C Decimal Precision

 C Decimal Precision (Beginner → Advanced)

Decimal precision in C language means how many digits are stored and displayed after the decimal point.
This depends on:

• the data type (float, double, long double)

• how you print the value (printf format)

• how floating-point numbers are stored internally (IEEE-754)

 Decimal Precision by Data Type

float Precision Example

Output (approx):

•  Extra digits are lost because float can’t store them accurately.

double Precision Example

Output:

• double preserves more digits.

 Controlling Decimal Places in printf() (Most Important)

Syntax

n = number of digits after decimal

Example

• Rounding happens automatically

 Decimal Precision with float vs double

Output (approx):

• Shows precision difference clearly

Why Decimal Precision Is Inexact?

C uses binary floating-point (IEEE-754).
Some decimal numbers cannot be represented exactly in binary.

Famous Example

Output:

• This is normal behavior, not a bug.

 Comparing Floating-Point Numbers

 Wrong Way

 Correct Way (Using Tolerance)

• Always compare using a small epsilon

 Decimal Precision in Calculations

•  Integer division happens before assignment
•  Type casting fixes precision

 Using long double for High Precision

•  Best for scientific & financial calculations

float, double, long double – When to Use?

 Common Mistakes

•  Expecting exact decimal values
•  Comparing floats with ==
•  Using float for money
•  Forgetting format precision in printf()

 Interview Questions (Very Important)

1. Difference between float and double precision?

2. Why 0.1 + 0.2 != 0.3?

3. How to print only 2 decimal places?

4. How to safely compare floating numbers?

5. When to use long double?

 Summary

•  Decimal precision depends on data type
• float ≈ 6 digits, double ≈ 15 digits
•  Use %.nf to control output precision
•  Floating numbers are approximate
•  Never compare floats using ==
•  Critical for accuracy, interviews & real programs