Infix to postfix conversion

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I write a C code to convert infix expression to postfix expresion. The expression may inclue '^' , "+", "-" , "/", "*" and all three types of brackets i.e." ({[ ". The code should first check whether the brackets are balanced or not and then do conversion. But the code is showing empty stack at the output.

What I have tried:

#include "stackar.h"
#include "fatal.h"
#include <stdlib.h>

#define EmptyTOS ( -1 )
#define MinStackSize ( 5 )

struct StackRecord
{
int Capacity;
int TopOfStack;
ElementType *Array;
};

/* START: fig3_48.txt */
int
IsEmpty( Stack S )
{
return S->TopOfStack == EmptyTOS;
}
/* END */

/* START: fig3_46.txt */
Stack
CreateStack( int MaxElements )
{
Stack S;

/* 1*/ if( MaxElements < MinStackSize )
/* 2*/ Error( "Stack size is too small" );

/* 3*/ S = malloc( sizeof( struct StackRecord ) );
/* 4*/ if( S == NULL )
/* 5*/ FatalError( "Out of space!!!" );

/* 6*/ S->Array = malloc( sizeof( ElementType ) * MaxElements );
/* 7*/ if( S->Array == NULL )
/* 8*/ FatalError( "Out of space!!!" );
/* 9*/ S->Capacity = MaxElements;
/*10*/ MakeEmpty( S );

/*11*/ return S;
}
/* END */

/* START: fig3_50.txt */
void
Push( ElementType X, Stack S )
{
if( IsFull( S ) )
Error( "Full stack" );
else
S->Array[ ++S->TopOfStack ] = X;
}
/* END */


/* START: fig3_51.txt */
ElementType
Top( Stack S )
{
if( !IsEmpty( S ) )
return S->Array[ S->TopOfStack ];
Error( "Empty stack" );
return 0; /* Return value used to avoid warning */
}
/* END */

/* START: fig3_52.txt */
void
Pop( Stack S )
{
if( IsEmpty( S ) )
Error( "Empty stack" );
else
S->TopOfStack--;
}
/* END */

/* START: fig3_53.txt */
ElementType
TopAndPop( Stack S )
{
if( !IsEmpty( S ) )
return S->Array[ S->TopOfStack-- ];
Error( "Empty stack" );
return 0; /* Return value used to avoid warning */
}
/* END */
/* MY CODE TO CONVERT INFIX TO POSTFIX STARTS FROM HERE. THE ABOVE FILE IS "stacker.c"*/

#include <stdio.h>
#include<ctype.h>
#include "stackar.h"

int ismatchingpair(char char1, char char2) // check whether the bracket are same or not
{
if (char1 == '(' && char2 == ')')
return 1;
if (char1 == '[' && char2 == ']')
return 1;
if (char1 == '{' && char2 == '}')
return 1;
else
return 0;
}
int bracketsBalance(char *str)
{
Stack S;
S = CreateStack( 20 );
int i;
for (int i = 0; str[i]; ++i)
{
if (str[i] == '(' || str[i] == '{' || str[i] == '[')// if opening bracket, push it in the stack
{
Push( str[i], S);
}
else if (str[i] == ')' || str[i] == '}' || str[i] == ']')// if closing bracket, check whether it matches the opening bracket
{
if (ismatchingpair(Top(S), str[i]))// if it matches the opening bracket, pop it in the stack
{
Pop(S);
}
}
}
if (IsEmpty(S)) // if stack is empty, return 1
{
return 1;
}
else // else return 0
{
return 0;
}
}

