Infix to Postfix conversion and evaluation

#include <stdio.h>
char stack[100];
int top = -1;
char ch;
void push(char elem){
    stack[++top] = elem;
}
char pop(){
   return (stack[top--]);
}
void evalutation(char postfix[50]){
    int j=0, operand1, operand2;
    while((ch=postfix[j++]) != '\0'){//jth character of postfix string will be stored in ch
        if(ch >= '0' && ch <= '9') push(ch - '0');//if ch is a number, number will be pushed on top of stack
        else{//if ch is an operator
            operand2=pop();//2 operands will be popped
            operand1=pop();
            switch(ch){//the operation will be done and pushed on top of the stack
                case '+':   push(operand1 + operand2); break;
                case '-':   push(operand1 - operand2); break;
                case '*':   push(operand1 * operand2); break;
                case '/':   push(operand1 / operand2); break;
            }
        }
    }
}
int precidence(char elem){
    switch (elem){ // | ( | < | +,- | < | *,/ |
        case '(':   return 1;
        case '+':
        case '-':   return 2;
        case '*':
        case '/':   return 3;
        default:    return 0;
    }
}
void convert(char infix[50], char postfix[50]){
    int i = 0, j = 0; //i and j will be indexes for the infix & postfix strings
    while ((ch = infix[i++]) != '\0'){//ith character will be stored in ch
        if (ch == '(')  push(ch);//if ch is (, it will be on top of the stack
        else if (ch >= '0' && ch <= '9')   postfix[j++] = ch;//if ch is 0-9, add to postfix string
        else if (ch == ')'){//if ch is ),
            while (stack[top] != '(')//Until stack's top is not (,
                postfix[j++] = pop();//pop elements from stack, add to postfix string
            pop();//pop the (
        } else{// if ch is an operator
            while (precidence(stack[top]) >= precidence(ch))//if top's character's precidence is >= ch's,
                postfix[j++] = pop();//pop the character on top of the stack and add it to the postfix string
            push(ch);//push operator on top of the stack
        }
    }
    while (top != -1)   postfix[j++] = pop();//Until the stack doesn't become emty, pop the top element and add to postfix string
    postfix[j] = '\0';//add null character at the end of postfix string to indicate end of string
}
int main(){
    char infix[50], postfix[50];//infix and postfix strings created
    printf("\nInfix Expression:\t");
    gets(infix);//infix string is inputted
    convert(infix, postfix);//call for converting infix string to postfix
    printf("\nPostfix Expression:\t%s\n", postfix);//print the postfix string
    evalutation(postfix);//call for evaluating postfix string
    printf("\nPostfix Evaluation:\t%d\n",stack[top]);//the value on top will be the answer
    return 0;
}
//Output:-
//>C:\programs\"in2post"

//Infix Expression:       7-(2*3+5)*(8-4/2)

//Postfix Expression:     723*5+842/-*-

//Postfix Evaluation:     -59