convert infix to postfix

This is a discussion on convert infix to postfix within the C Programming forums, part of the General Programming Boards category; Hi everyone.I got problem about my homework which contains converting to infix to postfix. For example : if the user ...

  1. #1
    Registered User
    Join Date
    May 2010
    Posts
    1

    Unhappy convert infix to postfix

    Hi everyone.I got problem about my homework which contains converting to infix to postfix.
    For example : if the user enters 5*6+8/4 as infix, its postfix will be 57*84/+
    Here is my code bu it doesn't work and I couldn't understand where my fault is.

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <ctype.h>
    struct yapi{
           char data;
           struct yapi *next;
           };
    typedef struct yapi *stack;
    void push(stack ptr,char x);
    char pop(stack ptr);
    int priority(char x) ;
    void infixtopostfix(char *infix,char *postfix,stack ptr);
    int main()
           { 
                            char in[50],post[50];
                            stack header;
                            header=(stack)malloc(sizeof(struct yapi));
                            header->next=NULL;
                            printf("Enter infix expression\n");
                            scanf("%s",in);
                            infixtopostfix(&in[0],&post[0],header);
                            printf("Postfix expression is %s\n",post);
                            system("pause");
                            return 0;
                            }
    void push(stack ptr,char x){ 
                stack newnode;
                newnode=(stack)malloc(sizeof(struct yapi));
                newnode->next=ptr->next;
                newnode->data=x;
                ptr->next=newnode;
                }                                  
    char pop(stack ptr){ 
         stack current; 
         char value;    
         ptr->next=current;
         value=current->data;
         ptr->next=ptr->next->next;
         free(current);
         return value;
         }                        
    int priority(char x){
         int pri=0;
         if(x=='+' || x=='-'){
                      pri=1;
                          }  
         if(x=='*' || x=='/'){
                  pri=2;
                  }   
                  return pri;
                  }                     
    void infixtopostfix(char *infix,char *postfix,stack ptr){
                 char *i,*p;
                 char character;
                 i=&infix[0];
                 p=&postfix[0];
                 while(*i!='\0')
                 {
                       if(isdigit(*i) || isalpha(*i)){
                                      *p=*i;
                                       p++;
                                       i++;
                                                      }
                       if(*i=='+' || *i=='-' || *i=='*' || *i=='/')
                       {
                                  if(ptr->next==NULL)
                                          {
                                          push(ptr,*i);
                                          }
                            else 
                            {
                       character=pop(ptr);
                       if(priority(character)>=priority(*i)) 
                       {
                       *p=character;
                       p++;
                       push(ptr,*i);
                       }
                              else{
                                     push(ptr,character); 
                                     push(ptr,*i);    
                                  }                                        
                            }   
                         i++;                 
                       }
                 }
                 while(ptr->next!=NULL){
                                       *p=pop(ptr);
                                       p++;
                                       }
                                       *p='\0';
                                        
                 }

  2. #2
    and the Hat of Guessing tabstop's Avatar
    Join Date
    Nov 2007
    Posts
    14,185
    Your compiler is giving you a big hint:
    Code:
    $ gcc -o i2p i2p.c -Wall -Wextra
    i2p.c: In function ‘pop’:
    i2p.c:36: warning: ‘current’ is used uninitialized in this function

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