Senior .NET Study Portal

Replace Int Without Variable

Excellent interview-style question 💡

You can swap two integer values without using a third variable using either arithmetic operations or bitwise XOR.

Let’s go through both 👇

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🧮 1️⃣ Method 1 — Using Arithmetic (Addition & Subtraction)

int a = 5;
int b = 10;

a = a + b; // a = 15
b = a - b; // b = 5
a = a - b; // a = 10

Console.WriteLine($"a = {a}, b = {b}");

✅ Output:

a = 10, b = 5

⚙️ How it works

• Step 1: a temporarily holds the sum of both.
• Step 2: Subtracting b from sum gives original a.
• Step 3: Subtracting new b gives original b.

⚠️ Caveat

• Risk of integer overflow if a + b exceeds the data type’s range.

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⚡ 2️⃣ Method 2 — Using Bitwise XOR

int a = 5;
int b = 10;

a = a ^ b;
b = a ^ b;
a = a ^ b;

Console.WriteLine($"a = {a}, b = {b}");

✅ Output:

a = 10, b = 5

🧠 How it works

XOR has a neat property:

• x ^ x = 0
• x ^ 0 = x
• x ^ y ^ y = x

So:

1. a = a ^ b → combined info of a & b
2. b = a ^ b → becomes original a
3. a = a ^ b → becomes original b

✅ Pros

• No overflow risk
• Works perfectly for integers

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🔍 3️⃣ Comparison

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🚀 Interview Tip

If asked in an interview, say:

“There are two main ways — arithmetic or bitwise XOR. XOR is safer because it avoids overflow and doesn’t need extra storage.”

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Would you like me to explain why the XOR trick mathematically guarantees that the two values swap back correctly (bitwise reasoning)?