Java String - 1 is not 1?

I'm a c++ programmer who wrote some php server code to be translated to java and pretty new to Java. But it feels quite common coming from c++.
I have a server up and running and this code:

String req = "1";
Document doc = loadXMLFromString (req);
Element root = doc.getDocumentElement();    // "xml"
// check version
Element e = firstChildElement (root, "version");
String result = e.getTextContent(); // returns "1"!
String expected = "1";
if (result != expected)
{
    out.printLn ("wrong version: (" + result + "), expected: (" + expected + ")!");
    return;
}

This prints "wrong version: (1), expected (1)". The same holds with if (result != "1").
Now even the debugger (eclipse) shows that result is actually "1", so I'm truly lost here.
I seem to be missing the obvious but just can't see it...can you help pls? Thanks!

c++ - Why does returning a floating-point value change its value?

The following code raises the assert on Red Hat 5.4 32 bits but works on Red Hat 5.4 64 bits (or CentOS).

On 32 bits, I must put the return value of millis2seconds in a variable, otherwise the assert is raised, showing that the value of the double returned from the function is different from the one that was passed to it.

If you comment the "#define BUG" line, it works.

Thanks to @R, passing the -msse2 -mfpmath options to the compiler make both variants of the millis2seconds function work.

/*
 * TestDouble.cpp
 */

#include 
#include 

#include 

static double millis2seconds(int millis) {
#define BUG
#ifdef BUG
    // following is not working on 32 bits architectures for any values of millis
    // on 64 bits architecture, it works
    return (double)(millis) / 1000.0;
#else
    //  on 32 bits architectures, we must do the operation in 2 steps ?!? ...

    // 1- compute a result in a local variable, and 2- return the local variable
    // why? somebody can explains?
    double result = (double)(millis) / 1000.0;
    return result;
#endif
}

static void testMillis2seconds() {
    int millis = 10;
    double seconds = millis2seconds(millis);


    printf("millis                  : %d\n", millis);
    printf("seconds                 : %f\n", seconds);
    printf("millis2seconds(millis)  : %f\n", millis2seconds(millis));
    printf("seconds <  millis2seconds(millis)  : %d\n", seconds < millis2seconds(millis));
    printf("seconds >  millis2seconds(millis)  : %d\n", seconds > millis2seconds(millis));
    printf("seconds == millis2seconds(millis)  : %d\n", seconds == millis2seconds(millis));

    assert(seconds == millis2seconds(millis));
}


extern int main(int argc, char **argv) {
    testMillis2seconds();
}

Answer

With the cdecl calling convention, which is used on Linux x86 systems, a double is returned from a function using the st0 x87 register. All x87 registers are 80-bit precision. With this code:

static double millis2seconds(int millis) {
    return (double)(millis) / 1000.0;

};

The compiler calculates the division using 80-bit precision. When gcc is using the GNU dialect of the standard (which it does by default), it leaves the result in the st0 register, so the full precision is returned back to the caller. The end of the assembly code looks like this:

fdivrp  %st, %st(1)  # Divide st0 by st1 and store the result in st0
leave
ret                  # Return

With this code,

static double millis2seconds(int millis) {
    double result = (double)(millis) / 1000.0;
    return result;
}

the result is stored into a 64-bit memory location, which loses some precision. The 64-bit value is loaded back into the 80-bit st0 register before returning, but the damage is already done:

fdivrp  %st, %st(1)   # Divide st0 by st1 and store the result in st0
fstpl   -8(%ebp)      # Store st0 onto the stack
fldl    -8(%ebp)      # Load st0 back from the stack
leave
ret                   # Return

In your main, the first result is stored in a 64-bit memory location, so the extra precision is lost either way:

double seconds = millis2seconds(millis);

but in the second call, the return value is used directly, so the compiler can keep it in a register:

assert(seconds == millis2seconds(millis));

When using the first version of millis2seconds, you end up comparing the value that has been truncated to 64-bit precision to the value with full 80-bit precision, so there is a difference.

On x86-64, calculations are done using SSE registers, which are only 64-bit, so this issue doesn't come up.

Also, if you use -std=c99 so that you don't get the GNU dialect, the calculated values are stored in memory and re-loaded into the register before returning so as to be standard-conforming.

.net - Regular expression for parsing links from a webpage?

I'm looking for a .NET regular expression extract all the URLs from a webpage but haven't found one to be comprehensive enough to cover all the different ways you can specify a link.

And a side question:

Is there one regex to rule them all? Or am I better off using a series of less complicated regular expressions and just using mutliple passes against the raw HTML? (Speed vs. Maintainability)

Answer

((mailto\:|(news|(ht|f)tp(s?))\://){1}\S+)

I took this from regexlib.com

[editor's note: the {1} has no real function in this regex; see this post]

bash - In the shell, what does " 2>&1 " mean?

