Thread: Infix to postfix problem..

Hi all, this is my fist post here and I am a beginner at C programming.
I have the following code for converting a list of t infix expressions into their postfix form:-

Code:

#include<stdio.h>
#include<string.h>
#include<ctype.h>
#define MAX 400
struct stack
{
    char arr[MAX];
    int top;
};
void initstack(struct stack *);
void push(struct stack *,char);
char pop(struct stack *);
void convert(struct stack *s,char *);
int priority(char e);
int main()
{
   int t,k=0;
   char str[MAX];
   struct stack s1;
   scanf("%d",&t);          //No of infix expressions to be converted in postfix//
   initstack(&s1);
   while(t)
   {
       t--;
       scanf("%s",str);
       convert(&s1,str);
   }
   return 0;
}
void initstack(struct stack *s)
{
    s->top=-1;
}
void push(struct stack *s,char data)
{
    if(s->top==MAX-1)
     return;
    else
    {
        s->top++;
        s->arr[s->top]=data;
    }
}
char pop(struct stack *s)
{
    char a;
    if(s->top==-1)
     return -1;
    else
    {
        a=s->arr[s->top];
        s->top--;
        return a;
    }
}
void convert(struct stack *s,char *exp)
{
    char op[400],d,p1;
    int i=0,j=0,pr;
    while(*(exp))
    {
        if(*(exp)=='(')
         {
          push(s,*(exp));
          exp++;
         }
        else if(*(exp)=='+'||*(exp)=='-'||*(exp)=='^'||*(exp)=='/'||*(exp)=='*')
         {
           if(s->top==-1)
            {
                push(s,*(exp));
                exp++;
            }
            else
            {
                p1=pop(s);
                if(priority(p1)<priority(*(exp)))
                {
                    push(s,p1);
                    push(s,*(exp));
                    exp++;
                }
                else
                {
                    while(priority(p1)>=priority(*(exp)))
                    {
                      op[i]=p1;
                      i++;
                      p1=pop(s);
                    }
                    push(s,p1);
                    push(s,*(exp));
                    exp++;
                }
            }
          }
          else if(*(exp)==')')
          {
              p1=pop(s);
              while((p1)!='(');
              {
                  op[i]=p1;
                  i++;
                  p1=pop(s);
              }
              exp++;
           }
           else if(isalpha(*(exp)))
           {
               while(isalpha(*(exp)))
               {
               op[i]=*(exp);
               exp++;
               i++;
               }
            }
        }
        while(s->top!=-1)
        {
            d=pop(s);
            op[i]=d;
            i++;
        }
        op[i]='\0';
        while(op[j])
        printf("%c",op[j++]);
        printf("\n");
}
int priority(char e)
{
    if(e=='^')
     return 3;
    else if(e=='*'||e=='/')
     return 2;
    else if(e=='+'||e=='-')
     return 1;
    else
     return 0;
}

I am getting correct outputs for non-bracketed inputs.But for bracketed
inputs I don't get any output. I have checked the code many times but
couldn't find as to why there is no output for bracketed inputs.Please
help

I am reposting the code again. Here it is without line numbers:-

Code:

#include<stdio.h>
#include<string.h>
#include<ctype.h>

#define MAX 400
struct stack
{
    char arr[MAX];
    int top;
};

void initstack(struct stack *);
void push(struct stack *,char);
char pop(struct stack *);
void convert(struct stack *s,char *);
int priority(char e);

int main()
{
   int t;
   char str[MAX];
   struct stack s1;
   scanf("%d",&t);          //No of infix expressions to be converted in postfix//
   initstack(&s1);
   while(t)
   {
       t--;
       scanf("%s",str);
       convert(&s1,str);
   }
   return 0;

}

void initstack(struct stack *s)
{
    s->top=-1;
}

void push(struct stack *s,char data)
{
    if(s->top==MAX-1)
     return;
    else
    {
        s->top++;
        s->arr[s->top]=data;
    }
}

char pop(struct stack *s)
{
    char a;
    if(s->top==-1)
     return -1;
    else
    {
        a=s->arr[s->top];
        s->top--;
        return a;
    }
}

void convert(struct stack *s,char *exp)
{
    char op[400],d,p1;
    int i=0,j=0,pr;
    while(*(exp))
    {
        if(*(exp)=='(')
         {
          push(s,*(exp));
          exp++;
         }
        else if(*(exp)=='+'||*(exp)=='-'||*(exp)=='^'||*(exp)=='/'||*(exp)=='*')
         {
           if(s->top==-1)
            {
                push(s,*(exp));
                exp++;
            }
            else
            {
                p1=pop(s);
                if(priority(p1)<priority(*(exp)))
                {
                    push(s,p1);
                    push(s,*(exp));
                    exp++;
                }
                else
                {
                    while(priority(p1)>=priority(*(exp)))
                    {
                      op[i]=p1;
                      i++;
                      p1=pop(s);
                    }
                    push(s,p1);
                    push(s,*(exp));
                    exp++;
                }
            }
          }
          else if(*(exp)==')')
          {
              p1=pop(s);
              while((p1)!='(');
              {
                  op[i]=p1;
                  i++;
                  p1=pop(s);
              }
              exp++;
           }
           else if(isalpha(*(exp)))
           {
               while(isalpha(*(exp)))
               {
               op[i]=*(exp);
               exp++;
               i++;
               }
            }
        }
        while(s->top!=-1)
        {
            d=pop(s);
            op[i]=d;
            i++;
        }
        op[i]='\0';
        while(op[j])
        printf("%c",op[j++]);
        printf("\n");
}


int priority(char e)
{
    if(e=='^')
     return 3;
    else if(e=='*'||e=='/')
     return 2;
    else if(e=='+'||e=='-')
     return 1;
    else
     return 0;
}

Well, I tried all I could do and that is why I didn't wait for a day.. There is segmentation fault in the code but I can't figure where does the code makes an invalid memory reference. I have been trying to run it since morning but no success. That is why I am here.

Originally Posted by gaurav_13191

There is segmentation fault in the code but I can't figure where does the code makes an invalid memory reference.

If you can't find it by reading, why not have the debugger tell you when the segfault happens?

I got the problem with ')' but still the program runs for some inputs and for some inputs it just outputs nothing. Still some other rectification is required so that it runs for all inputs.

Well, thanks all with your suggestions I was able to do it correctly. It was my first post but I really liked that I got lot of suggestions within no time and didn't have to wait for the whole day.