 

int pr(char symbol)
{ /* Function for precedence */

if(symbol == '^')/* exponent operator, highest precedence*/
{
return(3);
}
else if(symbol == '*' || symbol == '/')
{
return(2);
}
else if(symbol == '+' || symbol == '-') /* lowest precedence */
{
return(1);
}
else
{
return(0);
}
}
int openingbracket(char x)
{
if (x=='(' || x=='{' || x=='[')
{
return 1;
}
else {
return 0;
}
}
int closingbracket(char y)
{
if (y==')' || y=='}' || y==']')
{
return 1;
}
else{ return 0;}
}
int checkoperator(char z)
{
if (z=='^' || z=='+' || z=='-' || z=='/' || z=='*')
{
return 1;
}
else{ return 0;}
}
int infixToPostfix(char *infix, char *postfix, int str_size)
{
str_size = strlen(infix);
Stack S;
S = CreateStack( 20 );

char i;
for (i=0; i<str_size;i++){
if(!(checkoperator(infix[i]) || openingbracket(infix[i]) || closingbracket(infix[i]))){
strncat(postfix, &infix[i],1);
}
else if (openingbracket(infix[i])){
Push(infix[i],S);
}
else
{
checkoperator(infix[i]);
while(pr(Top(S))>=pr(i))
{
strcat(postfix, &infix[i]);
Pop(S);
}
Push(i, S);
}
}
while(Top(S)!='#')
{
TopAndPop(S);
strncat(postfix, &infix[i],1);
}
}
main()
{
int roll_no=5;
char in[1000]="[(62.1+9.0)*(84+roll_no)-2]*4)";
char post[1000];
if (bracketsBalance(in))
{
printf("Brackets in %s are balanced\n", in);
infixToPostfix(&in[0], &post[0],1);
printf("Postfix expressionof %s is: %s\n",in, &post[0]);
}
else{printf("Brackets unbalanced. No conversion performed");}
return 0;
}

Posted 17-May-21 7:47am

Mehreen Saleem

Updated 17-May-21 10:51am

PIEBALDconsult

v2

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jeron1 17-May-21 14:09pm

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OriginalGriff · Answer 1 · 2021-05-17T08:29:00

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C#

int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!

Patrice T · Answer 2 · 2021-05-17T08:30:00

Since brackets of input are not balanced:

C++

char in[1000]="[(62.1+9.0)*(84+roll_no)-2]*4)";

I fear your code do not try to build the postfix tree.
-----
Advice: Learn to indent properly your code, it show its structure and it helps reading and understanding. It also helps spotting structures mistakes.

C++

#include "stackar.h"

#include "fatal.h"

#include <stdlib.h>

#define EmptyTOS(-1)
#define MinStackSize(5)

struct StackRecord {
  int Capacity;
  int TopOfStack;
  ElementType * Array;
};

/* START: fig3_48.txt */
int
IsEmpty(Stack S) {
  return S -> TopOfStack == EmptyTOS;
}
/* END */

/* START: fig3_46.txt */
Stack
CreateStack(int MaxElements) {
  Stack S;

  /* 1*/
  if (MaxElements < MinStackSize)
    /* 2*/
    Error("Stack size is too small");

  /* 3*/
  S = malloc(sizeof(struct StackRecord));
  /* 4*/
  if (S == NULL)
    /* 5*/
    FatalError("Out of space!!!");

  /* 6*/
  S -> Array = malloc(sizeof(ElementType) * MaxElements);
  /* 7*/
  if (S -> Array == NULL)
    /* 8*/
    FatalError("Out of space!!!");
  /* 9*/
  S -> Capacity = MaxElements;
  /*10*/
  MakeEmpty(S);

  /*11*/
  return S;
}
/* END */

/* START: fig3_50.txt */
void
Push(ElementType X, Stack S) {
  if (IsFull(S))
    Error("Full stack");
  else
    S -> Array[++S -> TopOfStack] = X;
}
/* END */

/* START: fig3_51.txt */
ElementType
Top(Stack S) {
  if (!IsEmpty(S))
    return S -> Array[S -> TopOfStack];
  Error("Empty stack");
  return 0; /* Return value used to avoid warning */
}
/* END */

/* START: fig3_52.txt */
void
Pop(Stack S) {
  if (IsEmpty(S))
    Error("Empty stack");
  else
    S -> TopOfStack--;
}
/* END */

