02 - Infix to Postfix Conversion

#include <ctype.h> to get the isalnum() function to check alphabets and numbers which returns false for any special characters.

Logic for Variables

char stack[100]: A fixed-size array to store operators and operands for conversion.
int top = -1: This variable points to the top of the stack.
char exp[100]: Another array to store the user input expression.
char *e: A pointer used to traverse the exp[] array.
char x: A variable to store the element popped from the stack.

Implementation Logic

Declare a global stack (stack[100]) with top = -1 to track the top of the stack.
In the main function, declare the exp[100] array to capture the user input expression. Use scanf("%s", exp) to capture the entire expression (%c is for single char) (&exp is not needed as it is an array).
Declare e and Set e = exp to point to the start of the expression, enabling traversal.
Use x in the main function to capture popped elements.
Use a while loop (while(*e != '\0')) to traverse the expression by incrementing pointer e++.
Each iteration processes one character in exp[] and ends when the end of the array is reached which is signaled by *e == '\0'

Conversion Logic

As the while loop runs, e is incremented and it traverses the exp array elements.
e points at the location within the array and *e is element in that array location.

Alphanumeric Characters: if ( isalnum(*e) ) If the character *e is alphanumeric (number or letter), it is printed directly without being pushed to the stack.
Opening Parenthesis (: Always pushed onto the stack.
Closing Parenthesis ): Pop elements from the stack and print them until an opening parenthesis ( is encountered. while( (x=pop()) != '(' )
Operators: For other characters (operators), compare their priority with the operator at the top of the stack. Pop and print operators with equal or higher priority before pushing the current operator. while ( priority(stack[top]) >= priority(*e) )

Conditions for pushing in the operator.

If stack is empty, if (top == -1 ) then can be pushed in.
If not empty, then priority of the element at the top of the stack stack[top] has to be compared with the element from the expression *e.

After processing the entire expression, pop all remaining operators from the stack. After processing the entire expression the main while loop ends after *e reaches \0. There are still operators in the stack. pop all remaining operators from the stack. while (top != -1 ) print all the popped items.

Functions logic

Push

push need the char that has to be pushed which is in the *e
Increment the top to next place top++, and assign the value to stack[top]
top++ ; stack[top] = x or stack[++top]

Pop

Edge case is top = -1 then return -1 to calling function. (This allows to know the end if the pop value is used in any checking)
else, return the value which is at the stack[top] and then decrement top
stack[top--]

Priority

Takes a char as parameter.
Works by returning a value to reflect the operators precedence which allows the while loop to compare both operators based on their return value.
Operator with lowest precedence returns 0 and next would be 1, 2, 3
- (: Lowest precedence (0).

+, -: Medium precedence (1).
*, /: Highest precedence (2).

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

char stack[100];
int top = -1;

char pop();
void push(char x);
int priority(char x);

int main()
{
	char exp[100];
	char *e, x;
	
	printf("Enter the Infix Expression to be Converted: \n");
	scanf("%s", exp);
	e = exp;
	
	while( *e != '\0' )
	{
		if ( isalnum(*e) )
		{
			print("%c \n", *e );
		}
		else if ( *e == '(')
		{
			push(*e);
		}
		else if ( *e == ')')
		{
			while ( (x = pop()) != '(' )
			{
				printf("%c \n", x);
			}
		}
		else 
		{
			while ( priority(stack[top]) >= priority(*e) )
			{
				printf("%c \n", pop());
			}
			push(*e);
		}
		e++;
	}
	
	while ( top != -1)
	{
		printf("%c \n", pop());
	}
	
	return 0;
}


void push(char x)
{
	top++;
	stack[top] = x;
}
char pop()
{
	if (top == -1)
		return -1;
	else
		return stack[top--];
}
int priority(char x)
{
	if (x == '(')
		return 0;
	else if ( x == '+' || x == '-')
		return 1;
	else if ( x == '*' || x == '/')
		return 2;
	else 
		return 0;
}

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

char stack[100];
int top = -1;

char pop();
void push(char x);
int priority(char x);

int main() {
    char exp[100];  // Expression to be converted
    char *e, x;     // Pointer to traverse the expression and variable to store popped element

    printf("Enter the Infix Expression to be Converted: \n");
    scanf("%s", exp);  // Take input expression

    e = exp;  // Pointer to the start of expression

    while (*e != '\0')   // Loop until end of expression 
    {  
        if (isalnum(*e))  // If the character is a number/letter 
        {  
            printf("%c \n", *e);  // Print the character (operand)
        }
        else if (*e == '(')   // If opening parenthesis
        {
            push(*e);  // Push it onto the stack
        }
        else if (*e == ')')  // If closing parenthesis
        {
            while ((x = pop()) != '(')  // Pop until '(' is encountered
            {
                printf("%c \n", x);  // Print the popped operator
            }
        }
        else  // If the character is an operator
        {
            // Pop operators of higher or equal precedence and print them
            while (priority(stack[top]) >= priority(*e))
            {
                printf("%c \n", pop());
            }
            push(*e);  // Push the current operator onto the stack
        }
        e++;  // Move to the next character in the expression
    }

    // After processing the expression, pop all remaining operators from the stack
    while (top != -1) 
    {
        printf("%c \n", pop());  // Print and pop the remaining operators
    }

    return 0;
}

// Function to push an element onto the stack
void push(char x) 
{
    top++;  // Increment the top of the stack
    stack[top] = x;  // Assign the element to the stack
}

// Function to pop an element from the stack
char pop() 
{
    if (top == -1)  // Check if the stack is empty
        return -1;  // Return -1 if stack is empty
    else
        return stack[top--];  // Return the top element and decrement top
}

// Function to return the priority of operators
int priority(char x) 
{
    if (x == '(') return 0;  // '(' has the lowest precedence
    else if (x == '+' || x == '-') return 1;  // + and - have medium precedence
    else if (x == '*' || x == '/') return 2;  // * and / have high precedence
    else return 0;  // Default case for non-operators
}

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01 - Stack Operations

03 - Postfix Expression Evaluation

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