HP 15c - Manual (Page 24)

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Section 2
Numeric Functions
This section discusses the numeric functions of the HP-15C (excluding
statistics and advanced functions). The nonnumeric functions are discussed
separately (digit entry in section 1, stack manipulation in section 3, and
display control in section 5).
The numeric functions of the HP-15C are used in the same way whether
executed from the keyboard or in a program. Some of the functions (such as
a) are, in fact, primarily of interest for programming.
Remember that the numeric functions, like all functions except digit entry
functions, automatically terminate digit entry. This means a numeric
function does not need to be preceded or followed by v.
Pi
Pressing | $ places the first 10 digits of π into the calculator. $
does not need to be separated from other numbers by v.
Number Alteration Functions
The number alteration functions act upon the number in the display
(X-register).
Integer Portion. Pressing | ‘ replaces the number in the display
with the nearest integer of lesser or equal magnitude.
Fractional Portion. Pressing ´ q replaces the number in the display
with its fractional part (that is, the difference between the number and its
integer part).
Rounding. Pressing | & rounds all 10 internally held digits of the
mantissa of the displayed value to the number of digits specified by the
current •, i, or ^ display format.
Absolute Value. Pressing | a yields the absolute value of the
number in the display.