Frequent Questions
2,275,465 questions
2463
votes
23
answers
488k
views
How to make a great R reproducible example
When discussing performance with colleagues, teaching, sending a bug report or searching for guidance on mailing lists and here on Stack Overflow, a reproducible example is often asked and always ...
209
votes
12
answers
4.3m
views
What is a NullPointerException, and how do I fix it?
What are Null Pointer Exceptions (java.lang.NullPointerException) and what causes them?
What methods/tools can be used to determine the cause so that you stop the exception from causing the program ...
6742
votes
44
answers
2.2m
views
How do I return the response from an asynchronous call?
How do I return the response/result from a function foo that makes an asynchronous request?
I am trying to return the value from the callback, as well as assigning the result to a local variable ...
- 820k
2769
votes
27
answers
2.2m
views
How can I prevent SQL injection in PHP?
If user input is inserted without modification into an SQL query, then the application becomes vulnerable to SQL injection, like in the following example:
$unsafe_variable = $_POST['user_input'];
...
2355
votes
36
answers
4.0m
views
RegEx match open tags except XHTML self-contained tags
I need to match all of these opening tags:
<p>
<a href="foo">
But not self-closing tags:
<br />
<hr class="foo" />
I came up with this and wanted to make ...
1866
votes
26
answers
2.5m
views
What is a NullReferenceException, and how do I fix it?
I have some code and when it executes, it throws a NullReferenceException, saying:
Object reference not set to an instance of an object.
What does this mean, and what can I do to fix this error?
4028
votes
36
answers
617k
views
Is floating-point math broken?
Consider the following code:
0.1 + 0.2 == 0.3 -> false
0.1 + 0.2 -> 0.30000000000000004
Why do these inaccuracies happen?
- 45.9k
4226
votes
1
answer
3.4m
views
The Definitive C++ Book Guide and List
This question attempts to collect the few pearls among the dozens of bad C++ books that are published every year.
Unlike many other programming languages, which are often picked up on the go from ...
723
votes
23
answers
4.8m
views
How do I compare strings in Java?
I've been using the == operator in my program to compare all my strings so far.
However, I ran into a bug, changed one of them into .equals() instead, and it fixed the bug.
Is == bad? When should it ...
1830
votes
40
answers
1.0m
views
What is an undefined reference/unresolved external symbol error and how do I fix it?
What are undefined reference/unresolved external symbol errors? What are common causes, and how do I fix and prevent these errors?
- 260k
1373
votes
29
answers
2.2m
views
"Notice: Undefined variable", "Notice: Undefined index", "Warning: Undefined array key", and "Notice: Undefined offset" using PHP
I'm running a PHP script and continue to receive errors like:
Notice: Undefined variable: my_variable_name in C:\wamp\www\mypath\index.php on line 10
Notice: Undefined index: my_index C:\wamp\www\...
2738
votes
14
answers
263k
views
Why shouldn't I use mysql_* functions in PHP?
What are the technical reasons for why one shouldn't use mysql_* functions? (e.g. mysql_query(), mysql_connect() or mysql_real_escape_string())?
Why should I use something else even if they work on ...
- 176k
1673
votes
23
answers
1.1m
views
Event binding on dynamically created elements?
I have a bit of code where I am looping through all the select boxes on a page and binding a .hover event to them to do a bit of twiddling with their width on mouse on/off.
This happens on page ready ...
- 100k
2825
votes
30
answers
414k
views
Should I cast the result of malloc (in C)?
In this question, someone suggested in a comment that I should not cast the result of malloc. i.e., I should do this:
int *sieve = malloc(sizeof(*sieve) * length);
rather than:
int *sieve = (int *) ...
- 65.7k
651
votes
15
answers
417k
views
How can I avoid using Select in Excel VBA?
I've heard much about the understandable abhorrence of using .Select in Excel VBA, but I am unsure of how to avoid using it.
I am finding that my code would be more reusable if I were able to use ...
- 7,304
1920
votes
16
answers
650k
views
How to access the correct `this` inside a callback
I have a constructor function which registers an event handler:
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', function () {
alert(this.data);
...
- 820k
782
votes
21
answers
903k
views
PHP parse/syntax errors; and how to solve them
Everyone runs into syntax errors. Even experienced programmers make typos. For newcomers, it's just part of the learning process. However, it's often easy to interpret error messages such as:
PHP ...
