Write a program to swap two numbers using a third variable.

Exercises · Q14

Q. Write a program to swap two numbers using a third variable.

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Concept understanding — Variable Assignment Order

Variable Assignment Order — First Principles

Imagine you are following a recipe. The recipe says: "Take the number of eggs, call it $x$ . Then take $x$ , add 2, and call the result $y$ ." You would never dream of using $y$ before you have defined $x$ — that would be nonsense. Variable assignment order is simply the rule that a variable must be given a value before it can be used in any expression that defines another variable.

The Intuition

Think of a variable as a labelled box. You cannot take something out of a box that is empty. When you write:

x = 5

y = x + 3

you first put the number 5 into box $x$ , then you look into box $x$ , take out the 5, add 3, and put the result (8) into box $y$ . The order is: define first, use second.

Now consider what happens if you reverse the order:

y = x + 3

x = 5

At the moment you try to compute $y$ , box $x$ is empty. The expression $x + 3$ has no meaning — it is like asking "what is nothing plus 3?" In most programming languages and in mathematical reasoning, this is an error. The variable $x$ has not yet been assigned.

The Precise Statement

Important

Variable Assignment Order: In any sequence of assignments (or definitions), a variable must appear on the left-hand side of an assignment (i.e., be defined) before it appears on the right-hand side of any later assignment (i.e., before it is used). Violating this order produces an undefined variable error.

Formally, for a sequence of statements $S_1, S_2, \dots, S_n$ , if a variable $v$ is used in $S_k$ (on the right-hand side), then there must exist some $S_j$ with $j < k$ where $v$ appears on the left-hand side.

Why This Matters in Exams

In Indian board exams (CBSE, ICSE, state boards), especially in Computer Science and Mathematics, you will encounter questions that test this concept in two ways:

Tracing code: Given a sequence of assignments, you must compute the final values. The order is everything.
Spotting errors: A question may present a jumbled or incorrect sequence and ask you to identify why it fails.

A Worked Example

Consider this sequence:

a = 10
b = a + c
c = 5

At the second line, $c$ is used but has never been assigned. This violates assignment order. The correct order would be:

a = 10
c = 5
b = a + c

Now $b$ becomes $10 + 5 = 15$ .

A Common Pitfall …

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Use a temporary variable to hold one value while the overwrite happens:

num1 = int(input("Enter first number: "))
num2 = int(input("Enter second number: "))
print("Before swapping:", num1, num2)
temp = num1      # keep num1 safe
num1 = num2      # overwrite num1
num2 = temp      # restore the old num1 into num2
print("After swapping: ", num1, num2)
``` …

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Save one value in a temporary variable before overwriting it: temp = num1; num1 = num2; num2 = temp — inputs 5 and 10 print as 10 and 5 after the swap.

The concept is assignment order: an assignment replaces a variable's old value, so writing num1 = num2 first would destroy num1's original value forever. The third variable exists purely to keep a copy of that value alive during the exchange.

# Swap two numbers using a third variable
num1 = int(input("Enter first number: "))
num2 = int(input("Enter second number: "))
print("Before swapping:", num1, num2)

temp = num1      # step 1: park num1's value
num1 = num2      # step 2: num1 takes num2's value
num2 = temp      # step 3: num2 takes the parked value

print("After swapping: ", num1, num2)

Trace with num1 = 5, num2 = 10:

Step	Statement	num1	num2	temp
start	(inputs read)	5	10	—
1	temp = num1	5	10	5
2	num1 = num2	10	10	5

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Q.Write a program to swap two numbers using a third variable.

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