Tutorial by Examples: u

A macro can call itself, like a function recursion: macro_rules! sum { ($base:expr) => { $base }; ($a:expr, $($rest:expr),+) => { $a + sum!($($rest),+) }; } Let's go though the expansion of sum!(1, 2, 3): sum!(1, 2, 3) // ^ ^~~~ // $a $rest => 1 + sum!(2, 3)...

A macro can produce different outputs against different input patterns: /// The `sum` macro may be invoked in two ways: /// /// sum!(iterator) /// sum!(1234, iterator) /// macro_rules! sum { ($iter:expr) => { // This branch handles the `sum!(iterator)` case $iter.f...

(All of these are unstable, and thus can only be used from a nightly compiler.) log_syntax!() #![feature(log_syntax)] macro_rules! logged_sum { ($base:expr) => { { log_syntax!(base = $base); $base } }; ($a:expr, $($rest:expr),+) => { { log_syntax!(a = $...

Fortran 2003 introduced intrinsic modules which provide access to special named constants, derived types and module procedures. There are now five standard intrinsic modules: ISO_C_Binding; supporting C interoperability; ISO_Fortran_env; detailing the Fortran environment; IEEE_Exceptions, IEEE_...

extern crate serde; extern crate serde_json; macro_rules! enum_str { ($name:ident { $($variant:ident($str:expr), )* }) => { #[derive(Clone, Copy, Debug, Eq, PartialEq)] pub enum $name { $($variant,)* } impl ::serde::Serialize for $name {...

Say we have an enum DayOfWeek: enum DayOfWeek { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY; } An enum is compiled with a built-in static valueOf() method which can be used to lookup a constant by its name: String dayName = DayOfWeek.SUNDAY.name(); assert dayName.equal...

It's good practice to document your work for later use, especially if you are coding for a dynamic workload. Good comments should explain why the code is doing something, not what the code is doing. Function Bonus(EmployeeTitle as String) as Double If EmployeeTitle = "Sales" Then ...

The <label> element is used to reference a form action element. In the scope of User Interface it's used to ease the target / selection of elements like Type radio or checkbox. <label> as wrapper It can enclose the desired action element <label> <input type="checkb...

Given a sequence of steps we use repeatedly, it's often handy to store it in a function. Pipes allow for saving such functions in a readable format by starting a sequence with a dot as in: . %>% RHS As an example, suppose we have factor dates and want to extract the year: library(magrittr) ...

You should verify every received string as being valid UTF-8 before you try to store it or use it anywhere. PHP's mb_check_encoding() does the trick, but you have to use it consistently. There's really no way around this, as malicious clients can submit data in whatever encoding they want. $str...

If your application transmits text to other systems, they will also need to be informed of the character encoding. In PHP, you can use the default_charset option in php.ini, or manually issue the Content-Type MIME header yourself. This is the preferred method when targeting modern browsers. hea...

JSFiddle: https://jsfiddle.net/UnsungHero97/80qod7aL/2/ HTML <div class="box_shadow"></div> CSS .box_shadow { background-color: #1C90F3; width: 200px; height: 100px; margin: 50px; } .box_shadow:after { content: ""; width: 190px; height: ...

The following code will print the arguments to the program, and the code will attempt to convert each argument into a number (to a long): #include <stdlib.h> #include <stdio.h> #include <errno.h> #include <limits.h> int main(int argc, char* argv[]) { for (int ...

A unique index prevents the insertion of duplicated data in a table. NULL values can be inserted in the columns that form part of the unique index (since, by definition, a NULL value is different from any other value, including another NULL value) -- Creates a unique index for column 'name' in tabl...

There is a limit to the depth of possible recursion, which depends on the Python implementation. When the limit is reached, a RuntimeError exception is raised: def cursing(depth): try: cursing(depth + 1) # actually, re-cursing except RuntimeError as RE: print('I recursed {} times!'....

Suppose we want to write a factory function that accepts an arbitrary list of arguments and passes those arguments unmodified to another function. An example of such a function is make_unique, which is used to safely construct a new instance of T and return a unique_ptr<T> that owns the instan...

Defining a member block inside a resource creates a route that can act on an individual member of that resource-based route: resources :posts do member do get 'preview' end end This generates the following member route: get '/posts/:id/preview', to: 'posts#preview' # preview_post_p...

If you are building a function that may be heavy on the processor (either clientside or serverside) you may want to consider a memoizer which is a cache of previous function executions and their returned values. This allows you to check if the parameters of a function were passed before. Remember, p...

# Create a sample DF df = pd.DataFrame(np.random.randn(5, 3), columns=list('ABC')) # Show DF df A B C 0 -0.467542 0.469146 -0.861848 1 -0.823205 -0.167087 -0.759942 2 -1.508202 1.361894 -0.166701 3 0.394143 -0.287349 -0.978102 4 -0.160431 1.054736 -0.785250 ...

The specifier override has a special meaning in C++11 onwards, if appended at the end of function signature. This signifies that a function is Overriding the function present in base class & The Base class function is virtual There is no run time significance of this specifier as is mainl...