Evaluate a postfix expression using a stack and array in C -

i still new , not quick on picking coding c. assignment have evaluate postfix expression array using stack. while sure have several problems code feel basic structure good. compiles without errors, have warnings. run , printout printf statements in main not in evaluation far can tell. not looking cheat or complete fix guidance appreciated please respond assuming know little.

cheers

#include <stdio.h> #include <stdlib.h>  struct stacknode {     int data;     struct stacknode *nextptr; }; typedef struct stacknode stacknode; typedef stacknode *stacknodeptr;  int evaluatepostfixexpression( char *expr ); int calculate( int op1, int op2, char operator ); void push( stacknodeptr *topptr, int value ); int pop( stacknodeptr *topptr ); int isempty( stacknodeptr topptr ); void printstack( stacknodeptr topptr );  char postfix[50]; int answer;    void main() {     printf("print postfix expression\n");     scanf("%s", postfix);     evaluatepostfixexpression(postfix);     printf("the value of expression is: %i",answer); }  int evaluatepostfixexpression( char *expr )     //evaluate postfix expression. {      stacknode node;     stacknodeptr ptrnode;     ptrnode = &node;      int x;     int y;     int z;      strcat(ptrnode,'\0');/*append null character ('\0') end of postfix expression.     when null character encountered, no further processing necessary.*/      int i=0;     for(i; postfix[i]!='\0'; i++){      //while '\0' has not been encountered, read expression left right.         if(isdigit(postfix[i])){           //if current character digit,push integer value onto stack             push(&ptrnode, postfix[i]); //(the integer value of digit character value in computer’s character             printstack(ptrnode);                           //set minus value of '0' in computer’s character set).         }         else if(postfix[i]=='+'||postfix[i]=='-'||postfix[i]=='*'||postfix[i]=='/'||postfix[i]=='^'){                   //otherwise, if current character operator, pop 2 top elements of stack             x=pop(&ptrnode);    //variables x , y. calculate y operator x.             printstack(ptrnode);             y=pop(&ptrnode);             printstack(ptrnode);             z=calculate(x,y,postfix[i]);             push(&ptrnode, z);          /*push result of calculation onto stack.*/             printstack(ptrnode);         }         if (postfix[i]=='\0'){  //when null character encountered in expression, pop top value of             answer = pop(&ptrnode);//stack. result of postfix expression.             printstack(ptrnode);         }     } }  int calculate( int op1, int op2, char operator )     //evaluate expression op1 operator op2. {     if (operator=='+')         return op1+op2;      else if (operator=='-')         return op1-op2;      else if (operator=='*')         return op1*op2;      else if (operator=='/')         return op1/op2;      else if (operator=='^')         return op1^op2;        else{     return printf("calculation error");     } }  void push( stacknodeptr *topptr, int value )     //push value on stack. {     stacknodeptr temp; /* create new node */     temp =  malloc(sizeof(value));//need malloc because not remember     temp->data = value;     temp->nextptr = null; /* new node points null */       if(isempty(*topptr)==0) {     temp->nextptr = *topptr;     }  }  int pop( stacknodeptr *topptr )     //pop value off stack. {     char data ; /* used store data */     stacknodeptr tmp; /* used handling node*/                         /* going deleted */     tmp = *topptr; /* tmp has address of node */                         /* going deleted */     data = tmp->data;     *topptr = tmp->nextptr;      free(tmp);     return data; }  int isempty( stacknodeptr topptr )     //determine if stack empty. {     if(topptr->nextptr==null)         return 1;     else         return 0; }  void printstack( stacknodeptr topptr )     //print stack. {     while ((topptr->nextptr)!=null){         printf("%c",topptr->data);     }     if ((topptr->nextptr)==null)         printf("null");      printstack(topptr->nextptr); } 

one thing see was:

if(postfix[i]=="%i"){  //if current character digit,push integer value onto stack 

this incorrect. comparing character @ index address of string literal. should using function isdigit() instead.

also

else if(postfix[i]==('+')|('-')|('*')|('/')|('^')){  

should be:

else if(postfix[i]=='+' || postfix[i]=='-' || .....