Statements - Arangodb

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Lazy statements

The primitives and composite building blocks presented before are sufficiently powerful to construct quite elaborate structures and facilities. We have presented lazy-functions and lazy-operators. How about lazy-statements? First, an appetizer:

Print all odd-numbered contents of an STL container using std::for_each (sample4.cpp):

for_each(c.begin(), c.end(), if_(arg1 % 2 == 1) [ cout << arg1 << ' ' ] );

Huh? Is that valid C++? Read on...

Yes, it is valid C++. The sample code above is as close as you can get to the syntax of C++. This stylized C++ syntax differs from actual C++ code. First, the if has a trailing underscore. Second, the block uses square brackets instead of the familiar curly braces {}.

Here are more examples with annotations. The code almost speaks for itself.

1) block statement:

statement, statement, .... statement

Basically, these are comma separated statements. Take note that unlike the C/C++ semicolon, the comma is a separator put *in-between* statements. This is like Pascal's semicolon separator, rather than C/C++'s semicolon terminator. For example:

statement, statement, statement, // ERROR!

Is an error. The last statement should not have a comma. Block statements can be grouped using the parentheses. Again, the last statement in a group should not have a trailing comma.

statement, statement, ( statement, statement ), statement

Outside the square brackets, block statements should be grouped. For example:

for_each(c.begin(), c.end(), ( do_this(arg1), do_that(arg1) ) );

2) if_ statement:

We have seen the if_ statement. The syntax is:

if_(conditional_expression) [ sequenced_statements ]

3) if_ else_ statement:

The syntax is

if_(conditional_expression) [ sequenced_statements ] .else_ [ sequenced_statements ]

Take note that else has a prefix dot and a trailing underscore: .else_

Example: This code prints out all the elements and appends " > 5", " == 5" or " < 5" depending on the element's actual value:

for_each(c.begin(), c.end(), if_(arg1 > 5) [ cout << arg1 << " > 5\n" ] .else_ [ if_(arg1 == 5) [ cout << arg1 << " == 5\n" ] .else_ [ cout << arg1 << " < 5\n" ] ] );

Notice how the if_ else_ statement is nested.

4) while_ statement:

The syntax is:

while_(conditional_expression) [ sequenced_statements ]

Example: This code decrements each element until it reaches zero and prints out the number at each step. A newline terminates the printout of each value.

for_each(c.begin(), c.end(), ( while_(arg1--) [ cout << arg1 << ", " ], cout << val("\n") ) );

5) do_ while_ statement:

The syntax is:

do_ [ sequenced_statements ] .while_(conditional_expression)

Again, take note that while has a prefix dot and a trailing underscore: .while_

Example: This code is almost the same as the previous example above with a slight twist in logic.

for_each(c.begin(), c.end(), ( do_ [ cout << arg1 << ", " ] .while_(arg1--), cout << val("\n") ) );

6) for_ statement:

The syntax is:

for_(init_statement, conditional_expression, step_statement) [ sequenced_statements ]

It is again almost similar to C++ for statement. Take note that the init_statement, conditional_expression and step_statement are separated by the comma instead of the semi- colon and each must be present (i.e. for_(,,) is invalid).

Example: This code prints each element N times where N is the element's value. A newline terminates the printout of each value.

int iii; for_each(c.begin(), c.end(), ( for_(var(iii) = 0, var(iii) < arg1, ++var(iii)) [ cout << arg1 << ", " ], cout << val("\n") ) );

As before, all these are lazily evaluated. The result of such statements are in fact composites that are passed on to STL's for_each function. In the viewpoint of for_each, what was passed is just a functor, no more, no less.

| Unlike lazy functions and lazy operators, lazy statements always return void. |

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Use, modification and distribution is subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE\_1\_0.txt)