.. index:: assignment, ! delete, lvalue

Operators Involving LValues
===========================

If ``a`` is an LValue (i.e. a variable or something that can be assigned to), the following operators are available as shorthands:

``a += e`` is equivalent to ``a = a + e``. The operators ``-=``, ``*=``, ``/=``, ``%=``, ``|=``, ``&=`` and ``^=`` are defined accordingly. ``a++`` and ``a--`` are equivalent to ``a += 1`` / ``a -= 1`` but the expression itself still has the previous value of ``a``. In contrast, ``--a`` and ``++a`` have the same effect on ``a`` but return the value after the change.

delete
------

``delete a`` assigns the initial value for the type to ``a``. I.e. for integers it is
equivalent to ``a = 0``, but it can also be used on arrays, where it assigns a dynamic
array of length zero or a static array of the same length with all elements set to their
initial value. ``delete a[x]`` deletes the item at index ``x`` of the array and leaves
all other elements and the length of the array untouched. This especially means that it leaves
a gap in the array. If you plan to remove items, a mapping is probably a better choice.

For structs, it assigns a struct with all members reset. In other words, the value of ``a`` after ``delete a`` is the same as if ``a`` would be declared without assignment, with the following caveat:

``delete`` has no effect on mappings (as the keys of mappings may be arbitrary and are generally unknown). So if you delete a struct, it will reset all members that are not mappings and also recurse into the members unless they are mappings. However, individual keys and what they map to can be deleted: If ``a`` is a mapping, then ``delete a[x]`` will delete the value stored at ``x``.

It is important to note that ``delete a`` really behaves like an assignment to ``a``, i.e. it stores a new object in ``a``.
This distinction is visible when ``a`` is reference variable: It will only reset ``a`` itself, not the
value it referred to previously.

::

    pragma solidity >=0.4.0 <0.7.0;

    contract DeleteExample {
        uint data;
        uint[] dataArray;

        function f() public {
            uint x = data;
            delete x; // sets x to 0, does not affect data
            delete data; // sets data to 0, does not affect x
            uint[] storage y = dataArray;
            delete dataArray; // this sets dataArray.length to zero, but as uint[] is a complex object, also
            // y is affected which is an alias to the storage object
            // On the other hand: "delete y" is not valid, as assignments to local variables
            // referencing storage objects can only be made from existing storage objects.
            assert(y.length == 0);
        }
    }