c program to convert infix to postfix

// C program to convert infix expression to postfix  
#include <stdio.h> 
#include <string.h> 
#include <stdlib.h> 
  
// Stack type 
struct Stack 
{ 
    int top; 
    unsigned capacity; 
    int* array; 
}; 
  
// Stack Operations 
struct Stack* createStack( unsigned capacity ) 
{ 
    struct Stack* stack = (struct Stack*)  
           malloc(sizeof(struct Stack)); 
  
    if (!stack)  
        return NULL; 
  
    stack->top = -1; 
    stack->capacity = capacity; 
  
    stack->array = (int*) malloc(stack->capacity *  
                                   sizeof(int)); 
  
    return stack; 
} 
int isEmpty(struct Stack* stack) 
{ 
    return stack->top == -1 ; 
} 
char peek(struct Stack* stack) 
{ 
    return stack->array[stack->top]; 
} 
char pop(struct Stack* stack) 
{ 
    if (!isEmpty(stack)) 
        return stack->array[stack->top--] ; 
    return '$'; 
} 
void push(struct Stack* stack, char op) 
{ 
    stack->array[++stack->top] = op; 
} 
  
  
// A utility function to check if  
// the given character is operand 
int isOperand(char ch) 
{ 
    return (ch >= 'a' && ch <= 'z') ||  
           (ch >= 'A' && ch <= 'Z'); 
} 
  
// A utility function to return  
// precedence of a given operator 
// Higher returned value means  
// higher precedence 
int Prec(char ch) 
{ 
    switch (ch) 
    { 
    case '+': 
    case '-': 
        return 1; 
  
    case '*': 
    case '/': 
        return 2; 
  
    case '^': 
        return 3; 
    } 
    return -1; 
} 
  
  
// The main function that  
// converts given infix expression 
// to postfix expression.  
int infixToPostfix(char* exp) 
{ 
    int i, k; 
  
    // Create a stack of capacity  
    // equal to expression size  
    struct Stack* stack = createStack(strlen(exp)); 
    if(!stack) // See if stack was created successfully  
        return -1 ; 
  
    for (i = 0, k = -1; exp[i]; ++i) 
    { 
          
        // If the scanned character is  
        // an operand, add it to output. 
        if (isOperand(exp[i])) 
            exp[++k] = exp[i]; 
          
        // If the scanned character is an  
        // ‘(‘, push it to the stack. 
        else if (exp[i] == '(') 
            push(stack, exp[i]); 
          
        // If the scanned character is an ‘)’, 
        // pop and output from the stack  
        // until an ‘(‘ is encountered. 
        else if (exp[i] == ')') 
        { 
            while (!isEmpty(stack) && peek(stack) != '(') 
                exp[++k] = pop(stack); 
            if (!isEmpty(stack) && peek(stack) != '(') 
                return -1; // invalid expression              
            else
                pop(stack); 
        } 
        else // an operator is encountered 
        { 
            while (!isEmpty(stack) &&  
                 Prec(exp[i]) <= Prec(peek(stack))) 
                exp[++k] = pop(stack); 
            push(stack, exp[i]); 
        } 
  
    } 
  
    // pop all the operators from the stack 
    while (!isEmpty(stack)) 
        exp[++k] = pop(stack ); 
  
    exp[++k] = '\0'; 
    printf( "%s", exp ); 
} 
  
// Driver program to test above functions 
int main() 
{ 
    char exp[] = "a+b*(c^d-e)^(f+g*h)-i"; 
    infixToPostfix(exp); 
    return 0; 
}