detecting end by the evaluation function
EvalPostfix(postfix);
}

Answer 1

There are some problems in your code
your pop is not symmetric with your push, push post increment the index so pop must pre decrement the index, and because of that the first invalid index is not -1 but 0 :
char pop() // Pop char to top of stack
{
if (top == 0)
return -1;
else
return stack[--top];
}
priority does not return a value if all the tests are false, but probably the last test is useless
In
while (priority(stack[top]) >= priority( * e))
you missed to check if the stack is empty, must be :
while ((top != 0) && (priority(stack[top]) >= priority( * e))) {
Because the first invalid index for stack is 0 rather than -1
while (top != -1) // printing remaining elements in the stack
must be
while (top != 0) // printing remaining elements in the stack
When you make the postfix expression there is no separation between the numbers, for instance "12+3" becomes "123+" like "1+23", and in EvalPostfix you consider a number has only one digit (eval_push(ch - '0');), so you do not manage the numbers having more than 1 digit. To manage several digits add a separator after all numbers, for instance a space to have "12 3 +" or "1 23 +" and read the number with scanf etc
You do not make the right postfix expression in all the cases, for instance for 1+2*3 you make 12+3* but it must be 123*+
You do not detect the invalid infix expressions
in
while (priority(stack[top]) >= priority( * e))
I missed to say the top element is not stack[top] but stack[top - 1] so it must be replaced by
while ((top != 0) && (priority(stack[top - 1]) >= priority( * e))) {
adding that correction 1+2*3 produces the right postfix expression 123*+
Note it is more clear to introduce the function empty() and tops(), and in case of an invalid access into the stack print a message and exit rather than to return -1 as a char
int empty()
{
return (top == 0);
}
char tops()
{
if (top == 0) {
fputs("top() on the empty stack, abort", stderr);
exit(-1);
}
return stack[top - 1];
}
char pop() // Pop char to top of stack
{
if (top == 0) {
fputs("pop() on the empty stack, abort", stderr);
exit(-1);
}
return stack[--top];
}
also detect a possible overflow of the stack :
void push(char x) // Push char into stack
{
if (top == sizeof(stack)) {
fputs("stack overflow", stderr);
exit(-1);
}
stack[top++] = x;
}
so now you can do
while (!empty() && (priority(tops()) >= priority( * e))) {
Of course this is the same for the other stack
I need the code to account for white spaces and double digits
double digits is too restrictive, just manage any integer, for that you can extract the number using strtol. You cannot also read the full expression using scanf("%s", exp); because is stops on the first space, use fgets.