Statements

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.

Copyright © 2001-2002 Joel de Guzman

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)