Basics of C Programming

⯈Basic C Programs

Algorithm and Flowchart to swap two numbers

Swapping, exchange the values of two variables. The swapping is used in many algorithms, to name we use swapping in selection sort, bubble sort, etc.

The input to this algorithm is two numbers. Let the two numbers be A and B. After swapping is performed, the contents of A and B has to be exchanged.


Let A = 10 and B = 20

After swapping,

A = 20 and B = 10

There are two methods with which we can swap. (Actually three, the last one is using bitwise XOR, which we will discuss in next chapter)

Method 1: Using temporary variable.

Let, A = 10 and B = 20

swap without extra variable

If we write A = B and B = A, this will produce the wrong answer. Because the contents of A will be overwritten with B, and the value of A is lost. 

Check the diagram below for more detailed explanation. 

Note: A variable can only store one value at a given point of time, if we store the new value the previous value will be overwritten, variables say "Chhodo kal Ki Baatein"
Values of A and B for swap

The value of A becomes 20, and the value 10 is lost. Therefore, if we write B = A now, B will also be 20.

It means the value of A is formatted by the statement A = B. Before we format, we take the back up of A, so that we can use the value of A later to store in B.

Therefore, we write,

temp = A [taking back-up of variable A]

swap logic part 1

Now we can overwrite the value of A, since the backup is taken.

A = B

swap logic using temp

At last, we can clearly see that B should get the value of temp.

B = temp

swap using temp

At end we see that the values of A and B are swapped.


Name of Algorithm: To swap two numbers.

Step 1: Start

Step 2: Read two numbers as A and B.

Step 3: temp = A

             A = B

             B = temp

Step 4: Display A and B

Step 5: Stop


Flowchart to swap two numbers

In the next section we will see how to swap two integers without using extra variable.

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