Elements of Python programming - Revision history

WikiSysop at 21:20, 27 March 2018

2018-03-27T21:20:45Z

← Older revision		Revision as of 21:20, 27 March 2018
Line 383:		Line 383:

	For our purposes, text storage may be regarded as a sequence of bytes with each one representing a different character following the ASCII assignments. The number of bytes per character is left up to the encoding, and we have to be aware that data representing strings, and data representing bytes, are not equivalent.		For our purposes, text storage may be regarded as a sequence of bytes with each one representing a different character following the ASCII assignments. The number of bytes per character is left up to the encoding, and we have to be aware that data representing strings, and data representing bytes, are not equivalent.


	=== Integers ===		=== Integers ===
Line 422:		Line 423:
	factorial() returns the factorial of an integer		factorial() returns the factorial of an integer
	but you need the math package for that.		but you need the math package for that.


	=== Floating point and math ===		=== Floating point and math ===

WikiSysop: /* Integers */

2018-03-27T21:16:33Z

Integers

← Older revision		Revision as of 21:16, 27 March 2018
Line 414:		Line 414:
	There are other basic operations on integers in Python, among them		There are other basic operations on integers in Python, among them
	float() turns an integer or numeric string into a floating point		float() turns an integer or numeric string into a floating point
	% means "modulo", that is representation of the modulus of the value in that base, so 10%7 is 3.		% means "modulo", that is representation of the modulus of the value in that base, so 10%7 is 3.
	- negation		- negation
	** raise to a power		** raise to a power
	int() creates an integer from a string		int() creates an integer from a string
	long() creates a long integer from a string		long() creates a long integer from a string
	abs() returns the absolute value of an number		abs() returns the absolute value of an number
	factorial() returns the factorial of an integer		factorial() returns the factorial of an integer
			but you need the math package for that.

	=== Floating point and math ===		=== Floating point and math ===

WikiSysop: /* Binary numbers */

2018-03-27T21:07:57Z

Binary numbers

← Older revision		Revision as of 21:07, 27 March 2018
Line 338:		Line 338:
	To see the hexadecimal form of an integer, use formatted print this way		To see the hexadecimal form of an integer, use formatted print this way

	print(format(11,'x'))		print(format(12,'x'))
	b		c

			To see the binary form, use the 'b' format this way

			print(format(12,'b'))
			1100

	To make longer numbers we string together bytes, and for a 64-bit computer, we treat these in chunks of 8 bytes per word. You can think of computer memory of a linear space with bits one after another, organized as bytes, and words, and sorted out by programs into text and numbers that are stored in these bits. Normally, you would not even worry over such workings, unless you need to "set a bit", control a logical decision, or sometimes control an instrument by changing internal values in a word. There are some "gotchas" to be at least aware of.		To make longer numbers we string together bytes, and for a 64-bit computer, we treat these in chunks of 8 bytes per word. You can think of computer memory of a linear space with bits one after another, organized as bytes, and words, and sorted out by programs into text and numbers that are stored in these bits. Normally, you would not even worry over such workings, unless you need to "set a bit", control a logical decision, or sometimes control an instrument by changing internal values in a word. There are some "gotchas" to be at least aware of.

WikiSysop: /* Binary numbers */

2018-03-27T21:05:09Z

Binary numbers

← Older revision		Revision as of 21:05, 27 March 2018
Line 335:		Line 335:

	Binary numbers are often referred to in hexadecimal or "hex" code, which counts up to 15 and is given in powers of 16 rather than powers of 2. The counting sequence is 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F. So, 4 bits of a byte can be a hex code, and it takes two of them to represent an 8-bit byte. In this terminology, decimal 1 is binary 1 and hex 1. Decimal 255 is binary 11111111 and hex FF.		Binary numbers are often referred to in hexadecimal or "hex" code, which counts up to 15 and is given in powers of 16 rather than powers of 2. The counting sequence is 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F. So, 4 bits of a byte can be a hex code, and it takes two of them to represent an 8-bit byte. In this terminology, decimal 1 is binary 1 and hex 1. Decimal 255 is binary 11111111 and hex FF.

			To see the hexadecimal form of an integer, use formatted print this way

			print(format(11,'x'))
			b

	To make longer numbers we string together bytes, and for a 64-bit computer, we treat these in chunks of 8 bytes per word. You can think of computer memory of a linear space with bits one after another, organized as bytes, and words, and sorted out by programs into text and numbers that are stored in these bits. Normally, you would not even worry over such workings, unless you need to "set a bit", control a logical decision, or sometimes control an instrument by changing internal values in a word. There are some "gotchas" to be at least aware of.		To make longer numbers we string together bytes, and for a 64-bit computer, we treat these in chunks of 8 bytes per word. You can think of computer memory of a linear space with bits one after another, organized as bytes, and words, and sorted out by programs into text and numbers that are stored in these bits. Normally, you would not even worry over such workings, unless you need to "set a bit", control a logical decision, or sometimes control an instrument by changing internal values in a word. There are some "gotchas" to be at least aware of.

