PreIncrement¶

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description¶

PreIncrement[x]

or ++x increases the value of x by 1, returning the new value of x. ++x is equivalent to x = x + 1.

Notes

PreIncrement has attribute HoldFirst. In PreIncrement\[x\], x can be a symbol or a Part expression referring to an existing value. If x has no assigned value, PreIncrement::rvalue is emitted and the expression is left unevaluated.

Examples (3)¶

Every input below was run against the current Mathilda build and its output recorded.

Applications (3)¶

In[1]:= j = 5
Out[1]= 5

In[2]:= ++j
Out[2]= 6

In[3]:= j
Out[3]= 6

Implementation notes¶

builtin_preincrement (++x) calls the shared increment_core(lhs, 1, negate=false, pre=true, "PreIncrement"). The pre=true flag makes it return the new value: it reads the old OwnValue via evaluate, computes and writes back Plus[old, 1] through an evaluated Set (which preserves lvalue shape via Set's HoldFirst), then returns the new value and frees the old. Requires the target to already have an OwnValue, else emits PreIncrement::rvalue and returns NULL. Carries ATTR_HOLDFIRST | ATTR_PROTECTED.

Attributes: HoldFirst, Protected.

References¶

Source: src/core.c
Specification: docs/spec/builtins/assignment-and-rules.md
Tests: tests/test_increment.c

Notes & additional examples¶

Notes¶

++j (PreIncrement) increases j by 1 and returns the new value. It is equivalent to j = j + 1. Contrast with j++, which returns the old value.