Just wanted to know if someone can explain the difference between these two conditionals:
if ( !object )
if ( object == null )
where object is an instance of a user-defined class.
I'm sure that these two cannot be used in an interchangeable manner, or are they?
Thanks.
The effect is in practice the same, so I guess you could say they're interchangeable.
In a boolean context (such as a conditional), an expresion is evaluated to either true or false.
In Actionscript 3.0, the following values evaluate to false:
false
null
0
NaN
"" (the empty string)
undefined
void
Everything else evaluates to true.
A reference to an user-defined class instance can either be null or not null.
So, in this case:
if ( object == null )
Obviously, the condition is met only if object is null.
In this other case:
if ( !object )
The expression object will evaluate to false if object is null. If it is null, the expression is false. Since this is in turn negated, the final value will be true and so the condition will be satisfied. So, just like in the first case, if object is null, the condition is met. And like in the first case, again, if object is not null, the condition is not met.
There's no other option if your variable is typed to a user-defined class; such a variable can only contain a valid reference or null; i.e. it can't hold any value evaluable to false in a boolean context, except for null; so, again, it's either null or not null. Which is why both code samples have the same effect.
The first is making a boolean comparison. If the object is false, the not(!) operation will make the condition true, if the object has a value other than false the statement will fail.
The second conditional is evaluating if the object has the value of null or not.
The reason these may be interchangeable is that various languages allow some equivalence between 0, false, null (or "\0") and other values of similar meaning.
I do not know actionscript, but testing equivalence of false, null, 0 etc., or reading the docs on boolean values, will be of some benefit.
Sure not :)
The first one means that the proposition is true only if different from the object;
The second one is true only if the object equals to null.
"!" means "is not the object"
"==" means that the the object has to have the value equal to the one at the right of the symbol
Related
I have two parameters , let's say P1 and P2. The sample expression I used for P2 is
IIF(P1.Label="string", "null" ,Split(P1.Label," ").GetValue(0))
When the condition is false, the split expression is working fine. But if the condition is true, I'm getting 'Index was outside the bounds of the array' error. If the condition is true, I need to pass the value "null" as varchar type.
Can someone please advice on this?
The problem with the IIF function is that it is a function not a language construct. This means that it evaluates both parameters before passing the parameters to the function. Consequently, if you are trying to do a Split on a parameter that can't be split, you will still get the 'Index was outside the bounds of the array' error, even when it looks like that code shouldn't be executed due to boolean condition of the IIF statement.
The best way to solve this is to create a safe string splitter function in custom code where you can use real language constructs. Also, check that the string is splittable by checking it contains a space instead of checking for a special string:
Public Function SafeSplit(ByVal SplitThis As String) As String
If InStr(SplitThis, " ") Then
Return Split(SplitThis, " ")(0)
End If
Return "null"
End Function
and then use this in your report for the Value expression instead of IIF:
=Code.SafeSplit(Parameters!P1.Label)
I am modifying a test method from single parameter to multiple:
#ParameterizedTest
#NullSource
#ValueSource({"foo", "bar"..})
void shouldReturnFalse(String x) {
assertThat(someMethod(x)).isFalse();
}
#ParameterizedTest
#CsvSource({
"null, null",
"foo, bar"
})
void shouldReturnFalse(String x, String y) {
assertThat(someMethod(x, y)).isFalse();
}
null here is passed in as a String literal instead of a null literal. As a result, this test fails. This test previously works with single argument with #NullSource, but it gives following error when switched to multiple:
org.junit.jupiter.api.extension.ParameterResolutionException: No ParameterResolver registered for parameter...
I couldn't find a workaround for this and the solutions I saw were rather hacky and cumbersome. Is there a easier way for providing values with null?
#CsvSource has an attribute called nullValues.
See the documentation.
A list of strings that should be interpreted as null references.
#CsvSource(value= {"null, null",
"foo, bar"}
, nullValues={"null"})
The other option is to simply don't pass any value as stated in the previously linked documentation.
Please note that unquoted empty values will always be converted to null references regardless of the value of this nullValues attribute; whereas, a quoted empty string will be treated as an emptyValue().
#CsvSource({",",
"foo, bar"})
You can use nullValues as explained above by #Jan Schmitz, or you can just discard null values like this:
#CsvSource({
",",
"foo, bar"
})
In ActionScript 3, you can check if a value exists like this:
if (object) {
trace("This object is not null, undefined, or empty!")
}
I frequently use this as a shorthand for if (object != null)
Is there a proper term for evaluating objects for null in this fashion? I suppose it's a matter of the Boolean typecasting rules for the language but I'm not sure if there's a name for the resulting syntax.
If your object is always an actual object reference, e.g. a variable of any object type, then checking if (object) is a valid way to test for nulls. If it's a property of variant type, or a dynamic property that can potentially contain simple values (ints, strings etc) then the proper way to test for null will be explicit conversion, probably even with a strict type check if (object !== null).
Like most computer languages, Ecma-script supports Boolean data types;
values which can be set to true or false. In addition, everything in
JavaScript has an inherent Boolean value, generally known as either
truthy or falsy
list of falsy values (non truthy)
false
0 (zero)
"" (empty string)
null
undefined
NaN (a special Number value meaning Not-a-Number!)
regularly for checking if a variable is null
you may use : (a == null) this statement returns true if a is null, But also return true for all of the above list, because they'r all falsy values and (a == undefined) returns true, even a is not undefined but null or 0 or false.
so you should use Identity operator in this case. following evaluations just returns true when a is null
(typeof a === "null")
// or
(a === null)
I want to save a true/false in my MySQL database. I'm saving 1/0 in an INT column to do this. When I select it, I get the 1 or 0, but I want it to return true/false to my PHP code, without having to rewrite the database.