In a Unix shell, if I want to combine stderr and stdout into the stdout stream for further manipulation, I can append the following on the end of my command:

2>&1

So, if I want to use head on the output from g++, I can do something like this:

g++ lots_of_errors 2>&1 | head

so I can see only the first few errors.

I always have trouble remembering this, and I constantly have to go look it up, and it is mainly because I don't fully understand the syntax of this particular trick.

Can someone break this up and explain character by character what 2>&1 means?

Answer

File descriptor 1 is the standard output (stdout).
File descriptor 2 is the standard error (stderr).

Here is one way to remember this construct (although it is not entirely accurate): at first, 2>1 may look like a good way to redirect stderr to stdout. However, it will actually be interpreted as "redirect stderr to a file named 1". & indicates that what follows is a file descriptor and not a filename. So the construct becomes: 2>&1.

php - How to delete an array element based on key?

Possible Duplicate:
How to delete an element from an array in php?

For instance,

Array(      
    [0] => Array
        (
            [0] => hello
            [1] => open
        )

    [1] => Array
        (

            [0] => good
            [1] => center
        )

    [2] => Array
        (
            [0] => hello
            [1] => close
        )
)

I want to delete the element which key is 1, after the operation:

Array(
    [0] => Array
        (
            [0] => hello
            [1] => open
        )


    [2] => Array
        (
            [0] => hello
            [1] => close
        )
)

Answer

PHP

unset($array[1]);

r - How to create pair-wise data.frame

I have a data.frame with sample data from multiple sites and years. I would like to create a new "pairwise" data.frame that takes creates three columns, Site_Year, Value_2008, Value_Year, where Value_Year is the value for any other year in the dataset. For example, in my sample dataset the site "Anjan" is visited in 2008, 2009, 2010. The new data.frame to have two records for this site:

Site_Year Value_2008 Value_Year
Anjan.2009 0.11 0.96
Anjan.2010 0.11 -0.25

If there is a site that was sampled before 2008 the results would look like:

Site_Year Value_2008 Value_Year
Ljungdalen.2005 0.09 -0.40

Ljungdalen.2009 0.09 0.32
Ljungdalen.2011 0.09 -0.00

Ultimately, I am going to plot these results so that I can look at changes in these (mean) values between years and test them to see if they are significantly different.

Sample data:

WW_Site_Year_Data <- structure(list(Site_Year = c("Anjan.2008", "Anjan.2009", "Anjan.2010",
"Fittjebodarna.2008", "Fittjebodarna.2009", "Flatruet.2008",

"Flatruet.2009", "Flatruet.2010", "Flatruet.2011", "Fotingen.2008",
"Fotingen.2009", "Gestvallen.2008", "Gestvallen.2009", "Glen.2008",
"Glen.2009", "Glen.2010", "Gräftåvallen.2008", "Gräftåvallen.2009",
"Hallbodarna.2008", "Hallbodarna.2009", "Ljungdalen.2005", "Ljungdalen.2008",
"Ljungdalen.2009", "Ljungdalen.2011", "Ljungris.2008", "Ljungris.2009",
"Ljungris.2010", "Mårdsund.2008", "Mårdsund.2009", "Mårdsund.2010",
"Öster_Galåbodarna.2008", "Öster_Galåbodarna.2009", "Ramundberget.2008",
"Ramundberget.2009", "Rätan.2008", "Rätan.2009", "Rätan.2010",
"Särvfjället.2008", "Särvfjället.2009", "Storulvån.2008", "Storulvån.2009",
"Storulvån.2010", "Tångböle.2005", "Tångböle.2008", "Tångböle.2009",

"Tossåsen.2008", "Tossåsen.2009", "Vålådalen.2008", "Vålådalen.2009",
"Vålådalen.2010", "Vemdalsskalet.2002", "Vemdalsskalet.2008",
"Vemdalsskalet.2009"), Site_Name = c("Anjan", "Anjan", "Anjan",
"Fittjebodarna", "Fittjebodarna", "Flatruet", "Flatruet", "Flatruet",
"Flatruet", "Fotingen", "Fotingen", "Gestvallen", "Gestvallen",
"Glen", "Glen", "Glen", "Gräftåvallen", "Gräftåvallen", "Hallbodarna",
"Hallbodarna", "Ljungdalen", "Ljungdalen", "Ljungdalen", "Ljungdalen",
"Ljungris", "Ljungris", "Ljungris", "Mårdsund", "Mårdsund", "Mårdsund",
"Öster_Galåbodarna", "Öster_Galåbodarna", "Ramundberget", "Ramundberget",
"Rätan", "Rätan", "Rätan", "Särvfjället", "Särvfjället", "Storulvån",

