Postfix vs Prefix: Mastering Interviews

A code snippet generating confusion among postfix vs prefix operators in middle level languages.

Ace Your Interviews!

Ever bomb a C interview (or a particularly brutal viva, for all my student warriors out there) because the interviewer decided to unleash a cryptic expression like i++, ++i, ++i, i on you? It seems simple, right, Postfix vs Prefix in the same line? The same variable, is used multiple times in a single line – shouldn’t it just be, well, the same value repeated? Wrong! But yet, what is specified by most standards, is that modifying the same variable more than once in one operation is undefined behaviour in most compilers12? And that’s where the wonderful world of C operator precedence throws a wrench in the works. While the C standard might leave some wiggle room, modifying the same variable within a single expression is a recipe for undefined behaviour in most compilers. (Let’s be honest, who wants undefined behaviour in their code? Not exactly a recipe for interview success!)

Been there, done that, got the marked-down viva score.

In my first semester, this exact topic tripped me up too. These seemingly straightforward expressions can have surprising outputs for those unfamiliar with the intricacies of C operator precedence. Fear not, fellow programmers! This article is your guide to transforming from a confused code cadet into a bonafide operator precedence ninja. By the end of this journey, you’ll be dissecting these expressions with confidence and leaving interviewers (or professors) in awe of your C prowess.

This article dives deep into the world of postfix vs. prefix increment/decrement operators, taking you from the basics to mastery. We’ll break down these operators, explore common interview questions, and equip you with the knowledge to conquer even the trickiest C expressions. So, grab your keyboard, buckle up, and get ready to embark on your operator precedence hero’s journey!


Understanding Postfix vs. Prefix Operators

Ah, the infamous postfix and prefix increment/decrement operators. These little guys can turn a seemingly simple expression into a mind-bending code puzzle for the uninitiated. Buckle up, because we’re about to untangle this operator tango!

The Operators Themselves:

  • Postfix Increment/Decrement (a++, a–): Imagine you’re at a Walmart managing their stock. A customer buys an item, and you need to update the inventory. With postfix decrement (a--), you decrement the stock count (--stock) after noting the current quantity. Think of it like writing down the original number on a notepad before subtracting one. Similarly, postfix increment (a++) would be like noting the current stock (item++) before actually adding a new item to the shelves.
  • Prefix Increment/Decrement (++a, –a): Now, imagine using a fancy automatic inventory system. With prefix increment (++stock), you add one to the stock count (++stock) before registering the sale. This ensures the system always reflects the updated quantity. Prefix decrement (--a) works similarly, subtracting one (--itemCount) before removing an item from the shopping cart.

The Key Difference: Modification Timing

That’s the core difference! Postfix operators modify the variable’s value after the expression is evaluated, while prefix operators do it before. It’s like the difference between checking the current price tag (a) before marking it down (a--) and updating the price first (--a) before displaying it.

Why the Unary Fuss (When We Have =)?

Ah, the mighty unary operator (++ or --). It’s like a single, powerful tool in your coding toolbox. While you could use the assignment operator (=) to increment (x = x + 1;) or decrement (x -= 1;), unary operators offer several advantages:

  1. Shorthand Notation: Unary operators are like code shortcuts. They’re concise and efficient, making your code more readable (especially for those quick increments/decrements).
  2. Compactness: Using ++i is much cleaner than i = i + 1; in tight loops or scenarios where space is a premium.
  3. One-by-One Increments/Decrements: Unary operators are perfect for situations where you need to modify a variable by exactly one, each time.
  4. Potential Speed Optimization (Compiler Dependent): While not always guaranteed, some compilers might optimize the code using unary operators for speedier execution.

So, the next time you encounter these operators in an interview or your code, remember – it’s all about the timing of the modification! With a clear understanding of postfix vs. prefix operators, you’ll be well on your way to mastering C’s operator precedence and writing cleaner, more efficient code.


Mastering the Interview Beast: Decoding C’s Expression Evaluation Mystery

Alright, code warriors, buckle up! We’ve explored the basics of postfix vs. prefix operators, but now it’s time to face the real dragon – interview questions that test your understanding of expression evaluation in C. Here’s where things can get a little…well, mind-boggling.

The Challenge:

Let’s test your coding courage with this infamous interview question: (actually I created this question myself)

int a = 10;
printf ("%d %d %d %d %d", ++a, --a, a++, a++, ++a);
Code: C | © Naman Vrati

Think you have a solid answer in mind? Don’t peek yet! Challenge yourself to explain the output based on your understanding of operator precedence and evaluation order.

