Why is documentation necessary during program development.?

Answer 1

Documenting a program while it's being developed allows the documentation to fully and clearly reflect the original spirit and intent of the program. Also, writing documentation during development is worlds easier than tacking it on afterward. Documenting an algorithm in (relatively) plain English pseudocode allows it to be thoroughly checked before any code is written, and the pseudocode can easily be translated to programming code that is already documented. For instance, the pseudocode documentation:

// Create a new Foo.
// Twiddle the Foo.
// If twiddling fails:

// Frobnicate.

// Destroy the Foo.

Can be used to produce the already-documented code:

// Create a new Foo.
Foo* foo = new Foo();

// Twiddle the Foo.
foo->twiddle();

// If twiddling fails:
if (foo->twiddling_failed()) {

// Frobnicate.
frobnicate();

} // if

// Destroy the Foo.
delete foo;

Which is eminently readable, maintainable, and just plain awesome.