What Is the Purpose of the -M Switch

What is the purpose of a 'switch 0' statement in C?

switch(0) will always execute the block of code associated with the case 0: block; still, here there's no actually executed code - both cases are empty.

The point here is to make the compiler angry at compile time if the asserted expression (a) is not verified: in this case, the expanded macro will have two case 0: - the one provided explicitly, and the one that uses the result of the asserted expression (so, 0 in case it failed); this results in a switch with two identical case, which is not allowed and makes the compiler stop with an error at compile time.

This will also fail if the passed expression is not a constant evaluated at compile time (as you cannot have runtime-determined case values), which is also expected from a static_assert.

What exactly is switch used for in React Router?

Since you are new am going to take a bit more time to explain with examples and also add some few things about it you may want to have handy.

So like Iddler said, Switch is more or less like the "Switch case" condition in any other languages but usually ends with the first match it finds.

<Switch>
    <Route path="/home" component={Home} />
    <Route path="/about" component="{About} />
</Switch>

That is an example of its most basic use. Switch determines the start and end of the block or condition. Each Route checks for the current path. supposing we were working on "www.test.com". All of "www.test.com" is the root "/". So the Route checks for the path after the root. so if you had "www.test.com/home", "/home" comes after the root so the "Home" component will be loaded in our example above and if you had "www.test.com/about" the "About" component is loaded.

Be mindful that you can use any names. the components and paths do not need to be the same.

There are also cases when you might want to use exact to match an exact path. this is useful when you have similar paths. eg "/shop" and "/shop/shoes". using exact ensures Switch matches exact paths and not just the first.

Eg:

<Switch>
    <Route exact path="/shop" component={Shop} />
    <Route exact path="shop/shoes" component="{Shoes} />
</Switch>

You can also use <Route... /> without the <Switch>.

Eg:

<Route path="/home" component={Home} />

so unlike direct component loads where you just load a component like <Home /> Routers work with the URLs.

Lastly, the <Route... /> path can take arrays of url to load same component.

Eg:

<Switch>
    <Route path={[ "/home", "/dashboard", "/house", /start" ]} component={Home} />
    <Route exact path={[ "/about", "/about/management", "/about/branches" ]} component="{About} />
</Switch>

I hope this helps. Let me know if you need clarifications of any sort. :)

UPDATE:

You are not required to write Routers in this same format always. below is another format you could use;

<Router>
    <Switch>
      <Route path="/home">
        <Home />
      </Route>
      <Route path="/about">
        <About />
      </Route>
    </Switch>
</Router>

There are instances like am in now where you want to be able to handle what shows when a wrong URL is entered. like a 404page. you could use Router without a path. so like a regular switch statement, that becomes your default.

<Switch>
    <Route path="/home" component={Home} />
    <Route path="/about" component="{About} />
    <Route component="{PageNotFound} />
</Switch>

What is the purpose of boolean switch statements in JavaScript?

Yes, there's a difference. Taking your example into account,

var a = 0,
    b = 1;

Now let's look at the switch statement:

switch (a || b) {

When this switch statement is run, the expression a || b is evaluated. || is a short-circuit operator, it will return the left operand's value if it's "truthy", else it will return the right operand's value. In this case, a = 0, so b will be returned (1). Now look at the case statement:

case true:

When evaluating case statements, no type coercion is performed on either value and strict equality is assumed. In our example, this is the same as writing 1 === true, so the code following the case statement is never run. So let's take a look at the if statement:

if (a || b) {

For an if statement, the conditional expression a || b is evaluated and then the result is converted to a boolean. Internally, this looks like ToBoolean(a || b). Since a || b evaluates to 1 and coercion of 1 to a boolean is true, the condition passes and the block executes.

A better equivalent would be:

if ((a || b) === true) {
    // do some stuff
}
else {
    // do other stuff
}

As already pointed out, in situations where there are many cases and types could vary, a switch statement could be useful. Such a situation would be rare, however.

How to use switch with extern constants?

No. switch only works with fully defined integral type constants (including enum members and classes that unambiguously cast to integral type). here is a link to an old reference of MSDN, but what is says is still valid.

This link that I provided in a comment to another answer explains what optimizations compilers may perform to assembly code. If this was delayed to the linking step, it would not be easily possible.

You should therefore use if..else if in your case.

Why should we use svn switch?

Well, the SVN Switch functionality is not that useless.

There might be times when you have huge codebase located in multiple branches of a release.

Consider a scenario:

You have a branch named Release X and for some purpose, you create another branch Release X.1 from it.

If a developer wants to work with X.1 Branch now, he has two options:

Checkout the whole branch and the complete codebase (Time consumuing?)
SVN Switch on Release X working copy to point to Release X.1 (Considering there are no huge changes) - Works like a charm!
Once finished with the work, switch back seamlessly to previous branch...

This was one of the scenario ... What happens when you relocate your repository? The whole URL changes. Switching is intelligent rather checkout.

I believe I have made my point :)