CS 245 - Programming Languages

Lab 4

Sorting Go Slices

In this lab you will create and fill a slice containing instances of a struct. Then you will use the sort function in Go to set up sorthing on two distinct fields in the struct.

Setup

  1. Create a struct with 3 fields, an int a float and a string
  2. Create a slice that holds instances of the struct you created.
  3. Put into the slice about 10 instances of your struct; each instance should have values of the fields that are distinct from any other instance. Also, the values in each field should not be in a sorted order.
  4. Print the contents of your slice.

Sorting

Go has a concept of interfaces; they are differently implemented than in Java. Basically you do not have to declare that something implements an interface. Rather, at compile time the program is checked for uns of functions that require implementing an interface., If the required interface is implemented (usually as methods on a type), then the compile succeeds and the program runs.

Sorting requires implementing 3 methods on a type 

func (p S) Len() int {  }
func (p S) Swap(i, j int) {  }
func (p S) Less(i, j int) bool { }
Importantly the type that these methods are implemented on needs to be one that hold a slice. So, for instance, 
				type IntSlice []int
the somewhere else in your code you might have 
				aa := make(IntSlice,0)
Implement the methods and sort your slice instance using the functions in the Go "sort" package. (Do not write your own sort.)

Veryify that the sort works

Now, suppose you want to sort on some other field of your struct. You could do this by adding some branching inside the Less method and having a global variable to indicate the branch to use. But that is really ugly. Worse, it will slow down the sort. Instead, define a new type that is just a renaming of your struct/slice type. Then implement the Len, Swap and Less functions on your new type. Finally, cast your slice from the original type to your new type and sort that thing.

What to hand in

Send to gtowell245@cs.brynmawr.edu a copy of your go program. The program should include everything described above and a main function, that, when run, shows that different sorts can be done on the same slice by only casting the slice to a different types.