Consider this code:
x = 1 # 0001
x << 2 # Shift left 2 bits: 0100
# Result: 4
x | 2 # Bitwise OR: 0011
# Result: 3
x & 1 # Bitwise AND: 0001
# Result: 1
I can understand the arithmetic operators in Python (and other languages), but I never understood 'bitwise' operators quite well. In the above example (from a Python book), I understand the left-shift but not the other two.
Also, what are bitwise operators actually used for? I'd appreciate some examples.
Bitwise operators are operators that work on multi-bit values, but conceptually one bit at a time.
AND is 1 only if both of its inputs are 1, otherwise it's 0.
OR is 1 if one or both of its inputs are 1, otherwise it's 0.
XOR is 1 only if exactly one of its inputs are 1, otherwise it's 0.
NOT is 1 only if its input is 0, otherwise it's 0.
These can often be best shown as truth tables. Input possibilities are on the top and left, the resultant bit is one of the four (two in the case of NOT since it only has one input) values shown at the intersection of the inputs.
AND | 0 1 OR | 0 1 XOR | 0 1 NOT | 0 1
----+----- ---+---- ----+---- ----+----
0 | 0 0 0 | 0 1 0 | 0 1 | 1 0
1 | 0 1 1 | 1 1 1 | 1 0
One example is if you only want the lower 4 bits of an integer, you AND it with 15 (binary 1111) so:
201: 1100 1001
AND 15: 0000 1111
------------------
IS 9 0000 1001
The zero bits in 15 in that case effectively act as a filter, forcing the bits in the result to be zero as well.
In addition, >> and << are often included as bitwise operators, and they "shift" a value respectively right and left by a certain number of bits, throwing away bits that roll of the end you're shifting towards, and feeding in zero bits at the other end.
So, for example:
1001 0101 >> 2 gives 0010 0101
1111 1111 << 4 gives 1111 0000
Note that the left shift in Python is unusual in that it's not using a fixed width where bits are discarded - while many languages use a fixed width based on the data type, Python simply expands the width to cater for extra bits. In order to get the discarding behaviour in Python, you can follow a left shift with a bitwise and such as in an 8-bit value shifting left four bits:
bits8 = (bits8 << 4) & 255
With that in mind, another example of bitwise operators is if you have two 4-bit values that you want to pack into an 8-bit one, you can use all three of your operators (left-shift, and and or):
packed_val = ((val1 & 15) << 4) | (val2 & 15)
The & 15 operation will make sure that both values only have the lower 4 bits.
The << 4 is a 4-bit shift left to move val1 into the top 4 bits of an 8-bit value.
The | simply combines these two together.
If val1 is 7 and val2 is 4:
val1 val2
==== ====
& 15 (and) xxxx-0111 xxxx-0100 & 15
<< 4 (left) 0111-0000 |
| |
+-------+-------+
|
| (or) 0111-0100
One typical usage:
| is used to set a certain bit to 1
& is used to test or clear a certain bit
Set a bit (where n is the bit number, and 0 is the least significant bit):
unsigned char a |= (1 << n);
Clear a bit:
unsigned char b &= ~(1 << n);
Toggle a bit:
unsigned char c ^= (1 << n);
Test a bit:
unsigned char e = d & (1 << n);
Take the case of your list for example:
x | 2 is used to set bit 1 of x to 1
x & 1 is used to test if bit 0 of x is 1 or 0
what are bitwise operators actually used for? I'd appreciate some examples.
One of the most common uses of bitwise operations is for parsing hexadecimal colours.
For example, here's a Python function that accepts a String like #FF09BE and returns a tuple of its Red, Green and Blue values.
def hexToRgb(value):
# Convert string to hexadecimal number (base 16)
num = (int(value.lstrip("#"), 16))
# Shift 16 bits to the right, and then binary AND to obtain 8 bits representing red
r = ((num >> 16) & 0xFF)
# Shift 8 bits to the right, and then binary AND to obtain 8 bits representing green
g = ((num >> 8) & 0xFF)
# Simply binary AND to obtain 8 bits representing blue
b = (num & 0xFF)
return (r, g, b)
I know that there are more efficient ways to acheive this, but I believe that this is a really concise example illustrating both shifts and bitwise boolean operations.
