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(emacs-lisp-intro.info)Inc Example parts
The parts of the function definition
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The preceding analysis gives us the bones of our function definition:
first, we will need a variable that we can call `total' that will be
the total number of pebbles. This will be the value returned by the
function.
Second, we know that the function will require an argument: this
argument will be the total number of rows in the triangle. It can be
called `number-of-rows'.
Finally, we need a variable to use as a counter. We could call this
variable `counter', but a better name is `row-number'. That is because
what the counter does is count rows, and a program should be written to
be as understandable as possible.
When the Lisp interpreter first starts evaluating the expressions in
the function, the value of `total' should be set to zero, since we have
not added anything to it. Then the function should add the number of
pebbles in the first row to the total, and then add the number of
pebbles in the second to the total, and then add the number of pebbles
in the third row to the total, and so on, until there are no more rows
left to add.
Both `total' and `row-number' are used only inside the function, so
they can be declared as local variables with `let' and given initial
values. Clearly, the initial value for `total' should be 0. The
initial value of `row-number' should be 1, since we start with the
first row. This means that the `let' statement will look like this:
(let ((total 0)
(row-number 1))
BODY...)
After the internal variables are declared and bound to their initial
values, we can begin the `while' loop. The expression that serves as
the test should return a value of `t' for true so long as the
`row-number' is less than or equal to the `number-of-rows'. (If the
expression tests true only so long as the row number is less than the
number of rows in the triangle, the last row will never be added to the
total; hence the row number has to be either less than or equal to the
number of rows.)
Lisp provides the `<=' function that returns true if the value of
its first argument is less than or equal to the value of its second
argument and false otherwise. So the expression that the `while' will
evaluate as its test should look like this:
(<= row-number number-of-rows)
The total number of pebbles can be found by repeatedly adding the
number of pebbles in a row to the total already found. Since the
number of pebbles in the row is equal to the row number, the total can
be found by adding the row number to the total. (Clearly, in a more
complex situation, the number of pebbles in the row might be related to
the row number in a more complicated way; if this were the case, the
row number would be replaced by the appropriate expression.)
(setq total (+ total row-number))
What this does is set the new value of `total' to be equal to the sum
of adding the number of pebbles in the row to the previous total.
After setting the value of `total', the conditions need to be
established for the next repetition of the loop, if there is one. This
is done by incrementing the value of the `row-number' variable, which
serves as a counter. After the `row-number' variable has been
incremented, the true-or-false-test at the beginning of the `while'
loop tests whether its value is still less than or equal to the value
of the `number-of-rows' and if it is, adds the new value of the
`row-number' variable to the `total' of the previous repetition of the
loop.
The built-in Emacs Lisp function `1+' adds 1 to a number, so the
`row-number' variable can be incremented with this expression:
(setq row-number (1+ row-number))
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