GNU Emacs Lisp Reference Manual

3.5: Numeric Conversions

To convert an integer to floating point, use the function float.

Function: float number

This returns number converted to floating point. If number is already a floating point number, float returns it unchanged.

There are four functions to convert floating point numbers to integers; they differ in how they round. These functions accept integer arguments also, and return such arguments unchanged.

Function: truncate number

This returns number, converted to an integer by rounding towards zero.

Function: floor number &optional divisor

This returns number, converted to an integer by rounding downward (towards negative infinity).

If divisor is specified, number is divided by divisor before the floor is taken; this is the division operation that corresponds to mod. An arith-error results if divisor is 0.

Function: ceiling number

This returns number, converted to an integer by rounding upward (towards positive infinity).

Function: round number

This returns number, converted to an integer by rounding towards the nearest integer. Rounding a value equidistant between two integers may choose the integer closer to zero, or it may prefer an even integer, depending on your machine.