I think that the second part of the question:
Also, what are bitwise operators actually used for? I'd appreciate some examples.
Has been only partially addressed. These are my two cents on that matter.
Bitwise operations in programming languages play a fundamental role when dealing with a lot of applications. Almost all low-level computing must be done using this kind of operations.
In all applications that need to send data between two nodes, such as:
computer networks;
telecommunication applications (cellular phones, satellite communications, etc).
In the lower level layer of communication, the data is usually sent in what is called frames. Frames are just strings of bytes that are sent through a physical channel. This frames usually contain the actual data plus some other fields (coded in bytes) that are part of what is called the header. The header usually contains bytes that encode some information related to the status of the communication (e.g, with flags (bits)), frame counters, correction and error detection codes, etc. To get the transmitted data in a frame, and to build the frames to send data, you will need for sure bitwise operations.
In general, when dealing with that kind of applications, an API is available so you don't have to deal with all those details. For example, all modern programming languages provide libraries for socket connections, so you don't actually need to build the TCP/IP communication frames. But think about the good people that programmed those APIs for you, they had to deal with frame construction for sure; using all kinds of bitwise operations to go back and forth from the low-level to the higher-level communication.
As a concrete example, imagine some one gives you a file that contains raw data that was captured directly by telecommunication hardware. In this case, in order to find the frames, you will need to read the raw bytes in the file and try to find some kind of synchronization words, by scanning the data bit by bit. After identifying the synchronization words, you will need to get the actual frames, and SHIFT them if necessary (and that is just the start of the story) to get the actual data that is being transmitted.
Another very different low level family of application is when you need to control hardware using some (kind of ancient) ports, such as parallel and serial ports. This ports are controlled by setting some bytes, and each bit of that bytes has a specific meaning, in terms of instructions, for that port (see for instance http://en.wikipedia.org/wiki/Parallel_port). If you want to build software that does something with that hardware you will need bitwise operations to translate the instructions you want to execute to the bytes that the port understand.
For example, if you have some physical buttons connected to the parallel port to control some other device, this is a line of code that you can find in the soft application:
read = ((read ^ 0x80) >> 4) & 0x0f;
Hope this contributes.
I didn't see it mentioned above but you will also see some people use left and right shift for arithmetic operations. A left shift by x is equivalent to multiplying by 2^x (as long as it doesn't overflow) and a right shift is equivalent to dividing by 2^x.
Recently I've seen people using x << 1 and x >> 1 for doubling and halving, although I'm not sure if they are just trying to be clever or if there really is a distinct advantage over the normal operators.
I hope this clarifies those two:
x | 2
0001 //x
0010 //2
0011 //result = 3
x & 1
0001 //x
0001 //1
0001 //result = 1
Think of 0 as false and 1 as true. Then bitwise and(&) and or(|) work just like regular and and or except they do all of the bits in the value at once. Typically you will see them used for flags if you have 30 options that can be set (say as draw styles on a window) you don't want to have to pass in 30 separate boolean values to set or unset each one so you use | to combine options into a single value and then you use & to check if each option is set. This style of flag passing is heavily used by OpenGL. Since each bit is a separate flag you get flag values on powers of two(aka numbers that have only one bit set) 1(2^0) 2(2^1) 4(2^2) 8(2^3) the power of two tells you which bit is set if the flag is on.
Also note 2 = 10 so x|2 is 110(6) not 111(7) If none of the bits overlap(which is true in this case) | acts like addition.
Sets
Sets can be combined using mathematical operations.
The union operator | combines two sets to form a new one containing items in either.
The intersection operator & gets items only in both.
The difference operator - gets items in the first set but not in the second.
The symmetric difference operator ^ gets items in either set, but not both.
Try It Yourself:
first = {1, 2, 3, 4, 5, 6}
second = {4, 5, 6, 7, 8, 9}
print(first | second)
print(first & second)
print(first - second)
print(second - first)
print(first ^ second)
Result:
{1, 2, 3, 4, 5, 6, 7, 8, 9}
{4, 5, 6}
{1, 2, 3}
{8, 9, 7}
{1, 2, 3, 7, 8, 9}
This example will show you the operations for all four 2 bit values:
10 | 12
1010 #decimal 10
1100 #decimal 12
1110 #result = 14
10 & 12
1010 #decimal 10
1100 #decimal 12
1000 #result = 8
Here is one example of usage:
x = raw_input('Enter a number:')
print 'x is %s.' % ('even', 'odd')[x&1]
Another common use-case is manipulating/testing file permissions. See the Python stat module: http://docs.python.org/library/stat.html.
