Infix to Postfix Conversion Using Stack in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element onto the stack.

DSA Go

func push(stack []rune, element rune) []rune {
    stack = append(stack, [1])
    return stack
}

Drag options to blanks, or click blank then click option'

Aelement

Bstack

Crune

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Appending the whole stack instead of the element.

Using the wrong variable name.

Trying to append the type 'rune' instead of the variable.

To add an element to the stack, we append the element itself to the slice representing the stack.

2fill in blank

medium

Complete the code to check if the stack is empty.

DSA Go

func isEmpty(stack []rune) bool {
    return len(stack) [1] 0
}

Drag options to blanks, or click blank then click option'

A!=

B>

C<

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '=='.

Using '>' or '<' which do not correctly check emptiness.

The stack is empty when its length is equal to zero.

3fill in blank

hard

Fix the error in popping the top element from the stack.

DSA Go

func pop(stack []rune) ([]rune, rune) {
    if isEmpty(stack) {
        return stack, 0
    }
    top := stack[len(stack)[1]]
    stack = stack[:len(stack)-1]
    return stack, top
}

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A0

B+1

C-1

D-2

Attempts:

3 left

💡 Hint

Common Mistakes

Using len(stack) which is out of range.

Using len(stack)+1 which causes runtime error.

The top element is at index len(stack)-1 because slice indices start at 0.

4fill in blank

hard

Fill both blanks to correctly check operator precedence.

DSA Go

func precedence(op rune) int {
    if op == '+' || op == '-' {
        return [1]
    } else if op == '*' || op == '/' {
        return [2]
    }
    return 0
}

Drag options to blanks, or click blank then click option'

A1

B2

C0

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning higher precedence to '+' than '*'.

Using zero for all operators.

Addition and subtraction have precedence 1, multiplication and division have precedence 2.

5fill in blank

hard

Fill all three blanks to correctly convert infix to postfix inside the loop.

DSA Go

for _, ch := range expression {
    if (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z') {
        postfix += string([1])
    } else if ch == '(' {
        stack = push(stack, [2])
    } else if ch == ')' {
        for !isEmpty(stack) && stack[len(stack)-1] != [3] {
            var top rune
            stack, top = pop(stack)
            postfix += string(top)
        }
        stack, _ = pop(stack) // pop '('
    }
}

Drag options to blanks, or click blank then click option'

Ach

B'('

C')'

D'+'

Attempts:

3 left

💡 Hint

Common Mistakes

Pushing wrong character instead of '('.

Comparing with wrong character in the loop condition.

Adding wrong character to postfix.

We add the character itself to postfix if it's a letter, push '(' onto stack, and pop until '(' when we see ')'.

Practice

1. What is the main purpose of using a stack in Infix to Postfix Conversion?

easy

A. To temporarily hold operators and manage their precedence

B. To store the final postfix expression

C. To reverse the infix expression

D. To count the number of operands

Solution

1. Step 1: Understand the role of stack in conversion

   The stack is used to hold operators temporarily while processing the infix expression to respect operator precedence and associativity.

2. Step 2: Identify what is stored in stack

   Operands are directly added to the output, while operators are pushed to the stack to decide their order in postfix.

3. Final Answer:

   To temporarily hold operators and manage their precedence -> Option A

4. Quick Check:

   Stack holds operators for precedence [OK]

Hint: Stack holds operators, output holds operands [OK]

Common Mistakes:

• Thinking stack stores operands
• Confusing stack role with output
• Ignoring operator precedence

2. Which of the following Go code snippets correctly initializes an empty stack for infix to postfix conversion?

easy

A. var stack []rune = []rune{}

B. stack := make([]int, 0)

C. stack := []string{0}

D. var stack []int = nil

Solution

1. Step 1: Identify correct stack type for operators

   Operators are characters, so stack should be a slice of runes (characters) initialized empty.

2. Step 2: Check initialization correctness

   var stack []rune = []rune{} initializes an empty rune slice correctly. stack := make([]int, 0) uses int slice, not suitable. stack := []string{0} has syntax error (cannot use int 0 as string). var stack []int = nil is nil slice, which can cause runtime errors.

3. Final Answer:

   var stack []rune = []rune{} -> Option A

4. Quick Check:

   Empty rune slice for operator stack [OK]

Hint: Use rune slice for operator stack [OK]

Common Mistakes:

• Using int slice for operators
• Initializing with wrong type
• Using nil slice without initialization

3. What is the postfix expression output by the following Go code snippet for input "a+b*c"?

infix := "a+b*c"
// Assume correct infix to postfix conversion function called here
postfix := InfixToPostfix(infix)
fmt.Println(postfix)

medium

A. "a+bc*"

B. "ab+c*"

C. "abc*+"

D. "*abc+"

Solution

1. Step 1: Analyze operator precedence in expression

   Multiplication (*) has higher precedence than addition (+), so 'b*c' is evaluated before adding 'a'.

2. Step 2: Convert infix to postfix step-by-step

   Postfix is formed as 'a' 'b' 'c' '*' '+', which is "abc*+".

3. Final Answer:

   "abc*+" -> Option C

4. Quick Check:

   Multiplication before addition = "abc*+" [OK]

Hint: Multiply before add means * goes before + in postfix [OK]

Common Mistakes:

• Ignoring operator precedence
• Placing + before * in postfix
• Confusing operand order

4. Given this Go function snippet for infix to postfix conversion, what is the bug?

func precedence(op rune) int {
    if op == '+' || op == '-' {
        return 1
    } else if op == '*' || op == '/' {
        return 2
    }
    return 0
}

// InfixToPostfix function uses precedence(op) to compare operators
// but outputs incorrect postfix for input "a+b^c"

medium

A. Precedence function returns 0 for non-operators causing errors

B. Precedence function returns int instead of float

C. Precedence values for '+' and '-' are swapped

D. Precedence function does not handle '^' operator

Solution

1. Step 1: Check if all operators are handled

   The function handles '+', '-', '*', '/' but misses '^' (exponent) operator which is common in infix expressions.

2. Step 2: Understand impact on output

   If input contains '^', missing precedence causes wrong order in postfix output.

3. Final Answer:

   Precedence function does not handle '^' operator -> Option D

4. Quick Check:

   Missing '^' causes wrong postfix [OK]

Hint: Include all operators in precedence function [OK]

Common Mistakes:

• Ignoring missing operators
• Assuming 0 precedence is error
• Confusing precedence values

5. How would you modify the infix to postfix algorithm to correctly handle parentheses in the expression (a+b)*c?

hard

A. Ignore parentheses and treat them as operands

B. Push '(' onto stack and pop operators until '(' when ')' is found

C. Remove all parentheses before conversion

D. Treat '(' as higher precedence operator and never pop it

Solution

1. Step 1: Understand role of parentheses in infix

   Parentheses change normal precedence by grouping expressions to evaluate first.

2. Step 2: Modify stack operations for parentheses

   Push '(' onto stack when encountered. When ')' is found, pop operators until '(' is popped, discarding both parentheses.

3. Final Answer:

   Push '(' onto stack and pop operators until '(' when ')' is found -> Option B

4. Quick Check:

   Parentheses control stack push/pop [OK]

Hint: Push '(' and pop until '(' on ')' [OK]

Common Mistakes:

• Ignoring parentheses in algorithm
• Treating parentheses as operands
• Not popping until '(' on ')'