/* START: fig3_53.txt */
ElementType
TopAndPop(Stack S) {
  if (!IsEmpty(S))
    return S -> Array[S -> TopOfStack--];
  Error("Empty stack");
  return 0; /* Return value used to avoid warning */
}
/* END */
/* MY CODE TO CONVERT INFIX TO POSTFIX STARTS FROM HERE. THE ABOVE FILE IS "stacker.c"*/

#include <stdio.h>

#include<ctype.h>

#include "stackar.h"

int ismatchingpair(char char1, char char2) // check whether the bracket are same or not
{
  if (char1 == '(' && char2 == ')')
    return 1;
  if (char1 == '[' && char2 == ']')
    return 1;
  if (char1 == '{' && char2 == '}')
    return 1;
  else
    return 0;
}
int bracketsBalance(char * str) {
  Stack S;
  S = CreateStack(20);
  int i;
  for (int i = 0; str[i]; ++i) {
    if (str[i] == '(' || str[i] == '{' || str[i] == '[') // if opening bracket, push it in the stack
    {
      Push(str[i], S);
    } else if (str[i] == ')' || str[i] == '}' || str[i] == ']') // if closing bracket, check whether it matches the opening bracket
    {
      if (ismatchingpair(Top(S), str[i])) // if it matches the opening bracket, pop it in the stack
      {
        Pop(S);
      }
    }
  }
  if (IsEmpty(S)) // if stack is empty, return 1
  {
    return 1;
  } else // else return 0
  {
    return 0;
  }
}

int pr(char symbol) {
  /* Function for precedence */

  if (symbol == '^') /* exponent operator, highest precedence*/ {
    return (3);
  } else if (symbol == '*' || symbol == '/') {
    return (2);
  } else if (symbol == '+' || symbol == '-') /* lowest precedence */ {
    return (1);
  } else {
    return (0);
  }
}
int openingbracket(char x) {
  if (x == '(' || x == '{' || x == '[') {
    return 1;
  } else {
    return 0;
  }
}
int closingbracket(char y) {
  if (y == ')' || y == '}' || y == ']') {
    return 1;
  } else {
    return 0;
  }
}
int checkoperator(char z) {
  if (z == '^' || z == '+' || z == '-' || z == '/' || z == '*') {
    return 1;
  } else {
    return 0;
  }
}
int infixToPostfix(char * infix, char * postfix, int str_size) {
  str_size = strlen(infix);
  Stack S;
  S = CreateStack(20);

  char i;
  for (i = 0; i < str_size; i++) {
    if (!(checkoperator(infix[i]) || openingbracket(infix[i]) || closingbracket(infix[i]))) {
      strncat(postfix, & infix[i], 1);
    } else if (openingbracket(infix[i])) {
      Push(infix[i], S);
    } else {
      checkoperator(infix[i]);
      while (pr(Top(S)) >= pr(i)) {
        strcat(postfix, & infix[i]);
        Pop(S);
      }
      Push(i, S);
    }
  }
  while (Top(S) != '#') {
    TopAndPop(S);
    strncat(postfix, & infix[i], 1);
  }
}
main() {
  int roll_no = 5;
  char in [1000] = "[(62.1+9.0)*(84+roll_no)-2]*4)";
  char post[1000];
  if (bracketsBalance( in )) {
    printf("Brackets in %s are balanced\n", in );
    infixToPostfix( & in [0], & post[0], 1);
    printf("Postfix expressionof %s is: %s\n", in , & post[0]);
  } else {
    printf("Brackets unbalanced. No conversion performed");
  }
  return 0;
}

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Professional programmer's editors have this feature and others ones such as parenthesis matching and syntax highlighting.
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Enabling Open Innovation & Collaboration | The Eclipse Foundation[^]
-----
Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

1.11 — Debugging your program (stepping and breakpoints) | Learn C++[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

Infix to postfix conversion

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