"Storulvån", "Storulvån", "Tångböle", "Tångböle", "Tångböle",
"Tossåsen", "Tossåsen", "Vålådalen", "Vålådalen", "Vålådalen",
"Vemdalsskalet", "Vemdalsskalet", "Vemdalsskalet"), Year = c("2008",
"2009", "2010", "2008", "2009", "2008", "2009", "2010", "2011",
"2008", "2009", "2008", "2009", "2008", "2009", "2010", "2008",
"2009", "2008", "2009", "2005", "2008", "2009", "2011", "2008",
"2009", "2010", "2008", "2009", "2010", "2008", "2009", "2008",
"2009", "2008", "2009", "2010", "2008", "2009", "2008", "2009",
"2010", "2005", "2008", "2009", "2008", "2009", "2008", "2009",
"2010", "2002", "2008", "2009"), Value = c(0.112816109860291,

0.960290707474735, -0.257326331130005, 0.216427465038733, 0.956767099330118,
0.0526510364211729, 0.588996350906268, -0.465285757216318, -0.0437472490447301,
0.535560060909972, 0.389645985829418, 0.89391173396597, 0.240894790643034,
0.469695915206932, 0.251017199176266, -0.208323019946377, 0.0849050148677196,
0.184296870311739, -0.107309010266098, 0.36349491505071, -0.396608387512831,
0.0931374765423872, 0.32514153209616, -0.00466699456138238, 0.336789804880864,
0.806764888899387, -0.13955949253251, 0.108716189818012, 0.0978498660969545,
-0.160915801270199, 0.623151244760041, 0.713080174849265, -0.279733235253308,
0.427277452192635, 0.296839613563375, 0.659552054627706, 0.356154916318252,
0.659219370597927, 0.825734934055685, 0.183577169158829, 1.31979282562961,

0.0574714990570691, -0.517636804796056, 0.478950613513035, 0.504156229919797,
0.915335741962761, 1.11129338570452, 0.205145037964442, 0.829430613619136,
-0.187573312149385, -0.646545555669656, -0.034670665219269, 1.68156971430668
)), .Names = c("Site_Year", "Site_Name", "Year", "Value"), row.names = c(1L,
2L, 3L, 4L, 5L, 6L, 7L, 8L, 9L, 10L, 11L, 12L, 13L, 14L, 15L,
16L, 17L, 18L, 19L, 20L, 21L, 22L, 23L, 24L, 25L, 26L, 27L, 28L,
29L, 30L, 31L, 32L, 33L, 34L, 35L, 36L, 37L, 38L, 39L, 40L, 41L,
42L, 43L, 44L, 45L, 48L, 49L, 50L, 51L, 52L, 53L, 54L, 55L), class = "data.frame")

Answer

A very useful package in this situation is plyr:

ddply(WW_Site_Year_Data, .(Site_Name), transform,
      Value_2008 = if (2008 %in% Year) Value[Year==2008] else NA)

If records with no Value for 2008 are excluded from the data frame you could use

ddply(WW_Site_Year_Data, .(Site_Name), transform,
      Value_2008 = Value[Year==2008])

It you want to use base R, it's uglier:

unsplit(lapply(split(WW_Site_Year_Data, WW_Site_Year_Data$Site_Name),
        transform, Value_2008 = if (2008 %in% Year) Value[Year==2008] else NA),
        WW_Site_Year_Data$Site_Name)

javascript - Why console.log((!+[]+[]+![]).length) gives 9?

React Native/Javascript - How to know that object does not have the property?

Let say :

myVariable.color;
myVariable.height;
myVariable.width;

But sometimes, myVariable only have "color" and "height" property.

I have try this, but no luck :

if(myVariable.width == undefined) {
//but not filtered here
}

How to know if myVariable doesn't contain "width" property by code, is it possible ?

What does [:-1] mean/do in python?

Working on a python assignment and was curious as to what [:-1] means in the context of the following code: instructions = f.readline()[:-1]

Have searched on here on S.O. and on Google but to no avail. Would love an explanation!

Answer

It slices the string to omit the last character, in this case a newline character:

>>> 'test\n'[:-1]
'test'

Since this works even on empty strings, it's a pretty safe way of removing that last character, if present:

>>> ''[:-1]
''

This works on any sequence, not just strings.

For lines in a text file, I’d actually use line.rstrip('\n') to only remove a newline; sometimes the last line in the file doesn’t end in a newline character and using slicing then removes whatever other character is last on that line.

Cant connect my database with php

$con = mysqli_connect("localhost" "root" "") or die("Unable to connect");
 mysqli_select_db("logindb", $con);

 ?>

This is the configuration of the connecting.

require 'config.php';
 ?>

This is where is connect the database to a registration page.

Can you see something wrong? This is the error i get:
Parse error: syntax error, unexpected '"root"' (T_CONSTANT_ENCAPSED_STRING)

Thanks,
Realcookie

Answer

You are missing the commas bro. Here is the corrected one

$con = mysqli_connect("localhost", "root", "") or die("Unable to connect");
 mysqli_select_db($con, 'logindb'); 
 ?>

What is the yield keyword used for in C#?

In the How Can I Expose Only a Fragment of IList<> question one of the answers had the following code snippet:

IEnumerable