Classic Conversion of Infix to Postfix

I was doing a quick search on my name on Google and found quite a number of search results. One of them being a September 2002 posting on Experts-Exchange to help answer a C Programming question regarding conversion from Infix to Postfix notation.

The code snippet I posted below is exactly the same one I posted in year 2002. The same code was used during my Diploma days at Singapore Polytechnic.

/*****************************/
/* Programmer : Loh Hon Chun */
/* Adm No     : 9852152  */
/* Class  : 3A07     */
/* Description: Practical 3  */
/*****************************/

/*************************************************************/
/*************************************************************/
/*         ASSUMING INFIX EXPRESSION         */
/*             IS VALID          */
/*          !!!!!!!!          */
/*************************************************************/
/*************************************************************/

/* include necessary preprocessor header files */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

/* constants */
#define TRUE 1
#define FALSE 0

/* structure for stack */
typedef struct
{
    char data[20];  /* array to hold stack contents */
    int tos;    /* top of the stack pointer */
} STACK;


/* function prototypes */
void initStack(STACK *stack);
void get_infix(char infix[]);
void convertToPostfix(char infix[], char postfix[]);
int isOperator(char c);
int precedence(char operator1, char operator2);
int pred_level(char ch);
void push(STACK *stack, char value);
char pop(STACK *stack);
char stackTop(STACK *stack);
int isEmpty(STACK *stack);
int isFull(STACK *stack);
void printResult(char infix[], char postfix[]);
void print_msg(void);

/* program entry point */
int main(void)
{
    char infix[20], postfix[20]="";

    /* convert from infix to postfix main function */
    convertToPostfix(infix, postfix);
    /* display the postfix equivalent */
    infix[strlen(infix)-2] = '\0';
    printResult(infix, postfix);

    return EXIT_SUCCESS;
}

/* initalise the stack */
void initStack(STACK *stack)
{
    stack->tos = -1;  /* stack is initially empty */
}

/* get infix expression from user */
void get_infix(char infix[])
{
    int i;

    printf("Enter infix expression below (max 18 characters excluding spaces) : \n");
    fflush(stdin);
    /* to read in only 18 characters excluding spaces */
    for ( i=0; i<18; )
    {
        if ( (infix[i] = getchar()) == '\n' )
        {
            i++;
            break;
        }
        else if ( !(isspace(infix[i])) )
            i++;
    }

    infix[i] = '\0';
}

/* convert the infix expression to postfix notation */
void convertToPostfix(char infix[], char postfix[])
{
    int i, length;
    int j=0;
    char tos_ch;
    STACK stack;

    initStack(&stack); /* initialise stack */
    get_infix(infix);  /* get infix expression from user */
    length = strlen(infix);

    /* if strlen if infix is more than zero */
    if ( length )
    {  
        push(&stack, '(');
        strcat(infix, ")");
        length++;

        for ( i=0; i<length; i++ )
        {
            /* if current operator in infix is digit */
            if ( isdigit(infix[i]) )
            {
                postfix[j++] = infix[i];
            }
            /* if current operator in infix is left parenthesis */
            else if ( infix[i] == '(' )
            {
                push(&stack, '(');
            }
            /* if current operator in infix is operator */
            else if ( isOperator(infix[i]) )
            {
                while ( TRUE )
                {
                    /* get tos */
                    tos_ch = stackTop(&stack);

                    /* no stack left */
                    if ( tos_ch == '\0' )
                    {
                        printf("\nInvalid infix expression\n");
                        print_msg();
                        exit(1);
                    }
                    else
                    {
                        if ( isOperator(tos_ch) )
                        {
                            if ( pred_level(tos_ch) >= pred_level(infix[i]) )
                                postfix[j++] = pop(&stack);
                            else
                                break;
                        }
                        else
                            break;
                    }
                }
                push(&stack, infix[i]);
            }
            /* if current operator in infix is right parenthesis */
            else if ( infix[i] == ')' )
            {
                while ( TRUE )
                {
                    /* get tos */
                    tos_ch = stackTop(&stack);

                    /* no stack left */
                    if ( tos_ch == '\0' )
                    {
                        printf("\nInvalid infix expression\n");
                        print_msg();
                        exit(1);
                    }
                    else
                    {
                        if ( tos_ch != '(' )
                        {
                            postfix[j++] = tos_ch;
                            pop(&stack);
                        }
                        else
                        {
                            pop(&stack);
                            break;
                        }
                    }
                }
                continue;
            }
        }
    }

    postfix[j] = '\0';
}

/* determine if c is an operator */
int isOperator(char c)
{
    if ( c == '+' || c == '-' || c == '*' ||
        c == '/' || c == '%' || c == '^' )
    {
        return TRUE;
    }
    else
        return FALSE;
}

/* determine precedence level */
int pred_level(char ch)
{
    if ( ch == '+' || ch == '-' )
        return 1;
    else if ( ch == '^' )
        return 3;
    else
        return 2;
}

/* determine if the precedence of operator1 is less than,
equal to, greater than the precedence of operator2 */
int precedence(char operator1, char operator2)
{
    if ( pred_level(operator1) > pred_level(operator2) )
        return 1;
    else if ( pred_level(operator1) < pred_level(operator2) )
        return -1;
    else
        return 0;
}

/* push a value on the stack */
void push(STACK *stack, char value)
{
    if ( !(isFull(stack)) )
    {
        (stack->tos)++;
        stack->data[stack->tos] = value;
    }
}

/* pop a value off the stack */
char pop(STACK *stack)
{
    char ch;

    if ( !(isEmpty(stack)) )
    {
        ch = stack->data[stack->tos];
        (stack->tos)--;
        return ch;
    }
    else
        return '\0';
}

/* return the top value of the stack without popping the stack */
char stackTop(STACK *stack)
{
    if ( !(isEmpty(stack)) )
        return stack->data[stack->tos];
    else
        return '\0';
}

/* determine if stack is empty */
int isEmpty(STACK *stack)
{
    /* empty */
    if ( stack->tos == -1 )
        return TRUE;
    /* not empty */
    else
        return FALSE;
}

/* determine if stack is full */
int isFull(STACK *stack)
{
    /* full */
    if ( stack->tos == 19 )
        return TRUE;
    /* not full */
    else
        return FALSE;
}

/* display the result postfix expression */
void printResult(char infix[], char postfix[])
{
    /*system("cls");*/
    printf("\n\n");
    printf("Infix notation  : %s\n", infix);
    printf("Postfix notation: %s\n\n", postfix);
    print_msg();
}

/* print exit message */
void print_msg(void)
{
    printf("Hit <return> to exit......");
    fflush(stdin);
    getchar();
}