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doc: add a section about input parameters and output parameters

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Yoichi Hirai 2016-11-23 14:47:58 +01:00
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docs/control-structures.rst
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@ -2,6 +2,53 @@
Expressions and Control Structures
##################################
.. index:: ! parameter, parameter;input, parameter;output
Input Parameters and Output Parameters
======================================
Like in Javascript and in C, functions may take parameters as input;
unlike in Javascript and C, they may also return arbitrary number of
parameters as output.
Input Parameters
----------------
The input parameters are declared type followed
by name (though unused input parameters do not need names).
For example, suppose we want our contract to
accept one kind of external calls with two integers, we would write
something like::
contract Simple {
function taker(uint _a, uint _b) {
// do something with _a and _b.
}
}
Output Parameters
-----------------
The output parameters can be declared with the same syntax after the
``returns`` keyword. For example, suppose we wished to return two results:
the sum and the product of the two given integers, then we would
write::
contract Simple {
function arithmetics(uint _a, uint _b) returns (uint o_sum, uint o_product) {
o_sum = _a + _b;
o_product = _a * _b;
}
}
The names of output parameters can be omitted.
Return parameters are initialized to zero; if they are not explicitly
set, they stay to be zero.
Input parameters and output parameters can be used as expressions in
the function body. There, they are also usable in the left-hand side
of assignment.
.. index:: if, else, while, do/while, for, break, continue, return, switch, goto
Control Structures
@ -16,7 +63,15 @@ Parentheses can *not* be omitted for conditionals, but curly brances can be omit
around single-statement bodies.
Note that there is no type conversion from non-boolean to boolean types as
there is in C and JavaScript, so ``if (1) { ... }`` is *not* valid Solidity.
there is in C and JavaScript, so ``if (1) { ... }`` is *not* valid
Solidity.
Returning Multiple Values
-------------------------
When a function has multiple output parameters, ``return (v0, v1, ...,
vn)`` can return multiple values. The number of components must be
the same as the number of output parameters.
.. index:: ! function;call, function;internal, function;external