All Questions
2,211,585
questions
2464
votes
23
answers
469k
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
4.2m
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 ...
6709
votes
42
answers
2.1m
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 ...
- 811k
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'];
...
2275
votes
37
answers
3.8m
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 ...
1867
votes
26
answers
2.3m
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?
3969
votes
36
answers
589k
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?
- 45.3k
4229
votes
1
answer
3.2m
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
4.7m
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 ...
1815
votes
40
answers
990k
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?
- 257k
1363
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\...
2723
votes
14
answers
260k
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 ...
- 174k
1674
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 ...
- 98.7k
2798
votes
30
answers
393k
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 *) ...
- 65k
1914
votes
16
answers
636k
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);
...
- 811k
778
votes
20
answers
885k
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 ...
643
votes
15
answers
402k
views
How to 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 re-usable if I were able to use ...
- 7,194
3248
votes
45
answers
484k
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);
...
- 543k
3407
votes
41
answers
1.2m
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 ...
- 34.8k
916
votes
7
answers
305k
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....
- 69.9k
246
votes
5
answers
55k
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 ...
- 59.7k
52
votes
1
answer
212k
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 ...
1286
votes
39
answers
281k
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 ...
568
votes
30
answers
543k
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 ...
- 5,723
611
votes
7
answers
202k
views
Why does jQuery or a DOM method such as getElementById not find the element if I put the script element near the top of the page?
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 ...
- 811k
7750
votes
91
answers
2.7m
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 ...
1135
votes
32
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,
...
- 811k
1358
votes
13
answers
212k
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 ...
- 22.4k
63
votes
1
answer
60k
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 ...
- 16.9k
2300
votes
19
answers
808k
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?
(...
- 29.8k