For example, to compare a file's permissions to a desired permission set, you could do something like:
import os
import stat
#Get the actual mode of a file
mode = os.stat('file.txt').st_mode
#File should be a regular file, readable and writable by its owner
#Each permission value has a single 'on' bit. Use bitwise or to combine
#them.
desired_mode = stat.S_IFREG|stat.S_IRUSR|stat.S_IWUSR
#check for exact match:
mode == desired_mode
#check for at least one bit matching:
bool(mode & desired_mode)
#check for at least one bit 'on' in one, and not in the other:
bool(mode ^ desired_mode)
#check that all bits from desired_mode are set in mode, but I don't care about
# other bits.
not bool((mode^desired_mode)&desired_mode)
I cast the results as booleans, because I only care about the truth or falsehood, but it would be a worthwhile exercise to print out the bin() values for each one.
Bit representations of integers are often used in scientific computing to represent arrays of true-false information because a bitwise operation is much faster than iterating through an array of booleans. (Higher level languages may use the idea of a bit array.)
A nice and fairly simple example of this is the general solution to the game of Nim. Take a look at the Python code on the Wikipedia page. It makes heavy use of bitwise exclusive or, ^.
There may be a better way to find where an array element is between two values, but as this example shows, the & works here, whereas and does not.
import numpy as np
a=np.array([1.2, 2.3, 3.4])
np.where((a>2) and (a<3))
#Result: Value Error
np.where((a>2) & (a<3))
#Result: (array([1]),)
i didnt see it mentioned, This example will show you the (-) decimal operation for 2 bit values: A-B (only if A contains B)
this operation is needed when we hold an verb in our program that represent bits. sometimes we need to add bits (like above) and sometimes we need to remove bits (if the verb contains then)
111 #decimal 7
-
100 #decimal 4
--------------
011 #decimal 3
with python:
7 & ~4 = 3 (remove from 7 the bits that represent 4)
001 #decimal 1
-
100 #decimal 4
--------------
001 #decimal 1
with python:
1 & ~4 = 1 (remove from 1 the bits that represent 4 - in this case 1 is not 'contains' 4)..
Whilst manipulating bits of an integer is useful, often for network protocols, which may be specified down to the bit, one can require manipulation of longer byte sequences (which aren't easily converted into one integer). In this case it is useful to employ the bitstring library which allows for bitwise operations on data - e.g. one can import the string 'ABCDEFGHIJKLMNOPQ' as a string or as hex and bit shift it (or perform other bitwise operations):
>>> import bitstring
>>> bitstring.BitArray(bytes='ABCDEFGHIJKLMNOPQ') << 4
BitArray('0x142434445464748494a4b4c4d4e4f50510')
>>> bitstring.BitArray(hex='0x4142434445464748494a4b4c4d4e4f5051') << 4
BitArray('0x142434445464748494a4b4c4d4e4f50510')
the following bitwise operators: &, |, ^, and ~ return values (based on their input) in the same way logic gates affect signals. You could use them to emulate circuits.
To flip bits (i.e. 1's complement/invert) you can do the following:
Since value ExORed with all 1s results into inversion,
for a given bit width you can use ExOR to invert them.
In Binary
a=1010 --> this is 0xA or decimal 10
then
c = 1111 ^ a = 0101 --> this is 0xF or decimal 15
-----------------
In Python
a=10
b=15
c = a ^ b --> 0101
print(bin(c)) # gives '0b101'
You can use bit masking to convert binary to decimal;
int a = 1 << 7;
int c = 55;
for(int i = 0; i < 8; i++){
System.out.print((a & c) >> 7);
c = c << 1;
}
this is for 8 digits you can also do for further more.
How do I delete a character in 6502 basic? Like reverse print...
Would there be a way to channel the del key code into a program? I have tried looking at the reference but there is nothing there for a command.
