Wrong answer from a program (infix notation->postfix notation) in C - c

Question

I made a program which changes infix notation to postfix notation with using stack in C.
I made a function which prints the result of the postfix notation(from infix notaton).
But, the result of a notation is wrong. This should be '195' but its result is '-61'.
The results of the other notations are right but only the notation(op2) has this problem.
What should I do to fix this problem?
This is my code:
typedef char element;
typedef struct {
element stack[MAX_STACK_SIZE];
int top;
} StackType;
void init(StackType *s) {
s->top = -1;
}
int is_empty(StackType *s) {
return (s->top == -1);
}
int is_full(StackType *s) {
return (s->top == (MAX_STACK_SIZE - 1));
}
void push(StackType *s, element item) {
if (is_full(s)) {
fprintf(stderr, "FULL STACK ERROR\n");
return;
}
else s->stack[++(s->top)] = item;
}
element pop(StackType *s) {
if (is_empty(s)) {
fprintf(stderr, "EMPTY STACK ERROR\n");
exit(1);
}
else return s->stack[(s->top)--];
}
int eval(char exp[]) {
int op1, op2, value, i = 0;
int len = strlen(exp);
char ch;
StackType s;
init(&s);
for (i = 0; i < len; i++) {
ch = exp[i];
if (ch != '+' && ch != '-' && ch != '*' && ch != '/') {
value = ch - '0';
push(&s, value);
}
else {
op2 = pop(&s);
op1 = pop(&s);
switch (ch) {
case '+': push(&s, op1 + op2); break;
case '-': push(&s, op1 - op2); break;
case '*': push(&s, op1 * op2); break;
case '/': push(&s, op1 / op2); break;
}
}
}
return pop(&s);
}
char* infix_to_postfix(char exp[]) {
int i = 0, j = 0;
char ch, top_op;
int len = strlen(exp);
char *ex = (char *)malloc(sizeof(char)*(len + 1));
StackType s;
init(&s);
for (i = 0; i < len; i++) {
ch = exp[i];
switch (ch) {
case '+': case '-': case '*': case '/':
while (!is_empty(&s) && (prec(ch) <= prec(peek(&s)))) {
ex[j++] = pop(&s);
}
push(&s, ch);
break;
case '(':
push(&s, ch);
break;
case ')':
top_op = pop(&s);
while (top_op != '(') {
ex[j++] = top_op;
top_op = pop(&s);
}
break;
default:
ex[j++] = ch;
break;
}
}
while (!is_empty(&s)) {
ex[j++] = pop(&s);
}
ex[j] = NULL;
return ex;
}
void main() {
char *op1 = "(9-(3+2))*3+4*((4+2)/3)-1";
char *op2 = "(4*5-3)/3+((2+5*7)-8+9)*5";
char *op3 = "7*3-7-4+1/3+6-8*2";
char *pos1, *pos2, *pos3;
pos1 = infix_to_postfix(op1);
pos2 = infix_to_postfix(op2);
pos3 = infix_to_postfix(op3);
printf(" Result : %d\n", eval(pos1));
printf(" Result : %d\n", eval(pos2));
printf(" Result : %d\n", eval(pos3));
}
[RESULT]
Result : 19
Result : -61 // This should be '195'.
Result : 0

Answer 1

The clue is 61+195 = 256. Which means that somewhere, your computations are being saved to chars. Looks like it is element.
typedef element int;