All Questions

2468
votes
23answers
379k 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
12answers
3.5m 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 ...
6252
votes
42answers
1.8m views

How to return the response from an asynchronous call

I have a function foo which makes an asynchronous request. How can I return the response/result from foo? I am trying to return the value from the callback, as well as assigning the result to a local ...
2773
votes
28answers
2.0m 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']; ...
1912
votes
37answers
3.3m 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 these: <br /> <hr class="foo" /> I came up with this and wanted to make sure I've got it right. I am ...
1873
votes
27answers
1.8m 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?
3519
votes
31answers
437k 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?
724
votes
23answers
4.3m 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 ...
4235
votes
1answer
2.7m 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 ...
1310
votes
30answers
2.0m views

"Notice: Undefined variable", "Notice: Undefined index", 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\...
1673
votes
37answers
746k 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 to fix/prevent them?
2638
votes
13answers
242k 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 ...
731
votes
20answers
727k 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 ...
1675
votes
23answers
995k 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 ...
2630
votes
28answers
293k views

Do I cast the result of malloc?

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(int) * length); rather than: int *sieve = (int *) ...

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