What is wrong with my calculate function? All other functions work fine but when I try to calculate a post fix expression, it adds some weird symbols to my stack.

void push (char operand)
{       ++top;
        stack [top] = operand;
        printf("Stack: %s \n", stack);

}
int pop (char operand)
{
int ret;
    if (top == -1)
    {   ret = 0;
    }
    else
    {
        ret = stack [top];
--top;
    }
return ret;
}

int checkErrors(char array[10][100]){
    int l;
    int m;
    for(l=0; l<i;>    for(m=0; m<100; m++){
        if((array[l][m] != '\0')&&((array[l][m]=='$')|| (array[l][m]=='@'))){
            m++;
    printf("Equation %d is invalid\n", l);
    return 1;
        }
    }
    }

}

void postfix(char eqArray[100]){
    int j;
    int k=0;
    char op1;
    memset(&pf,'\0',100);
    memset(&stack,'\0',20);
    top=-1;
    int length=strlen(eqArray);
    //if(!checkErrors) &&&&&& add the paranthesis stuff
    for(j=0; j<length;>    {
        op1=eqArray[j];
        if(op1==')'){
           while(stack[top]!='(')
                    pop(stack[top]);
                    //pf[k]=stack[top];
                    //k++;
        }
        else if(eqArray[j]!='+'&&eqArray[j]!='-'&&eqArray[j]!='*'&&eqArray[j]!='/'&&eqArray[j]!=' '&&eqArray[j]!='('){
                pf[k]= eqArray[j];
                k++;
        }
        else if(top==-1)
        {
            push(op1);
        }
        else if (isPrior(op1,stack[top]))
        {
                pf[k]=stack[top];
                pop(stack[top]);
                push(op1);
                k++;
        }
        else
            push(op1);
    }
    while(top!=-1){
        pf[k]=stack[top];
        pop(stack[top]);
        k++;
    }

printf("Postfix : %s \n",  pf);
}

int isPrior(char op1, char op2)
{if((op1=='*')||(op1=='/'))
 return 1;
else if((op1=='+')||(op1=='-')||(op1=='('))
    return 0;
}

double calculate(char pf[100]){
    int q;
    char num;
    int answer;
    memset(&stack,'\0',20);
    top=-1;
    int length2=strlen(pf);
    printf("%s\n", pf);
    char op1,op2;
    //if(!checkErrors) && add the paranthesis stuff
    for(q=0; q<length2;>    {   num=pf[q];
        if(isdigit(pf[q]))
        {push(num);
        }
        else
            {
            op2=stack[top];
            pop(stack[top]);
            op1=stack[top];
            pop(stack[top]);
    switch(num){
        case '+':
            pop(stack[top]);
            push((op1+op2) - '0' );
            break;
        case '-':
            push((op1-op2) - '0');
            break;
        case '*':
            push((op1*op2) - '0');
            break;
        case '/':
            push((op1/op2) - '0');
            break;
            }
            }

    }
return 0;
}