3263
votes
45
answers
489k
views
JavaScript closure inside loops – simple practical example
var funcs = [];
// let's create 3 functions
for (var i = 0; i < 3; i++) {
// and store them in funcs
funcs[i] = function() {
// each should log its value.
console.log("My value:", i);
...
- 548k
3457
votes
39
answers
1.3m
views
What's the problem with "using namespace std;"?
I have heard using namespace std; is wrong, and that I should use std::cout and std::cin directly instead.
Why is this? Does it risk declaring variables that share the same name as something in the ...
- 35.3k
253
votes
5
answers
57k
views
How to make good reproducible pandas examples
Having spent a decent amount of time watching both the r and pandas tags on SO, the impression that I get is that pandas questions are less likely to contain reproducible data. This is something that ...
- 60.6k
918
votes
7
answers
309k
views
Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference
Given the following examples, why is outerScopeVar undefined in all cases?
var outerScopeVar;
var img = document.createElement('img');
img.onload = function() {
outerScopeVar = this.width;
};
img....
- 70.3k
52
votes
1
answer
221k
views
Reference - What does this regex mean?
What is this?
This is a collection of common Q&A. This is also a Community Wiki, so everyone is invited to participate in maintaining it.
Why is this?
regex is suffering from give me ze code type ...
1292
votes
39
answers
286k
views
Reference - What does this error mean in PHP?
What is this?
This is a number of answers about warnings, errors, and notices you might encounter while programming PHP and have no clue how to fix them. This is also a Community Wiki, so everyone is ...
574
votes
31
answers
556k
views
PHP mail function doesn't complete sending of e-mail
<?php
$name = $_POST['name'];
$email = $_POST['email'];
$message = $_POST['message'];
$from = 'From: yoursite.com';
$to = '[email protected]';
$subject = 'Customer ...
- 5,783
617
votes
7
answers
208k
views
Why does jQuery or a DOM method such as getElementById not find the element placed later in markup?
What are the possible reasons for document.getElementById, $("#id") or any other DOM method / jQuery selector not finding the elements?
Example problems include:
jQuery silently failing to ...
- 820k
185
votes
2
answers
32k
views
What is a debugger and how can it help me diagnose problems?
This is intended to be a general-purpose question to assist new programmers who have a problem with a program, but who do not know how to use a debugger to diagnose the cause of the problem.
This ...
- 49.2k
1141
votes
31
answers
1.3m
views
How can I access and process nested objects, arrays, or JSON?
I have a nested data structure containing objects and arrays. How can I extract the information, i.e. access a specific or multiple values (or keys)?
For example:
var data = {
code: 42,
...
- 820k
7743
votes
87
answers
2.8m
views
Is Java "pass-by-reference" or "pass-by-value"?
I always thought Java uses pass-by-reference. However, I read a blog post which claims that Java uses pass-by-value. I don't think I understand the distinction the author is making.
What is the ...
1371
votes
14
answers
221k
views
UTF-8 all the way through
I'm setting up a new server and want to support UTF-8 fully in my web application. I have tried this in the past on existing servers and always seem to end up having to fall back to ISO-8859-1.
Where ...
- 22.6k
64
votes
1
answer
65k
views
Official locator strategies for the webdriver
In the official W3C webdriver documentation, it's clearly stated that the location strategies are:
State Keyword
-----------------------------------------------
CSS selector ...
- 17.2k
2340
votes
20
answers
848k
views
Why can templates only be implemented in the header file?
Quote from The C++ standard library: a tutorial and handbook:
The only portable way of using templates at the moment is to implement them in header files by using inline functions.
Why is this?
(...
- 30.3k
802
votes
13
answers
339k
views
When to use single quotes, double quotes, and backticks in MySQL
I am trying to learn the best way to write queries. I also understand the importance of being consistent. Until now, I have randomly used single quotes, double quotes, and backticks without any real ...
- 29.1k
2325
votes
32
answers
482k
views
How do you parse and process HTML/XML in PHP?
How can one parse HTML/XML and extract information from it?
765
votes
23
answers
1.1m
views
Asking the user for input until they give a valid response
I am writing a program that accepts user input.
#note: Python 2.7 users should use `raw_input`, the equivalent of 3.X's `input`
age = int(input("Please enter your age: "))
if age >= 18:
...
566
votes
19
answers
282k
views
How do I create variable variables?
