Multiple distinct objects: Difference between revisions
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(Added Java, changed some code tags to tt tags) |
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<code lisp>(make-list n :initial-element (make-the-distinct-thing)) |
<code lisp>(make-list n :initial-element (make-the-distinct-thing)) |
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(make-array n :initial-element (make-the-distinct-thing))</code> |
(make-array n :initial-element (make-the-distinct-thing))</code> |
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which are incorrect since < |
which are incorrect since <tt>(make-the-distinct-thing)</tt> is only evaluated once. A common correct version is: |
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<code lisp>(loop repeat n collect (make-the-distinct-thing))</code> |
<code lisp>(loop repeat n collect (make-the-distinct-thing))</code> |
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which evaluates <tt>(make-the-distinct-thing)</tt> <var>n</var> times and collects each result in a list. |
which evaluates <tt>(make-the-distinct-thing)</tt> <var>n</var> times and collects each result in a list. |
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in an appropriate do block. If it is distinguished by, say, a numeric label, one could write |
in an appropriate do block. If it is distinguished by, say, a numeric label, one could write |
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<code haskell>map makeTheDistinctThing [1..n]</code> |
<code haskell>map makeTheDistinctThing [1..n]</code> |
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=={{header|Java}}== |
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{{works with|Java|1.5+}} |
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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 <tt>String</tt> of the class name. |
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<code java5>public static LinkedList<Object> getNNewObjects(int n, Class c){ |
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LinkedList<Object> ans = new LinkedList<Object>(); |
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try { |
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for(int i=0;i<n;i++) |
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ans.add(c.newInstance()); |
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} catch (InstantiationException e) { |
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e.printStackTrace(); |
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} catch (IllegalAccessException e) { |
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e.printStackTrace(); |
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} |
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return ans; |
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} |
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public static LinkedList<Object> getNNewObjects(int n, String className) |
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throws ClassNotFoundException{ |
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return getNNewObjects(n, Class.forName(className)); |
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}</code> |
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=={{header|Python}}== |
=={{header|Python}}== |
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The mistake is often written as: |
The mistake is often written as: |
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<code python>[Foo()] * n # here Foo() can be any expression that returns a new object</code> |
<code python>[Foo()] * n # here Foo() can be any expression that returns a new object</code> |
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which is incorrect since < |
which is incorrect since <tt>Foo()</tt> is only evaluated once. A common correct version is: |
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<code python>[Foo() for i in xrange(n)]</code> |
<code python>[Foo() for i in xrange(n)]</code> |
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which evaluates <tt>Foo()</tt> <var>n</var> times and collects each result in a list. This last form is also discussed [[Two-dimensional array (runtime)#Python|here]], on the correct construction of a two dimensional array. |
which evaluates <tt>Foo()</tt> <var>n</var> times and collects each result in a list. This last form is also discussed [[Two-dimensional array (runtime)#Python|here]], on the correct construction of a two dimensional array. |
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<code ruby>[Foo.new] * n # here Foo.new can be any expression that returns a new object |
<code ruby>[Foo.new] * n # here Foo.new can be any expression that returns a new object |
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Array.new(n, Foo.new)</code> |
Array.new(n, Foo.new)</code> |
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which are incorrect since < |
which are incorrect since <tt>Foo.new</tt> is only evaluated once. A common correct version is: |
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<code ruby>Array.new(n) { Foo.new }</code> |
<code ruby>Array.new(n) { Foo.new }</code> |
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which evaluates <tt>Foo.new</tt> <var>n</var> times and collects each result in an Array. This last form is also discussed [[Two-dimensional array (runtime)#Ruby|here]], on the correct construction of a two dimensional array. |
which evaluates <tt>Foo.new</tt> <var>n</var> times and collects each result in an Array. This last form is also discussed [[Two-dimensional array (runtime)#Ruby|here]], on the correct construction of a two dimensional array. |
Revision as of 16:29, 29 January 2009
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.