HTDP problem 9.5.5

Develop the function convert. It consumes a list of digits and produces
the corresponding number. The first digit is the least significant, and
so on.

Ok I know the formula for converting 123 would be 1 x 10 expt 0 + 2 x
10 expt 1 + 3 x 10 expt 2.

But I can't conceive a programmatic solution (based on the material
taught up to that point) that does not involve passing the expt to
apply as a parameter.

wooks wrote:

> Develop the function convert. It consumes a list of digits and produces=

> the corresponding number. The first digit is the least significant, and=

> so on.
>=20
> Ok I know the formula for converting 123 would be 1 x 10 expt 0 + 2 x
> 10 expt 1 + 3 x 10 expt 2.
>=20
> But I can't conceive a programmatic solution (based on the material
> taught up to that point) that does not involve passing the expt to
> apply as a parameter.

1234 =3D 4 + 10*( 3 + 10*( 2 + 10*( 1 ) ) )

--=20
Jens Axel S=F8gaard

wooks writes:

> Develop the function convert. It consumes a list of digits and
> produces the corresponding number. The first digit is the least
> significant, and so on.
>
> Ok I know the formula for converting 123 would be 1 x 10 expt 0 + 2
> x 10 expt 1 + 3 x 10 expt 2.

A list of digits, least significant first, would be (3 2 1) or maybe
(#\3 #\2 #\1). 123 is already a number, written with most significant
digit first.

> But I can't conceive a programmatic solution (based on the material
> taught up to that point) that does not involve passing the expt to
> apply as a parameter.

This is a simple recursion. To convert (5 1 4 1 3), convert the tail
to 3141, combine with the head to get 31415. Can you see how I got
31415 from 3141 and 5? Can you see what value is needed for () to get
3 out of (3)?

I didn't check HtDP, but very little is needed here. Don't use apply.
If you have fold, work it out with that. It would be from right, with
the above base case and recursion rule as arguments.