WikiSysop at 20:57, 27 March 2018

2018-03-27T20:57:04Z

← Older revision		Revision as of 20:57, 27 March 2018
Line 484:		Line 484:
	>> ord('X')		>> ord('X')
	88		88
	>> ~~char~~(88)		>> chr(88)
	'X'		'X'

WikiSysop: /* Binary numbers */

2018-03-22T07:38:15Z

Binary numbers

← Older revision		Revision as of 07:38, 22 March 2018
Line 370:		Line 370:
	98 62 01100010 b		98 62 01100010 b

	In modern UTF-8, the Unicode standard for the World Wide Web, the mapping may change with choice of language. It is backward compatible with ASCII, but uses 64-bits by using from one to four bytes. ~~For our purposes~~, ~~text storage~~ may be ~~regarded as a sequence of~~ 8-bit ~~bytes~~ with ~~each one representing a different character following the ASCII assignments~~.		In modern UTF-8, the Unicode standard for the World Wide Web, the mapping may change with choice of language. It is backward compatible with ASCII, but uses 64-bits by using from one to four bytes. Python 3 introduced a distinction between strings, which may be encoded with, for example UTF-8, and bytes storing 8-bit data. This distinction can cause issues when strings are managed in Python 3 with older Python 2 code. However, it really helps when reading files that may be encoded, which for Python 2 would often cause confusion or failure.

			For our purposes, text storage may be regarded as a sequence of bytes with each one representing a different character following the ASCII assignments. The number of bytes per character is left up to the encoding, and we have to be aware that data representing strings, and data representing bytes, are not equivalent.

	=== Integers ===		=== Integers ===

WikiSysop: /* Printing to the display */

2018-03-22T07:32:22Z

Printing to the display

← Older revision		Revision as of 07:32, 22 March 2018
Line 148:		Line 148:


	When you have data or text to display, you'd use a "print" function to have the data appear on the console as the program executes. (In Windows, you may follow this with a ~~raw_input() (in Python 2.7, or~~ input(~~) in Python 3~~) so that the console will not disappear before you read it.) In Linux or MacOS, you would usually run Python from a console, and the printed information appears on the display and remains visible after the program has finished. Unix-like environments "print" to the standard output, stdout, and may be redirected to a file. For example running a Python program from the command line that generates output, you could write		When you have data or text to display, you'd use a "print" function to have the data appear on the console as the program executes. (In Windows, you may follow this with a input() so that the console will not disappear before you read it.) In Linux or MacOS, you would usually run Python from a console, and the printed information appears on the display and remains visible after the program has finished. Unix-like environments "print" to the standard output, stdout, and may be redirected to a file. For example running a Python program from the command line that generates output, you could write

	python myprogram.py >> myfile.txt		python myprogram.py >> myfile.txt
Line 161:		Line 161:
	To print text the command is		To print text the command is

	print 'This will print on the screen.\n'		print ('This will print on the screen.\n')

	where the quoted (' and " have the same effect) text is sent. The '\n' is a line feed.		where the quoted (' and " have the same effect) text is sent. The '\n' is a line feed.
Line 178:		Line 178:
	1 2 3 Help me!!		1 2 3 Help me!!

	Of course, printing can be formatted. If you are familiar with C or Fortran, you'll find similarities that will help creating formatted output. ~~I in~~ this instance, it's also helpful to remember that the "print" function is converting internal data into displayed "text", so the formatting is really a way of controlling how some text and numerical data are mapped onto text that is then displayed.		Of course, printing can be formatted. If you are familiar with C or Fortran, you'll find similarities that will help creating formatted output. In this instance, it's also helpful to remember that the "print" function is converting internal data into displayed "text", so the formatting is really a way of controlling how some text and numerical data are mapped onto text that is then displayed.

	Formatting is available in both versions 2.7 and 3 in two ways		Formatting is available in both versions 2.7 and 3 in two ways
Line 262:		Line 262:

	Generally the most commonly used is the % expression which is embodied in Python 2.7 and in Python 3.		Generally the most commonly used is the % expression which is embodied in Python 2.7 and in Python 3.


	=== Input from a file and writing to files===		=== Input from a file and writing to files===

WikiSysop: /* Input from a console and keyboard */

2018-03-22T07:27:24Z

Input from a console and keyboard

← Older revision		Revision as of 07:27, 22 March 2018
Line 16:		Line 16:
	=== Input from a console and keyboard ===		=== Input from a console and keyboard ===

	To have a Python program accept data from a user at a console, include lines like these in Python ~~2.7~~		To have a Python program accept data from a user at a console, include lines like these in Python 3


	newtext = ~~raw_input~~()		newtext = input()
	print newtext		print newtext

	to take the ~~raw~~ input as text and print it. You can prompt for the input too		to take the input as text and print it. You can prompt for the input too

	newtext = ~~raw_input~~('Write what you like: ')		newtext = input('Write what you like: ')
	print 'This is what I like: ', newtext		print 'This is what I like: ', newtext


