Infix to Postfix program doesn't work as intended - c

Question

This is my attempt to write a program that can convert any expression in infix to postfix format. I assign -1 to the top to indicate the stack is empty. When push, the top is incremented, when pop, the top is decremented. However, when I type in a+b, the output only gives me ab without the + operator, while when I type (a+b), it says segmentation fault. I reckon there's something wrong with my stack, but couldn't figure out what went wrong.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#define SIZE 30
typedef struct Stack
{
int top;
int capacity;
char* storage;
} stack;
int isEmpty(stack* a);
char topelement(stack* a);
char pop(stack* a);
void push(stack* a,char b);
bool isOperand(char a);
int Precedence(char a);
stack* NewStack(char* a);
void InfixPostfix(char* a);
int main(void)
{
char expression[SIZE];
printf("Please enter an expression:");
scanf("%s",expression);
InfixPostfix(expression);
printf("\n");
}
int isEmpty(stack* a)
{
if(a->top==-1)
{
return 1;
}
else
return 0;
}
char topelement(stack* a)
{
return a->storage[a->top];
}
char pop(stack* a)
{
if(isEmpty(a)==1)
{
printf("Stack is Empty\n");
return '$';
}
else
return a->storage[a->top];
--(a->top);
}
void push(stack* a,char b)
{
++(a->top);
a->storage[a->top]=b;
}
bool isOperand(char a)
{
if ( (a >= 'a' && a<= 'z') ||(a>='A' && a<='Z'))
{
return 1;
}
else
return 0;
}
int Precedence(char a)
{
if(a=='+' || a=='-')
{
return 1;
}
if(a=='*' || a=='/')
{
return 2;
}
if(a=='^')
{
return 3;
}
else
return -1;
}
stack* NewStack(char* a)
{
stack* b= malloc(sizeof(stack));
if(b!=NULL)
{
b->top=-1;
b->storage=malloc((strlen(a))*sizeof(char));
return b;
}
else
return NULL;
}
void InfixPostfix(char* a)
{
int i; int j=-1;
stack* b=NewStack(a);
if(b!=NULL)
{
for(i=0; i<strlen(a) ;i++)
{
if(isOperand(a[i]))
{
a[++j]=a[i];
}
if(a[i]=='(')
{
push(b, a[i]);
}
if(a[i]==')')
{
while(isEmpty(b)==0 && topelement(b)!= '(')
{
a[++j]= pop(b);
}
}
else
{
while(isEmpty(b)==0 && Precedence(a[i]) <= Precedence(topelement(b)))
{
a[++j]=pop(b);
push(b,a[i]);
}
}
}
while(isEmpty(b)==0)
{
a[++j]=pop(b);
}
a[++j]='\0';
printf("%s",a);
}
}

Answer 1

Besides the already suggested malloc correction, there are a few more to make.
In pop() there's
return a->storage[a->top];
--(a->top);
where the last code line isn't reached; change that to return a->storage[a->top--];
In InfixPostfix(), else are missing before if(a[i]=='(') and if(a[i]==')').
In InfixPostfix(), after the loop
while(isEmpty(b)==0 && topelement(b)!= '(')
{
a[++j]= pop(b);
}
a pop(b); is missing - the element '(' must as well be removed from the stack.
In InfixPostfix(), the push(b,a[i]); must be removed from the loop
while(isEmpty(b)==0 && Precedence(a[i]) <= Precedence(topelement(b)))
{
a[++j]=pop(b);
push(b,a[i]);
}
and placed after that loop - the operator in a[i] has to be put onto the stack only once.