As OldBoyCoder suggests, it can be done - but the character depends on the platform.
10 PRINT "AB";
20 PRINT "C"
Output on Commodore PET or Apple II:
ABC
For Commodore PET add:
15 PRINT CHR$(20);
Output:
AC
On the Apple II you'd need to use 8 instead of 20 because the I/O firmware is different.
You can also use the string manipulation commands, such as LEFT$, RIGHT$ and MID$, here's an untested example from memory (for the Commodore PET and other Commodore 8-bit machines, probably also works on other variants of BASIC):
0 A$="ABC"
1 PRINT LEFT$(A$,2): REM PRINTS AB
2 PRINT RIGHT$(A$,2): REM PRINTS BC
3 PRINT MID$(A$,2,1): REM PRINTS B
Otherwise, you can over-write characters as long as you know where they are located on the screen.
0 D$=CHR$(17): REM CURSOR DOWN
1 U$=CHR$(145): REM CURSOR UP
2 R$=CHR$(29): REM CURSOR RIGHT
3 L$=CHR$(157): REM CURSOR LEFT
4 PRINT "ABC";L$;" "
I'm using CHR$ codes here for clarity. If you open a string with a double-quote, you can press the UP/DOWN and LEFT/RIGHT keys which will show inversed characters. Printing these inversed characters will do the same as moving the cursor around with the cursor keys.
I have a table of values (actually written in LaTeX markup) and I can select one column only using Ctrl+Q
For example,
% Select the second column only ....
1.75130211563 & 0.0299693961831 \\
1.72144412106 & 0.0181406611688 \\
1.92102386609 & 0.0247758598737 \\
1.56512790839 & 0.0137107006809 \\
1.75263937567 & 0.017155656704 \\
1.99501744437 & 0.39550649953 \\
1.96862597164 & 0.030198328601 \\
...
I'd like to reduce the number of decimal places of the selected numbers only (i.e. for each number selected I apply round(NUMBER * 100)/100 to get, for example, the number rounded to 2 decimal places). To do this, I need to first have a variable to refer to NUMBER (the number on that line) as well as replace the current selection with the output.
How do I do this?
Also if this isn't possible, I can copy the column into an actual spreadsheet program and edit it there, but how do I paste it back in place?
Update: I've accepted an answer. It isn't as neat as I had hoped, but it does make sense. Thanks!
Another update: To paste a column in from an external spreadsheet, paste the column into a :new buffer, select the data using Ctrl+Q and yank into a register. Move to the top line and appropriate column in the table of data and paste in with P.
Are you looking for this ?
:'<,'>s#\%V\d*\.\d\+#\=round(str2float(submatch(0))*100)/100#g
This seems to work:
:'<,'>:s/\d\+\.\d\+/\=printf("%.2f", str2float(submatch(0)))/
I.e. visually select the first column and issue the command above.
% Select the second column only ....
1.75 & 0.0299693961831 \\
1.72 & 0.0181406611688 \\
1.92 & 0.0247758598737 \\
1.57 & 0.0137107006809 \\
1.75 & 0.017155656704 \\
2.00 & 0.39550649953 \\
1.97 & 0.030198328601 \\
I am creating a report in LaTeX which involves a few tables. I'm stuck on that as my cell data in the table is exceeding the width of the page. Can I somehow wrap the text so that it falls into the next line in the same cell of the table?
Is it somehow related to the table's width? But as it's overshooting the page's width, will it make a difference?
Use p{width} for your column specifiers instead of l/r/c.
\begin{tabular}{|p{1cm}|p{3cm}|}
This text will be wrapped & Some more text \\
\end{tabular}
EDIT: (based on the comments)
\begin{table}[ht]
\centering
\begin{tabular}{p{0.35\linewidth} | p{0.6\linewidth}}
Column 1 & Column2 \\ \hline
This text will be wrapped & Some more text \\
Some text here & This text maybe wrapped here if its tooooo long \\
\end{tabular}
\caption{Caption}
\label{tab:my_label}
\end{table}
we get:
With the regular tabular environment, you want to use the p{width} column type, as marcog indicates. But that forces you to give explicit widths.
