`AddTo` (`+=`)¶

Increments a variable in place.

$ wo 'x = 5; x += 3; x'
8

The whole in-place family — AddTo, SubtractFrom, TimesBy, DivideBy, Increment, Decrement, AppendTo, PrependTo — needs a target that already has a value. One that does not is reported and left alone:

$ wo 'AddTo[5, 1]'

AddTo::rvalue: 5 is not a variable with a value, so its value cannot be changed.
5 += 1

$ wo 'AppendTo[{1, 2}, 3]'

AppendTo::rvalue: {1, 2} is not a variable with a value, so its value cannot be changed.
AppendTo[{1, 2}, 3]

That includes a symbol that has simply never been set:

$ wo 'AppendTo[q, 3]'

AppendTo::rvalue: q is not a variable with a value, so its value cannot be changed.
AppendTo[q, 3]

A Part target works when the location holds something extendable, and is reported against the part expression as written when it does not:

$ wo 'k = {{1}, {2}}; AppendTo[k[[1]], 9]'
{1, 9}

$ wo 'm = {1, 2}; AppendTo[m[[1]], 9]'

AppendTo::normal: Nonatomic expression expected at position 1 in AppendTo[m[[1]], 9].
AppendTo[m[[1]], 9]

A literal operand parses; it is only at evaluation that there turns out to be nothing to modify:

$ wo '5 += 1'

AddTo::rvalue: 5 is not a variable with a value, so its value cannot be changed.
5 += 1

The target may also be a function call — the shape a symbol used as a lookup table or a record has. A definition supplying the starting value is what makes the first += legal:

$ wo 'count[_] := 0; count["a"] += 1; count["a"] += 1; {count["a"], count["b"]}'
{2, 0}

$ wo 'state["pos"] = 0; state["pos"] += 8; state["pos"]'
8

$ wo 'ClearAll[f]; f[1] += 5'

AddTo::rvalue: f[1] is not a variable with a value, so its value cannot be changed.
f[1] += 5

A postfix ++ binds to its operand before juxtaposition, so 2 a++ is 2 (a++):

$ wo 'a = 5; 2 a++'
10

$ wo 'a = 5; a++ 2'
10

Because ++ is a single atomic token, 1++2 is Increment[1] 2 rather than 1 + (+2); separating the signs keeps it arithmetic:

$ wo '1++2'

Increment::rvalue: 1 is not a variable with a value, so its value cannot be changed.
2*1++

$ wo '1 + +2'
3

AddTo (+=)¶

`AddTo` (`+=`)¶