12.3 Floating-Point Constants

A floating-point constant must have either a decimal point, an exponent-of-ten, or both; they distinguish it from an integer constant.

To indicate an exponent, write ‘e’ or ‘E’. The exponent value follows. It is always written as a decimal number; it can optionally start with a sign. The exponent n means to multiply the constant’s value by ten to the nth power.

Thus, ‘1500.0’, ‘15e2’, ‘15e+2’, ‘15.0e2’, ‘1.5e+3’, ‘.15e4’, and ‘15000e-1’ are six ways of writing a floating-point number whose value is 1500. They are all equivalent in principle.

Here are more examples with decimal points:

1.0
1000.
3.14159
.05
.0005

For each of them, here are some equivalent constants written with exponents:

1e0, 1.0000e0
100e1, 100e+1, 100E+1, 1e3, 10000e-1
3.14159e0
5e-2, .0005e+2, 5E-2, .0005E2
.05e-2

A floating-point constant normally has type double. You can force it to type float by adding ‘f’ or ‘F’ at the end. For example,

3.14159f
3.14159e0f
1000.f
100E1F
.0005f
.05e-2f

Likewise, ‘l’ or ‘L’ at the end forces the constant to type long double.

You can use exponents in hexadecimal floating constants, but since ‘e’ would be interpreted as a hexadecimal digit, the character ‘p’ or ‘P’ (for “power”) indicates an exponent.

The exponent in a hexadecimal floating constant is an optionally signed decimal integer that specifies a power of 2 (not 10 or 16) to multiply into the number.

Here are some examples:

0xAp2        // 40 in decimal
0xAp-1       // 5 in decimal
0x2.0Bp4     // 32.6875 decimal
0xE.2p3      // 113 decimal
0x123.ABCp0  // 291.6708984375 in decimal
0x123.ABCp4  // 4666.734375 in decimal
0x100p-8     // 1
0x10p-4      // 1
0x1p+4       // 16
0x1p+8       // 256

See Floating-Point Data Types.