### Evaluate a Postfix and Prefix equation program in c

In this tutorial post, we are going to write to evaluate a postfix and prefix equation program in c programming.

``````#define MAXSTK 10
struct stack
{
int top;
int data[MAXSTK];
};

void push(struct stack *, int );
int pop(struct stack *);
int isoperator(char);
int evalpost(char []);
int evalpre(char []);
int eval(char, int, int);

void main()
{
char post[50],pre[50];
int ans;
clrscr();
printf("Enter string in postfix form");
gets(post);
ans = evalpost(post);
printf("Result of post eq is %d\n", ans);
printf("Enter string in prefix form");
gets(pre);
ans = evalpre(pre);
printf("Result of pre eq is %d\n", ans);
getch();
}

void push(struct stack *p, int item)
{
if(p -> top == MAXSTK-1)
{
printf("Overflow");
exit(0);
}
p->top++;
p->data[p->top] = item;
}

int pop(struct stack *p)
{
int item;
if(p-> top == -1)
{
printf("Underflow");
exit(0);
}
item = p->data[p->top];
p->top--;
return(item);
}

int isoperator(char ch)
{
if(ch == '+' || ch == '-' || ch == '*' || ch == '/' || ch == '%')
return(1);
else
return(0);
}

int eval(char op, int op1, int op2)
{
switch(op)
{
case '+': return(op1+op2);
case '-': return(op1-op2);
case '*': return(op1*op2);
case '/': return(op1/op2);
default: return(0);
}
}

int evalpost(char post[])
{
struct stack s1;
int i,a,b,ans;
s1.top = -1;
i=0;

while(post[i] != '\0')
{
if(isoperator(post[i]))
{
b = pop(&s1);
a = pop(&s1);
ans = eval(post[i],a,b);
push(&s1,ans);
}
else
push(&s1,post[i] - '0');
i++;
}
ans = pop(&s1);
return(ans);
}

int evalpre(char pre[])
{
struct stack s1;
int i,a,b,ans;
s1.top = -1;
i=0;

while(pre[i] != '\0')
i++;
i--;

while(i >= 0)
{
if(isoperator(pre[i]))
{
a = pop(&s1);
b = pop(&s1);
ans = eval(pre[i],a,b);
push(&s1,ans);
}
else
push(&s1,pre[i] - '0');
i--;
}
ans = pop(&s1);
return(ans);
}``````