A floating-point constant consists of:

• Decimal integer
• Decimal point
• Decimal fraction
• e or E and a signed integer exponent (optional)
• Type suffix: f or F or l or L (optional)

You can omit either the decimal integer or the decimal fraction (but not both). You can omit either the decimal point or the letter e (or E) and the signed integer exponent (but not both). These rules allow for conventional and scientific (exponent) notations. 

Negative floating constants are taken as positive constants with the unary operator minus (-) prefixed. 

Here are some examples:

In the absence of any suffixes, floating-point constants are of type double. However, you can coerce a floating constant to be of type float by adding an f or F suffix to the constant. Similarly, the suffix l or L forces the constant to be data type long double. The table below shows the ranges available for float, double, and long double. 

Floating-point constant sizes and ranges