When should comparator === be used over ==?_问答_开发者

So I've been actively programming bot in school and work the past 5 years, but I never tried to find out the difference between == and ===.

I can see the differenc开发者_如何转开发e of a comparator using a single =, it'll look at the value of the left handed variable through the loop, ex:

while($line = getrow(something))

So what's the difference between == and === in statements such as:

if ($var1 === $var2)
//versus
if ($var1 ==  $var2)

Likewise:

if ($var1 !== $var2)
//versus
if ($var1 != $var2)

I have always used double equals, I have never used tripple. The languages I use are :php, vb.net, java, javascript, c/c++.

I'm interested in learning systematically what is going on in a tripple quote that is different than that of a double quote.

When should one be used over another? Thanks for appeasing to my curiosity :)

Typically, == looks at equality of value only. So, for instance...

5 == 5.0  //true

However, === also considers value and type (in the languages I am familiar with).

var five = 5;
var five_float = (float)5.0;

five === 5;        //true - both int, both equal to 5
five_float === 5;  //false - both equal 5 but one is an int and one is a float

FYI, the = operator (usually called the assignment operator) is used to set the value of the left side parameter to the right side. This is pretty obvious. However, in most languages, this will also return true if the assignment is successful. You want to avoid using = where you mean to use == (or ===) because it will look like a comparison, but it's not - and it will return true unexpectedly.

For instance, lets say you want to check if a number is equal to 10...

myNumber = 7;
if(myNumber = 10)
{
    //will always be true and execute this code because myNumber will successfully
    //be assigned the value of 10 instead of checking to see if the number is 10.
    //oops!
}

A final note - this is true in PHP and JavaScript. I don't think there is a === operator in C++ or Java and == has a slightly different meaning as well.

$a === $b TRUE if $a is equal to $b, and they are of the same type. (introduced in PHP 4)

$a !== $b TRUE if $a is not equal to $b, or they are not of the same type. (introduced in PHP 4)

Reference

== will check the value only (equality operator), where === checks the data type as well (strict equality operator).

1 == '1' is true.
1 === '1' is false - the first is an Integer, the second is a String.

1 == true is true.
1 === true is false - the first is an Integer, the second is a Boolean.

Generally you want to use == (equality operator) but sometimes you want to make sure things are of certain types. I'm sure someone can provide an example, I can't think of one off the top of my head, but I've definitely used it.

In PHP and JavaScript (I'm not sure of other languages where the triple === syntax is valid) the difference is that === is a strict comparison. While == is loose. That means that === compares value and type, but == just compares value. A perfect example of this is the buggy PHP code below:

$str = 'Zebraman stole my child\'s pet lime!';

// Search for zebra man
if(strpos($str, 'Zebraman')){
    echo 'The string contains "Zebraman"';
}else{
    echo 'The string doesn\'t contain "Zebraman"';
}

Example Here

Since strpos($str, 'Zebraman') returns 0 (The index of the string Zebraman), and since 0 is falsy. That code will output The string doesn't contain "Zebraman". The correct code uses a strict comparison with false:

$str = 'Zebraman stole my child\'s pet lime!';

// Search for zebra man
if(strpos($str, 'Zebraman') !== false){
    echo 'The string contains "Zebraman"';
}else{
    echo 'The string doesn\'t contain "Zebraman"';
}

Example Here

See the PHP man page on strpos

I don't know if this holds true for all languages but in javascript the === stands for type comparison.