Description
Objectives
1. Defining functions in terms of other functions
2. Solve some recursion problems
3. Use the special forms if and cond
Activities
1. Warm up. Let’s define some functions having to do with circles and spheres.
(a) First thing, let us define a function pi (with no parameters) for π, so whenever we need π
we simply evaluate (pi).
We could define it as equal to a constant, as in the following:
( define ( pi )
3.14)
This is a pretty poor approximation: yours should be better. Hint: Scheme has inverse
trig functions asin, acos, and atan; these will give you the angle (in radians) for a given
sine, cosine, or tangent value. You should be able to calculate π using one of these.
(b) The area of a circle with radius r is πr2
. Define a function (area-of-circle r) that uses
PI that you defined as above.
(c) The surface area of a sphere with radius r is 4πr2
. Define a function for (surface-area-of-sphere
r) that gives the surface area of a sphere with radius r. This function should use your
area-of-circle function.
(d) The volume of a sphere is 4
3
πr3
. Define function for (volume-of-sphere r) that gives the
volume of a sphere with radius r. This function should use your surface-area-of-sphere
function.
2. The first three values of a particular sequence are 1, 2, 3. The remaining values in the sequence
can be calculated as a function of the three preceding values in the sequence sum of the three
preceding values in the sequence as follows:
Sn =
1 if n = 1;
2 if n = 2;
3 if n = 3;
Sn−3 − Sn−2 + Sn−1 otherwise.
So, the fourth value in the sequence would be 1 − 2 + 3 = 2. Write a recursive Scheme function
(s n) that computes the n
th value in the sequence.
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3. Write a recursive function, named zeno, that computes the sum of the first n terms of the
following series from Zeno’s Dichotomy Paradox, Zn =
1
2 +
1
4 +
1
8 + · · · +
1
2n . You may use the
built-in exponentiion function (expt x n) which evaluates to x
n
.
4. We can determine whether a non-negative number is even using the following recursive definition:
(even-nn-int? n) =
#t if n = 0;
#f if n = 1;
(even-nn-int? (- n 2)) otherwise.
(a) Write a recursive function, named even-nn-int? that determines whether a non-negative
number is even or not using the above definition. (Note: do not call it even?, as it would
clash with an existing function. Don’t use the existing even?, odd?, or modulo functions.)
(b) Using even-nn-int?, write a function even-int? that determines whether any integer is
even.
(c) Finally, we define odd-int? that determines whether any integer is odd.
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