## HtDP Problem Set | ## Section 9 |

Problem 1:

Develop the function

between?, which consumes three numbers and produces`true`

if the last is between the first two, otherwise it returns`false`

.Use

between?to develop the functionthree-between?, which consumes two numbers and a list of three numbers and determines if all three numbers in the list are between the first two.

Problem 2:

Develop a function that consumes a list of booleans and determines whether one of them is

`true`

. Consider whether`empty`

contains a`true`

element.

Problem 3:

Develop a function that consumes a list of numbers and computes the difference between the number of positive numbers in the list and the number of negative numbers in the list.

Problem 4:

Develop the function

string-append-n*, which consumes a non-empty list of strings and produces a single string resulting from appending all of the strings together. Use the built-in functionstring-append:`String String -> String`

, which appends two strings.Now develop the more general function

string-append*, which consumes any list of strings and produces a single string as described above. This new function must handleemptyin reasonable way.

Problem 5:

Develop the function

order-toy, which consumes a symbol representing a toy, a number representing how many of that particular toy you wish to have in stock, and a list of symbols representing an inventory of toys. It produces the number copies that need to be ordered. Consider using thehow-many-symbolsfunction from Exercise 9.5.2 as a helper.

Jamie Raymond | Matthias Felleisen |

01 december 2003 |