## 6.1 Numeric literals

The simplest form of expression is a literal that denotes a primitive numerical value.

### Integer literals

Integer literals represent integers of type int. Integer literals are written in base 10 without any separators. Integer literals may contain a single negative sign. (The expression --1 is interpreted as the negation of the literal -1.)

The following list contains well-formed integer literals.

0, 1, -1, 256, -127098, 24567898765

Integer literals must have values that fall within the bounds for integer values (see section).

Integer literals may not contain decimal points (.). Thus the expressions 1. and 1.0 are of type real and may not be used where a value of type int is required.

### Real literals

A number written with a period or with scientific notation is assigned to a the continuous numeric type real. Real literals are written in base 10 with a period (.) as a separator and optionally an exponent with optional sign. Examples of well-formed real literals include the following.

0.0, 1.0, 3.14, -217.9387, 2.7e3, -2E-5, 1.23e+3.

The notation e or E followed by a positive or negative integer denotes a power of 10 to multiply. For instance, 2.7e3 and 2.7e+3 denote $$2.7 \times 10^3$$, whereas -2E-5 denotes $$-2 \times 10^{-5}$$.

### Imaginary literals

A number followed by the character i denotes an imaginary number and is assigned to the numeric type complex. The number preceding i may be either a real or integer literal and determines the magnitude of the imaginary number. Examples of well-formed imaginary literals include the following.

1i, 2i, -325.786i, 1e10i, 2.87e-10i.

Note that the character i by itself is not a well-formed imaginary literal. The unit imaginary number must be written as 1i.

### Complex literals

Stan does not include complex literals directly, but a real or integer literal can be added to an imaginary literal to derive an expression that behaves like a complex literal. Examples include the following.

1 + 2i, -3.2e9 + 1e10i

These will be assigned the type complex, which is the result of adding a real or integer and a complex number. They will also function like literals in the sense that the C++ compiler is able to reduce them to a single complex constant at compile time.