C#•2y ago

✅ Operator ?? functions

I came across ?? operators whilst I try to understand solutions like https://leetcode.com/problems/n-ary-tree-preorder-traversal/solutions/3065645/not-fast-but-good-solution-without-any-recursion/ For example there is this line of code

root.children[0].children = new List<Node>(root.children[0].children ?? new List<Node>()) {root.children[i]};

root.children[0].children = new List<Node>(root.children[0].children ?? new List<Node>()) {root.children[i]};

` In this case the ?? operator is ordering the IDE like "hey, the value root.children[0].children can be the value of root.children at the ith position ?

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9 Replies

rotor_•2y ago

The ?? operator just means "if that was null then try this" So var val = nullableType ?? defaultValueThatIsNotNull; Or if you're feeling adventurous return a ?? b ?? c ?? throw new NullReferenceException(); Although you might not be permitted an exception there let me check that Looks like you can't use an exception there pepehands

Anton•2y ago

here's what the code actually does

var firstChild = root.children[0];
if (firstChild.children is not null)
    firstChild.children = MakeShallowCopy(firstChild.children);
else
    firstChild.children = MakeShallowCopy(new List<Node>(root.children[i]));

var firstChild = root.children[0];
if (firstChild.children is not null)
    firstChild.children = MakeShallowCopy(firstChild.children);
else
    firstChild.children = MakeShallowCopy(new List<Node>(root.children[i]));

it honestly doesn't make much sense

rotor_•2y ago

I disagree. The code reads

root.children[0].children = new List<Node>(
        root.children[0].children ?? new List<Node>()
    ) {
        root.children[i]
    };

root.children[0].children = new List<Node>(
        root.children[0].children ?? new List<Node>()
    ) {
        root.children[i]
    };

i.e.

var firstChild = root.children[0];
var newChildren = firstChild.children is not null
    ? firstChild.children
    : new List<Node>()
var firstChildChildren = new List<Node>(newChildren);

firstChildChildren.Add(root.children[i]);

root.children[0].children = firstChildChildren;

var firstChild = root.children[0];
var newChildren = firstChild.children is not null
    ? firstChild.children
    : new List<Node>()
var firstChildChildren = new List<Node>(newChildren);

firstChildChildren.Add(root.children[i]);

root.children[0].children = firstChildChildren;