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日期:2020-08-18 10:49

Test whether the function works for x = numeric(0) and for any other numeric

vector x.

3. The Body Mass Index (BMI) is used to determine a person’s corpulence. It is calculated

using the formula:

BMI = Weight (kg)

Height2

(m)

(a) [4 marks] Write an R function that calculates your BMI and round it to 3 decimal

places.

(b) [6 marks] Consider the following BMI and weight categories:

?????

Underweight if BMI < 18.5

Normal weight if BMI ∈ [18.5, 25)

Overweight if BMI ∈ [25, 33)

Obesity if BMI ≥ 33

Write an R function which takes the result of (3a) as a single argument BMI, a

and returns the corresponding category. For example:

> weight.category(21.133)

This corresponds to category: normal weight

4. Consider an asymmetric random walk when the probability of going one step up is 0.55

and the probability of going one step down is 0.45. Let Xn denotes the n-th step in this

random walk. Then, X1, X2, · · · are independent and identically random variables with

Xn =

(

+1 w.p. 0.55

?1 w.p. 0.45

Assume that {Dn}n≥0 is a sequence showing the distance from the origin (starting

point). So,

D0 = 0

Dn = Dn?1 + Xn for n = 1, 2, · · ·

(a) [6 marks] Write an R function which takes the number of steps as its single argument

and returns a vector of distances (for instance shows a sequence of realization

of distances D0, D1, · · · , D10 for the argument n = 10).

Hint: sample() function is helpful.

(b) [6 marks] By using the function in (4a) write a new R function (with single

argument) which generates one realization of the walk (starting from the origin)

and counts the total number of steps taken in the right hand side of the origin

2

(for instance, one example could be the case of starting from the origin with the

sequence of steps

X1 = +1, X2 = ?1, X3 = ?1, X4 = +1, X5 = ?1, X6 = +1.

Then, the sequence of distances is

D0 = 0, D1 = 1, D2 = 0, D3 = ?1, D4 = 0, D5 = ?1, D6 = 0.

Here, your function should returns 2 as the number of steps taken in the right hand

side of the origin).

3


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