Can I use another column type? Should I save it differently?
Update: My question is about not wanting to rewrite the returned value. I'm getting a lot of results from my database. Many of those are true/false, but some are 0s because the price is 0, so I don't want to universally rewrite all 1s and 0s. I also don't want to manually rewrite 10 columns.
To follow up my comment, here's a more detailed response which also covers the PHP side, although this probably belongs on StackOverflow.
I've always just used tinyint, although you can use bool/boolean which are synonyms for tinyint(1)
However as of MySQL 5.0.3 you can use the bit type:
As of MySQL 5.0.3, the BIT data type is used to store bit-field values. A type of BIT(M) enables storage of M-bit values. M can range from 1 to 64.
Next, assuming you have an active column, perhaps to store if a user is active, you could use PHP's automatic type conversion to handle this quite simply.
// Obviously you'd replace this with your database call
$results = [['active' => 1], ['active' => 0]];
foreach($results as $row) {
if ($row['active'] == true) {
echo "true\n";
}
else {
echo "false\n";
}
}
You don't strictly need to do anything.
PHP does not, and can not, use strongly typed variables. So, if you receive an (int) 1 from your query results, you can simply use this 1 as a boolean without rewriting or changing anything.
$intOne = (int) 1; //explicitly treat the variable as an integer
var_dump((bool) $intOne); //treat the variable as a boolean
When used in any boolean context, like if ($variable)... then any of these types will be considered to be false by PHP:
the boolean FALSE itself
the integer 0 (zero)
the float 0.0 (zero)
the empty string, and the string "0"
an array with zero elements
an object with zero member variables (PHP 4 only)
the special type NULL (including unset variables)
SimpleXML objects created from empty tags
... And, most importantly;
Every other value is considered TRUE (including any resource).
Source: PHP Manual > Booleans (english)
So while you can change the storage type of your column in mysql, this won't really change the way PHP handles the variable retrieved from your results at all.
Historically, I've always used a column of type TINYINT(1) to store boolean values in mysql, and as Tom Green points out, recent mysql versions provide a new BIT type, which might be appropriate. To the best of my knowledge, mysql does not currently have an actual boolean data type.
You could just as easily use a column of type VARCHAR(1), though, because PHP can and will use any value as a boolean, thanks to the glorious, majestic, and sometimes maddening, PHP Type Juggling.
If you're trying to use the values you're retrieving for boolean logic, just use the values you receive from mysql like booleans and it will work:
if ($valueFromResults) {
//The value was something like true
} else {
//The value was something like false
}
If you're trying to actually echo out the words "true" and "false", then you're probably best served by explicitly echoing the words out yourself, like this;
if ($valueFromResults) {
echo "true";
} else {
echo "false";
}
or, in my preferred shorthand;
echo ($valueFromResults) ? "true" : "false" ;
Update You mentioned in a comment that you want to pass the values through json_encode() and use them in javascript.
JavaScript treats any real value, like int 1, as true and any empty value, like int 0, or an empty string, as false. So if your json_encode() output gets used in actual JavaScript, the int values will still work as boolean values. So the integer values from your database should still work as they are.
Just check that your integer results are encoded as integers by PHP and not as strings - they should be encoded correctly by default - because "0" == true in javascript, but 0 == false.
For a boolean value (true/false), you should use the mySql type bit or tinyint(1).
$boolean = $mysql_data ? true : false;
I wanted to look for records where a certain string field was not blank or null, so I simply wrote SELECT ... FROM myTable WHERE x, assuming that blank and null strings would evaluate to false, but that doesn't appear to be the case.
The string "02306" is true, whereas "N06097EIP" is somehow false.
What's going on?
Edit: I'm aware of the workarounds, I simply want to know how the casting works.
In these expression string are first converted to numbers. "02306" is converted to 2306 which is >0 and therefore considered true, while "N06097EIP" (starting with non-digit) is converted to 0, which is evaluated as false.
Compare results of:
select convert("N06097EIP",signed)
and
select convert("02306",signed)
In a boolean context, such as
WHERE x
the expression x will be evaluated as an integer value. Note that MySQL considers a BOOLEAN as a numeric type.
http://dev.mysql.com/doc/refman/5.1/en/numeric-type-overview.html
It doesn't matter what type the expression x is; it's either an INTEGER type, or it will be converted to an INTEGER type, according to the documented conversion rules.
The end result is that the expression x will be evaluated to be either NULL, integer zero, or integer non-zero.
And those correspond to the boolean "truthiness" values of NULL, FALSE and TRUE.
The reason '02306' is considered TRUE is because this string converts to integer value 2306, which is non-zero.
The reason 'N06097EIP' is considered FALSE is because this string converts to integer value 0.
To can run a simple test case to verify:
SELECT IF( 0 = 'N06097EIP', 'is zero', 'is not zero')
SELECT 0 = 'N06097EIP'
The behavior you observe is entirely expected. It's all pretty straightforward. You may have been caught unawares, because the normative pattern is for us to avoid this type of evaluation and to instead use "workarounds" (as you put it) to return a boolean.
Don't try to be too cute about this with syntactic shortcuts. If nothing else, it's always harder on the next developer who has to figure out what you were doing.
Just spell out what you want.
SELECT *
FROM myTable
WHERE x IS NOT NULL AND x <> '';
or, if you'd prefer:
SELECT *
FROM myTable
WHERE COALESCE(x, '') <> '';