All Questions
2,211,267
questions
2464
votes
23
answers
468k
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 ...
6708
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 ...
- 810k
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'];
...
2273
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?
3967
votes
36
answers
587k
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?
- 45.2k
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 ...
1812
votes
40
answers
988k
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\...
2722
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
2797
votes
29
answers
392k
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
643
votes
15
answers
401k
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
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);
...
- 810k
778
votes
20
answers
884k
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 ...
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
3406
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
245
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 ...
- 59.6k
52
votes
1
answer
211k
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
542k
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
610
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 ...
- 810k
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,
...
- 810k
1358
votes
13
answers
211k
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
62
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
2297
votes
19
answers
807k
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
794
votes
13
answers
319k
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 ...
- 27.4k
175
votes
2
answers
28k
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 ...
- 48k
2316
votes
32
answers
478k
views
How do you parse and process HTML/XML in PHP?
How can one parse HTML/XML and extract information from it?
234
votes
3
answers
42k
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....
- 6,675
695
votes
5
answers
99k
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 ...
- 26.5k
556
votes
18
answers
269k
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
760
votes
22
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:
...
849
votes
31
answers
568k
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 ...
- 8,681
680
votes
6
answers
297k
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 ...
- 209k
572
votes
3
answers
58k
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; ?>;
...
- 518k
2537
votes
8
answers
386k
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?
- 261k
956
votes
19
answers
80k
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
>...
- 19.9k
361
votes
25
answers
1.0m
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 <= ...
- 11.6k
1496
votes
22
answers
468k
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 ...
- 46.8k
4609
votes
38
answers
3.1m
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 ...
- 80.1k
162
votes
1
answer
77k
views
What to do with mysqli problems? Errors like mysqli_fetch_array(): Argument #1 must be of type mysqli_result and such
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 ...
- 1,783
959
votes
25
answers
876k
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 ...
- 10.5k
788
votes
4
answers
315k
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(...
- 82.8k
1545
votes
14
answers
1.8m
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", 2), rep("Ohio", 1)))
...
- 24.7k
628
votes
5
answers
75k
views
How can I pivot a dataframe? [closed]
What is pivot?
How do I pivot?
Long format to wide format?
I've seen a lot of questions that ask about pivot tables, even if they don't know it. It is virtually impossible to write a canonical ...
- 292k