Tutorial by Examples: is

What is SFINAE

SFINAE stands for Substitution Failure Is Not An Error. Ill-formed code that results from substituting types (or values) to instantiate a function template or a class template is not a hard compile error, it is only treated as a deduction failure. Deduction failures on instantiating function templa...

C++ • SFINAE (Substitution Failure Is Not An Error)

Empty message is not printed

When NSLog is asked to print empty string, it omits the log completely. NSString *name = @""; NSLog(@"%@", name); // Resolves to @"" The above code will print nothing. It is a good practice to prefix logs with labels: NSString *name = @""; NSLog(@&quo...

Objective-C Language • Logging

This is confusing. Why not just always use parentheses?

Parentheses are used to enclose the arguments of function calls. Using them for procedure calls can cause unexpected problems. Because they can introduce bugs, both at run-time by passing a possibly unintended value to the procedure, and at compile-time by simply being invalid syntax. Run-time Re...

VBA • Procedure Calls

Lexicographical comparison

Two std::strings can be compared lexicographically using the operators ==, !=, <, <=, >, and >=: std::string str1 = "Foo"; std::string str2 = "Bar"; assert(!(str1 < str2)); assert(str > str2); assert(!(str1 <= str2)); assert(str1 >= str2); assert...

C++ • std::string

Attach object to another existing object (association)

It's possible to attach an object to an existing object as if there was a new property. This is called association and allows one to extend existing objects. It can be used to provide storage when adding a property via a class extension or otherwise add additional information to an existing object. ...

Objective-C Language • Low-level Runtime Environment

List all contributions grouped by author name

git shortlog summarizes git log and groups by author If no parameters are given, a list of all commits made per committer will be shown in chronological order. $ git shortlog Committer 1 (<number_of_commits>): Commit Message 1 Commit Message 2 ... Committer 2 (<number_of_...

Git • Browsing the history

exec statement is a function in Python 3

In Python 2, exec is a statement, with special syntax: exec code [in globals[, locals]]. In Python 3 exec is now a function: exec(code, [, globals[, locals]]), and the Python 2 syntax will raise a SyntaxError. As print was changed from statement into a function, a __future__ import was also added. ...

Python Language • Incompatibilities moving from Python 2 to Python 3

Discovering which virtual environment you are using

If you are using the default bash prompt on Linux, you should see the name of the virtual environment at the start of your prompt. (my-project-env) user@hostname:~$ which python /home/user/my-project-env/bin/python

Python Language • Virtual environments

List packages

Prints all packages, optionally only those whose package name contains the text in <FILTER>. adb shell pm list packages [options] <FILTER> All <FILTER> adb shell pm list packages Attributes: -f to see their associated file. -i See the installer for the packages. -u to ...

Android • adb shell

Granting & revoking API 23+ permissions

A one-liner that helps granting or revoking vulnerable permissions. granting adb shell pm grant <sample.package.id> android.permission.<PERMISSION_NAME> revoking adb shell pm revoke <sample.package.id> android.permission.<PERMISSION_NAME> Granting all run...

Android • adb shell

Iterating over a list

Python supports using a for loop directly on a list: my_list = ['foo', 'bar', 'baz'] for item in my_list: print(item) # Output: foo # Output: bar # Output: baz You can also get the position of each item at the same time: for (index, item) in enumerate(my_list): print('The item i...

Python Language • List

Change list

Let's say you have a simple myblog app with the following model: from django.conf import settings from django.utils import timezone class Article(models.Model): title = models.CharField(max_length=70) slug = models.SlugField(max_length=70, unique=True) author = models.ForeignKe...

Django • Administration

List and Details views

Template views are fine for static page and you could use them for everything with get_context_data but it would be barely better than using function as views. Enter ListView and DetailView app/models.py from django.db import models class Pokemon(models.Model): name = models.CharField(max...

Django • Class based views

Satisfy one of multiple constraints using switch

You can create a tuple and use a switch like so: var str: String? = "hi" var x: Int? = 5 switch (str, x) { case (.Some,.Some): print("Both have values") case (.Some, nil): print("String has a value") case (nil, .Some): print("Int has a value&...

Swift Language • Switch

jQuery UI Sortable - Drop Placeholder

This example of the Sortable using a Placeholder is common usage. Sortable is applied to a group of DOM elements, allowing the user to move items around in the list via Drag'n Drop style actions. <!doctype html> <html lang="en"> <head> <meta charset="utf-8&q...

jQuery UI Library • jquery ui sortable

Lens and Prism

A Lens' s a means that you can always find an a within any s. A Prism' s a means that you can sometimes find that s actually just is a but sometimes it's something else. To be more clear, we have _1 :: Lens' (a, b) a because any tuple always has a first element. We have _Just :: Prism' (Maybe a) a...

Haskell Language • Lens

Latest Value, Value History and Graph

Using Playground it is easy to see that happens inside loops or objects while the change is happening. For example, in the code below, the value of x will change from 1 to 4. import UIKit for x in [1, 2, 3, 4] { x } (1) Clicking on the eye symbol on the right will give us a quick look. (2...

Xcode • Playgrounds

List Files Sorted by Size

The ls command's -S option sorts the files in descending order of file size. $ ls -l -S ./Fruits total 444 -rw-rw-rw- 1 root root 295303 Jul 28 19:19 apples.jpg -rw-rw-rw- 1 root root 102283 Jul 28 19:19 kiwis.jpg -rw-rw-rw- 1 root root 50197 Jul 28 19:19 bananas.jpg When used with the -r o...

Bash • Listing Files

Checking if an object is an Array

Array.isArray(obj) returns true if the object is an Array, otherwise false. Array.isArray([]) // true Array.isArray([1, 2, 3]) // true Array.isArray({}) // false Array.isArray(1) // false In most cases you can instanceof to check if an object is an Array. []...

JavaScript • Arrays

Persisting

In addition to reading data with Eloquent, you can also use it to insert or update data with the save() method. If you have created a new model instance then the record will be inserted; otherwise, if you have retrieved a model from the database and set new values, it will be updated. In this exa...

Laravel • Eloquent