//using upper case letters for variables

double var_arr[26];   //26 letters;

case '=':getvar(var_arr);break;

//getvar assign number to variable
void getvar(double arr[]){ 
   int c;
   
    switch(c=getchar()){
        case 'A'...'Z':scanf("%lg",&arr[c-'A']);break;  
        default:printf("error: %c unknown variable\n",c);
    }
}

case 'A'...'Z':push(var_arr[type-'A']);break; 


now we can print:
=A 50  =B 60
A B + '\n'

       110