Another solution is the tabularx environment:
\usepackage{tabularx}
...
\begin{tabularx}{\linewidth}{ r X }
right-aligned foo & long long line of blah blah that will wrap when the table fills the column width\\
\end{tabularx}
All X columns get the same width. You can influence this by setting \hsize in the format declaration:
>{\setlength\hsize{.5\hsize}} X >{\setlength\hsize{1.5\hsize}} X
but then all the factors have to sum up to 1, I suppose (I took this from the LaTeX companion). There is also the package tabulary which will adjust column widths to balance row heights. For the details, you can get the documentation for each package with texdoc tabulary (in TeXlive).
Another option is to insert a minipage in each cell where text wrapping is desired, e.g.:
\begin{table}[H]
\begin{tabular}{l}
\begin{minipage}[t]{0.8\columnwidth}%
a very long line a very long line a very long line a very long line
a very long line a very long line a very long line a very long line
a very long line a very long line a very long line %
\end{minipage}\tabularnewline
\end{tabular}
\end{table}
I like the simplicity of tabulary package:
\usepackage{tabulary}
...
\begin{tabulary}{\linewidth}{LCL}
\hline
Short sentences & \# & Long sentences \\
\hline
This is short. & 173 & This is much loooooooonger, because there are many more words. \\
This is not shorter. & 317 & This is still loooooooonger, because there are many more words. \\
\hline
\end{tabulary}
In the example, you arrange the whole width of the table with respect to \textwidth. E.g 0.4 of it. Then the rest is automatically done by the package.
Most of the example is taken from http://en.wikibooks.org/wiki/LaTeX/Tables .
Simple like a piece of CAKE!
You can define a new column type like (L in this case) while maintaining the current alignment (c, r or l):
\documentclass{article}
\usepackage{array}
\newcolumntype{L}{>{\centering\arraybackslash}m{3cm}}
\begin{document}
\begin{table}
\begin{tabular}{|c|L|L|}
\hline
Title 1 & Title 2 & Title 3 \\
\hline
one-liner & multi-line and centered & \multicolumn{1}{m{3cm}|}{multi-line piece of text to show case a multi-line and justified cell} \\
\hline
apple & orange & banana \\
\hline
apple & orange & banana \\
\hline
\end{tabular}
\end{table}
\end{document}
If you want to wrap your text but maintain alignment then you can wrap that cell in a minipage or varwidth environment (varwidth comes from the varwidth package). Varwidth will be "as wide as it's contents but no wider than X". You can create a custom column type which acts like "p{xx}" but shrinks to fit by using
\newcolumntype{M}[1]{>{\begin{varwidth}[t]{#1}}l<{\end{varwidth}}}
which may require the array package. Then when you use something like \begin{tabular}{llM{2in}} the first two columns we be normal left-aligned and the third column will be normal left aligned but if it gets wider than 2in then the text will be wrapped.
To change the text AB into A \r B in a table cell, put this into the cell position: \makecell{A \\ B}.
Before doing that, you also need to include package makecell.
The new tabularray makes wrapping text in cells easier then ever before.
The package supports all the traditional used column names like c, l, r, etc., but also has its own Q column which accepts various keys to control the width and vertical and horizontal alignment. It also provides an X column, as known from tabularx` which will automatically calculate the width of the column to fit the table into the available text width.
Another nice feature is that all the settings can also be done for individual cells.
\documentclass{article}
\usepackage{tabularray}
\begin{document}
\begin{table}
\begin{tblr}{|c|Q[2cm,valign=m]|X[j,valign=m]|}
\hline
Title 1 & Title 2 & Title 3 \\
\hline
one-liner & multi-line text & multi-line piece of text to show case a multi-line and justified cell \\
\hline
apple & orange & banana \\
\hline
\SetCell{h,2cm} wrapping text only in a single cell & orange & banana \\
\hline
\end{tblr}
\end{table}
\end{document}
(thanks to Shayan Amani for providing a MWE in their answer!)
\begin{table}
\caption{ Example of force text wrap}
\begin{center}
\begin{tabular}{|c|c|}
\hline
cell 1 & cell 2 \\ \hline
cell 3 & cell 4 & & very big line that needs to be wrap. \\ \hline
cell 5 & cell 6 \\ \hline
\end{tabular}
\label{table:example}
\end{center}
\end{table}