Ready for the Reveal? (Brace Yourselves)

The output might surprise you (and roughly 80% of interviewees!):

OUTPUT SPOILER!
13 13 12 11 13

Hold on, what sorcery is this? Why this seemingly random order? The answer lies in the murky world of undefined behaviour. This expression modifies the same variable (a) multiple times within a single printf statement and the C standard allows compilers to take liberties in how they evaluate such expressions. Different compilers might choose different orders, and even the same compiler can be unpredictable across different runs.

Why Learn This Madness?

Even though this is technically undefined behaviour, some interviewers might still ask you to explain the output based on a specific evaluation order. So, fear not, intrepid coder! We’ll equip you with the knowledge to tackle these questions with confidence.


My Personal Recipe for Interview Success (3 Lines or Less!)

Here’s my secret weapon for conquering these confusing expression evaluation questions:

  1. Value Line: Write down the value of a after each operation (prefix/postfix increment/decrement) from right to left (remember, printf arguments are evaluated right-to-left).
  2. Print Line 1: Simulate the first evaluation and printing of printf arguments, going right-to-left again. Remember, postfix operators print the current value before incrementing/decrementing.
  3. Print Line 2 (Fill in the Blanks): Based on the remaining spots in the printf statement, fill them with the final value of a (obtained from step 1) going from left to right.

Let’s see this magic recipe in action!

int a = 10;
printf ("%d %d %d %d %d", ++a, --a, a++, a++, ++a); // a = 10
// Value:                 13   12   13   12   11
// Prints (1):                      12   11
// Prints (2):            13   13             13
// FINAL ANSWER:          13   13   12   11   13
Code: C | © Naman Vrati

Feeling Confident? Let’s Solve Another One!

Now that you’ve mastered this recipe, here’s another challenge to test your skills:

int a = 10; 
printf("%d %d %d %d %d %d %d %d", a, a++, ++a, --a, a--, a++, ++a, a); 
Code: C | © Naman Vrati
Did you guess the answer?
12 11 12 12 12 11 12 12

If so, you’re a coding ninja in the making! If not, no worries, let’s break it down using the same recipe:

int a = 10; 
printf("%d %d %d %d %d %d %d %d", a, a++, ++a, --a, a--, a++, ++a, a); 
// Value:                         12  12   11   10   11  12   11   10
// Prints (1):                        11             12  11   
// Prints (2):                    12       12   12            12   12
// FINAL ANSWER:                  12  11   12   12   12  11   12   12
Code: C | © Naman Vrati

Pro Tips for Mastering Operator Precedence: Conquering the Expression Maze

Feeling like you’ve just navigated a code labyrinth blindfolded? Don’t worry, mastering operator precedence takes practice. Here are some battle-tested tips to help you conquer those interview expressions and write cleaner, more efficient C code:

  • Befriend the Manual: The C Programming Language by Kernighan and Ritchie is your operator precedence bible. Spend some quality time with it to truly understand the rules of the game.
  • Practice Makes Perfect: Don’t just read – experiment! Write your own code examples with different operators and expressions. Test them out, analyze the results, and solidify your understanding.
  • Visualize the Journey: Diagrams and flowcharts can be your allies. Visualize the evaluation order of complex expressions to break them down step-by-step.
  • Online Resources are Your Friends: The internet is a treasure trove of C tutorials and quizzes. Utilize online resources to test your knowledge and learn from others.
  • Seek Help When Lost: Don’t be afraid to ask for clarification! Reach out to experienced programmers or online forums if you get stuck on a particularly perplexing expression.

Conclusion: Own the Expression Arena!

Mastering C operator precedence, especially those tricky postfix vs. prefix shenanigans, is a valuable skill for any programmer. It will not only strengthen your understanding of C fundamentals but also make you a confident warrior in those coding interviews. Remember, the key lies in understanding the evaluation order, operator associativity, and how these operators interact with other elements in your code. So, keep practising, stay curious, and conquer those coding challenges with the power of operator precedence at your fingertips!

Before going, let me leave you on a cliffhanger! Try to solve this question, it is a bit different and let’s say has its own style! Leave down the answer to this question in the comments section with an explanation of why!

int a = 10;
a = --a + --a + --a;
printf("%d", a); // ?
Code: C | © Naman Vrati

P.S. And hey, if you encounter an undefined behaviour question in an interview, use my 3-line recipe to break it down and showcase your problem-solving skills! Even if the exact output might vary, understanding the thought process behind evaluating the expression will leave a positive impression on the interviewer.

  1. Undefined behaviour of Postfix vs Prefix increment operators – StackOverflow. ↩︎
  2. Printing Postfix and Prefix increment operations – StackOverflow. ↩︎

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