Multiple distinct objects
You are encouraged to solve this task according to the task description, using any language you may know.
Create a sequence (array, list, whatever) consisting of n distinct items of the same type. n should be determined at runtime.
By distinct we mean that if they are mutable, changes to one do not affect all others; if there is an appropriate equality operator they are considered unequal; etc. The code need not specify a particular kind of distinction, but do not use e.g. a numeric-range generator which does not generalize.
This task was inspired by the common error of intending to do this, but instead creating a sequence of nreferences to the same mutable object; it might be informative to show the way to do that as well.
This task mostly makes sense for languages operating in the pass-references-by-value style (most object-oriented or 'dynamic' languages).
C
foo *foos = malloc(n * sizeof(*foos));
for (int i = 0; i < n; i++)
init_foo(&foos[i]);
(Or if no particular initialization is needed, skip that part, or use calloc.)
Common Lisp
The mistake is often written as one of these:
(make-list n :initial-element (make-the-distinct-thing))
(make-array n :initial-element (make-the-distinct-thing))
which are incorrect since (make-the-distinct-thing) is only evaluated once. A common correct version is:
(loop repeat n collect (make-the-distinct-thing))
which evaluates (make-the-distinct-thing) n times and collects each result in a list.
Haskell
If the creator of the distinct thing is in some monad, then one can write
replicateM n makeTheDistinctThing
in an appropriate do block. If it is distinguished by, say, a numeric label, one could write
map makeTheDistinctThing [1..n]
Java
It's not pretty but it gets the job done. The first method here is the one that does the work. The second method is a convenience method so that you can pass in a String of the class name.
public static LinkedList<Object> getNNewObjects(int n, Class c){
LinkedList<Object> ans = new LinkedList<Object>();
try {
for(int i=0;i<n;i++)
ans.add(c.newInstance());
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
return ans;
}
public static LinkedList<Object> getNNewObjects(int n, String className)
throws ClassNotFoundException{
return getNNewObjects(n, Class.forName(className));
}
Python
The mistake is often written as:
[Foo()] * n # here Foo() can be any expression that returns a new object
which is incorrect since Foo() is only evaluated once. A common correct version is:
[Foo() for i in xrange(n)]
which evaluates Foo() n times and collects each result in a list. This last form is also discussed here, on the correct construction of a two dimensional array.
Ruby
The mistake is often written as one of these:
[Foo.new] * n # here Foo.new can be any expression that returns a new object
Array.new(n, Foo.new)
which are incorrect since Foo.new is only evaluated once. A common correct version is:
Array.new(n) { Foo.new }
which evaluates Foo.new n times and collects each result in an Array. This last form is also discussed here, on the correct construction of a two dimensional array.