Ch8 Breaking Down Giant Expressions

1. Another Creative Way to Simplify Expressions

 

void AddStats(const Stats& add_from, Stats* add_to) {

 add_to->set_total_memory(add_from.total_memory() + add_to->total_memory());
 add_to->set_free_memory(add_from.free_memory() + add_to->free_memory());
 add_to->set_swap_memory(add_from.swap_memory() + add_to->swap_memory());
 add_to->set_status_string(add_from.status_string() + add_to->status_string());
 add_to->set_num_processes(add_from.num_processes() + add_to->num_processes());
 ...

 

 }

 

 After ten seconds of careful scrutiny, you might realize that each line is doing the same thing,
 just to a different field each time:

 

 add_to->set_XXX(add_from.XXX() + add_to->XXX())

 

 In C++, we can define a macro to implement this:

 

 void AddStats(const Stats& add_from, Stats* add_to) {
 #define ADD_FIELD(field) add_to->set_##field(add_from.field() + add_to->field())
 ADD_FIELD(total_memory);
 ADD_FIELD(free_memory);
 ADD_FIELD(swap_memory);
 ADD_FIELD(status_string);
 ADD_FIELD(num_processes);
 ...
 #undef ADD_FIELD
 }

 

2. Breaking Down Giant Statements

 

var update_highlight = function (message_num) {

    if ($("#vote_value" + message_num).html() === "Up") {
        $("#thumbs_up" + message_num).addClass("highlighted");
        $("#thumbs_down" + message_num).removeClass("highlighted");
    } else if ($("#vote_value" + message_num).html() === "Down") {
        $("#thumbs_up" + message_num).removeClass("highlighted");
        $("#thumbs_down" + message_num).addClass("highlighted");
    } else {
        $("#thumbs_up" + message_num).removeClass("highighted");
        $("#thumbs_down" + message_num).removeClass("highlighted");
    }
};

 

//his is also an instance of the DRY—Don’t Repeat Yourself—principle):

var update_highlight = function (message_num) {

    var thumbs_up = $("#thumbs_up" + message_num);
    var thumbs_down = $("#thumbs_down" + message_num);
    var vote_value = $("#vote_value" + message_num).html();

    var hi = "highlighted";

    if (vote_value === "Up") {
        thumbs_up.addClass(hi);
        thumbs_down.removeClass(hi);
    } else if (vote_value === "Down") {
        thumbs_up.removeClass(hi);
        thumbs_down.addClass(hi);
    } else {
        thumbs_up.removeClass(hi);
        thumbs_down.removeClass(hi);
    }
};

 

3. Explaining Variables

 

Here is an example:
    if line.split(':')[0].strip() == "root":
...
Here is the same code, now with an explaining variable:
    username = line.split(':')[0].strip()
    if username == "root":

 

4. Summary Variables

 

var CleanReply = function (request,  reply)
{
    //For example, consider the expressions in this code:
    if (request.user.id == document.owner_id) {
        // user can edit this document...
    }

    if (request.user.id != document.owner_id) {
        // document is read-only...
    }

    //stated more clearly by adding a summary variable:
    var user_owns_document = (request.user.id == document.owner_id);

 

    if (user_owns_document) {
        // user can edit this document...
    }

    if (!user_owns_document) {
        // document is read-only...
    }

}

 

5.Using De Morgan’s Laws (positive is easy to readable)

 

var Contains = function (request,  reply)
{
/*    1) not (a or b or c) ⇔ (not a) and (not b) and (not c)
    2) not (a and b and c) ⇔ (not a) or (not b) or (not c)*/

    //You can sometimes use these laws to make a boolean expression more readable. For instance,
    //if your code is:
    if (!(file_exists && !is_protected)) Error("Sorry, could not read file.");

    //It can be rewritten to:
    if (!file_exists || is_protected) Error("Sorry, could not read file.");
}