Consider:
echo 50 >> 4;
Output:
3
Why does it output 3?
8 Answers
33
votes
50 in binary is 11 0010, shift right by 4 yields 11 which is equal to 3.
See PHP documentation and Wikipedia.
21
votes
As documented on php.org, the >> operator is a bitwise shift operator which shifts bits to the right:
$a >> $b - Shift the bits of $a $b steps to the right (each step means "divide by two")
50 in binary is 110010, and the >> operator shifts those bits over 4 places in your example code. Although this happens in a single operation, you could think of it in multiple steps like this:
- Step 1 -
00011001 - Step 2 -
00001100 - Step 3 -
00000110 - Step 4 -
00000011
Since binary 11 is equal to 3 in decimal, the code outputs 3.
3
votes
3
votes
The >> operator is called a binary right shift operator.
Shifting bits to the right 4 times is the same as dividing by two, four times in a row. The result, in this case would be 3.125. Since 50 is an int, bit shifting will return the floor of this, which is 3.
Put another way, 50 is 0b110010 in binary. Shifted 4 times we have 0b11, which is 3 in decimal.
3
votes
>> is the binary right-shift operator.
Your statement shifts the bits in the numeric value 50 four places to the right. Because all integers are represented in two's complement, this equals 3. An easy way to remember this is that one shift to the right is the same as dividing by 2, and one shift to the left is the same as multiplying by 2.
1
votes
It's called a right shift. 'The bits of the left operand are shifted right by the number of positions of the right operand. The bit positions vacated on the left are filled with the sign bit, and bits shifted out on the right are discarded.'
Information can be found on it here: http://php.comsci.us/etymology/operator/rightshift.php
1
votes
0
votes
For your convenience, one of the fastest ways to calculate the outputted value from a bitwise shift is to multiply or divide by 2.
For example echo 50 >> 4;
Given that this is a bitwise right, it literally means that the value will be decrease, then we can get the output by divide 50 for 2 and 4 times.
echo 50 >> 4; // 50/(2*2*2*2) ~ 3.
Given that (from) 48 -> (to) 63/16(2*2*2*2), the result will be more than 2 and less than 4. Then
echo 48 >> 4; // 48/(2*2*2*2) ~ 3.
echo 63 >> 4; // 63/(2*2*2*2) ~ 3.
However, when bitwise left, the result will be totally different as it multiplies by 2 with n times:
If echo 50 << 4; // 50*(2*2*2*2) ~ 800
If echo 51 << 4; // 51*(2*2*2*2) ~ 816
Live example: https://3v4l.org/1hbJe