I know that some other languages, such as PHP, support a concept of "variable variable names" - that is, the contents of a string can be used as part of a variable name.
I heard that this is ...
Pyornide
695
votes
5
answers
103k
views
Why is iostream::eof inside a loop condition (i.e. `while (!stream.eof())`) considered wrong?
I just found a comment in this answer saying that using iostream::eof in a loop condition is "almost certainly wrong". I generally use something like while(cin>>n) - which I guess implicitly ...
- 26.6k
235
votes
3
answers
44k
views
How to use ThreeTenABP in Android Project
I'm using Android Studio 2.1.2 and my Java setup is the following:
>java -version
> openjdk version "1.8.0_91"
> OpenJDK Runtime Environment (build 1.8.0_91-8u91-b14-3ubuntu1~15.10....
- 6,765
850
votes
31
answers
593k
views
How to test multiple variables for equality against a single value?
I'm trying to make a function that will compare multiple variables to an integer and output a string of three letters. I was wondering if there was a way to translate this into Python. So say:
x = 0
y ...
- 8,701
684
votes
6
answers
305k
views
Why is “while( !feof(file) )” always wrong?
What is wrong with using feof() to control a read loop? For example:
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char **argv)
{
char *path = "stdin";
FILE ...
- 214k
575
votes
3
answers
60k
views
What is the difference between client-side and server-side programming?
I have this code:
<script type="text/javascript">
var foo = 'bar';
<?php
file_put_contents('foo.txt', ' + foo + ');
?>
var baz = <?php echo 42; ?>;
...
- 525k
2560
votes
8
answers
402k
views
What is The Rule of Three?
What does copying an object mean?
What are the copy constructor and the copy assignment operator?
When do I need to declare them myself?
How can I prevent my objects from being copied?
- 265k
968
votes
18
answers
83k
views
List of lists changes reflected across sublists unexpectedly
I created a list of lists:
>>> xs = [[1] * 4] * 3
>>> print(xs)
[[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1, 1]]
Then, I changed one of the innermost values:
>>> xs[0][0] = 5
>...
- 20.2k
1499
votes
22
answers
475k
views
How does the "this" keyword work, and when should it be used?
I am looking to find a clear explanation of what the "this" keyword does, and how to use it correctly.
It seems to behave strangely, and I don't fully understand why.
How does this work and ...
- 47.4k
363
votes
25
answers
1.1m
views
What causes a java.lang.ArrayIndexOutOfBoundsException and how do I prevent it?
What does ArrayIndexOutOfBoundsException mean and how do I get rid of it?
Here is a code sample that triggers the exception:
String[] names = { "tom", "bob", "harry" };
for (int i = 0; i <= ...
- 11.7k
4694
votes
38
answers
3.2m
views
How slicing in Python works
How does Python's slice notation work? That is: when I write code like a[x:y:z], a[:], a[::2] etc., how can I understand which elements end up in the slice?
See Why are slice and range upper-bound ...
- 81.2k
972
votes
25
answers
908k
views
Scanner is skipping nextLine() after using next() or nextFoo()?
I am using the Scanner methods nextInt() and nextLine() for reading input.
It looks like this:
System.out.println("Enter numerical value");
int option;
option = input.nextInt(); // Read ...
- 10.6k
795
votes
4
answers
326k
views
SQL injection that gets around mysql_real_escape_string()
Is there an SQL injection possibility even when using mysql_real_escape_string() function?
Consider this sample situation. SQL is constructed in PHP like this:
$login = mysql_real_escape_string(...
- 84.2k
164
votes
1
answer
80k
views
What to do with mysqli problems? Errors such as mysqli_fetch_array(): Argument #1 must be of type mysqli_result
In my local/development environment, the MySQLi query is performing OK. However, when I upload it on my web host environment, I get this error:
Fatal error: Call to a member function bind_param() on ...
- 1,803
1570
votes
14
answers
1.9m
views
How to join (merge) data frames (inner, outer, left, right)
Given two data frames:
df1 = data.frame(CustomerId = c(1:6), Product = c(rep("Toaster", 3), rep("Radio", 3)))
df2 = data.frame(CustomerId = c(2, 4, 6), State = c(rep("Alabama&...
- 25k
631
votes
5
answers
78k
views
How can I pivot a dataframe? [closed]
How do I pivot the pandas dataframe below such that the col values become columns, row values become the index, and mean of val0 becomes the values? (In some cases this is called transforming from ...
- 296k