Frequent Questions
2,273,809 questions
2464
votes
23
answers
483k
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 ...
6738
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 ...
- 818k
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'];
...
2324
votes
36
answers
3.9m
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
2.4m
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?
4017
votes
35
answers
609k
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?
- 45.8k
4228
votes
1
answer
3.3m
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 ...
1828
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?
- 259k
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\...
2736
votes
14
answers
262k
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 ...
- 175k
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 ...
- 99.7k
2820
votes
30
answers
408k
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 *) ...
- 65.5k
649
votes
15
answers
413k
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 ...
- 7,274