# r's and (r-1)'s Complement of Numbers

In this tutorial, we will learn about the r's complement and (r-1)'s complement of the numbers with the help of examples. By Saurabh Gupta Last updated : May 10, 2023

For a number system having its base/radix as **r**, we can define two types of complement for the corresponding number system which are as follows:

## 1) (r-1)'s complement

The **(r-1)'s complement of a number** in any number system with base **r** can be found out by subtracting every single digit of a number by **r-1**.

**For Example:** In the binary number system, the base is **2**. Hence, its **(r-1)'s** i.e., **(2-1 =1)'s** complement can be obtained by subtracting each bit from 1, i.e., **1's** complement for **001** can also be calculated by subtracting **001** from **111** which will be **(111-001) = (110) _{2}**.

Similarly, in the octal number system, the base is **8** so its **7's** complement can be calculated by subtracting each bit by **7**, i.e., **7's** complement for **347** in octal number system can be calculated by subtracting **347** from **777** which will yield **(777 – 347) = (430) _{8}**.

## 2) r's complement

The **r's complement** of a non-zero number in any number system with base **r** can be calculated by adding **1** to the LSB of its **(r-1)'s** complement.

**For Example:** In binary number system, **2's** complement of **001** can be calculated by adding 1 to the LSB of its 1'complement (i.e., **110 + 1**) = **(111) _{2}**.

Similarly, in octal number system, **8's** complement of **347** can be calculated by adding 1 to the LSB of its **7'complement** (i.e., **430 + 1**) = **(431) _{8}**.

### 9's and 10's complement in Decimal Number System

We already know that the decimal number system has its base as 10, As we have already discussed above, 9's complement of decimal number can be found out by subtracting its each by 9.

**Example 1:** Calculate 9's complement of (2457)^{10}

**Solution:** 9999 – 2457 = (7542)^{10}

Now, 10's complement of a decimal number can be calculated by adding 1 to the LSB of the 9's complement.

**Example 2:** Calculate 10's complement of (2457)^{10}

**Solution:** 10's complement for (2457)^{10} is calculated by adding 1 to (7542)^{10} which is the 9's complement. Therefore, 10's complement of (2457)^{10} is (7543)^{10}.

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