	In Python 2.7 there is also a Python command "input()" which treats incoming text as Python code. That is, if you input "1+2" it will return "3". With Python 3.0 this has changed, so some care is needed if you write for the new Python. ~~In that case, rather than raw_input() as you did in Python 2, you would use input(), to get the~~ effect of ~~the~~ Python 2's "input()" that evaluated the expression, in Python 3 you would explicitly ask for that outcome with eval(input()). You can see why using the newer Python 3.0 with older programs can raise some problems, though they are usually easy to fix.		In Python 2.7 there is also a Python command "input()" which treats incoming text as Python code. That is, if you input "1+2" it will return "3". With Python 3.0 this has changed, so some care is needed if you write for the new Python. The effect of Python 3's "input" is Python2's "raw_input". To have Python2's "input()" that evaluated the expression, in Python 3 you would explicitly ask for that outcome with eval(input()). You can see why using the newer Python 3.0 with older programs can raise some problems, though they are usually easy to fix.

	The input is text, but suppose we want a number instead. If we know it's a number, then		The input is text, but suppose we want a number instead. If we know it's a number, then

	newtext = ~~raw_input~~('Input a number >> ')		newtext = input('Input a number >> ')
	x = float(newtext)		x = float(newtext)
	print 'My number was ', x		print 'My number was ', x
Line 39:		Line 39:
	should do it. But, if you try this and input text that is not a number, the program will generate an error and respond with something like this		should do it. But, if you try this and input text that is not a number, the program will generate an error and respond with something like this

	~~python~~ input_number.py		python3 input_number.py
	Input a number >> x		Input a number >> x
	Traceback (most recent call last):		Traceback (most recent call last):
Line 48:		Line 48:
	How would we know if we have a number, given arbitrary text in the data, and avoid this error? One way is to use isdigit() --		How would we know if we have a number, given arbitrary text in the data, and avoid this error? One way is to use isdigit() --

	newtext = ~~raw_input~~('Input a number >> ')		newtext = input('Input a number >> ')
	if newtext.isdigit():		if newtext.isdigit():
	x = float(newtext)		x = float(newtext)
	print 'My number was ', x		print ('My number was ', x)
	else:		else:
	print 'That is not a number.'		print ('That is not a number.')

	In this you see that isdigit() tests whether newtext is a number. It returns a True or False which is used by the "if" statement to control what to do with the data. We will look at such flow control more later.		In this you see that isdigit() tests whether newtext is a number. It returns a True or False which is used by the "if" statement to control what to do with the data. We will look at such flow control more later.
Line 59:		Line 59:
	You may also want to read data by splitting a line of text into pieces, for example with something like this --		You may also want to read data by splitting a line of text into pieces, for example with something like this --

	newtext = ~~raw_input~~('Input the item name and quantity >>')		newtext = input('Input the item name and quantity >>')
	print newtext.split()		print newtext.split()

Line 73:		Line 73:
	print (x)		print (x)

	From here on if you see the older version of print that's a good clue the code was written for Python 2. Here is our example updated to Python 3:		From here on if you see the older version of print without parentheses that's a good clue the code was written for Python 2. Here is our example updated to Python 3:

	~~newtext = input('Input a number >> ')~~
	~~if newtext.isdigit():~~
	~~x = float(newtext)~~
	~~print ('My number was ', x)~~
	~~else:~~
	~~print ('That is not a number.')~~


	=== Style: Indentation, comments, and names of variables and functions ===		=== Style: Indentation, comments, and names of variables and functions ===

WikiSysop at 06:20, 27 February 2018

2018-02-27T06:20:18Z

← Older revision		Revision as of 06:20, 27 February 2018
Line 180:		Line 180:
	z = 3		z = 3
	h = 'Help me!!'		h = 'Help me!!'
	print x,y,z,h,'\n'		print (x,y,z,h,'\n')

	will display the values of x, y, or z regardless of whether they are numbers or text.		will display the values of x, y, or z regardless of whether they are numbers or text.
Line 247:		Line 247:
	In a program, rather than interactively, statements like these work in a print command		In a program, rather than interactively, statements like these work in a print command

	>> print '%6.5e'%x		>> print ('%6.5e'%x)
	1.2346e+00		1.2346e+00

Line 270:		Line 270:

	Generally the most commonly used is the % expression which is embodied in Python 2.7 and in Python 3.		Generally the most commonly used is the % expression which is embodied in Python 2.7 and in Python 3.


	=== Input from a file and writing to files===		=== Input from a file and writing to files===
Line 387:		Line 388:

	>> x = 1./3.		>> x = 1./3.
	>> print x		>> print (x)
	0.333333333333		0.333333333333
	>> y = 1/3		>> y = 1/3
	>> print y		>> print (y)
	0		0
	>> y = x		>> y = x
	>> print y		>> print (y)
	0.333333333333		0.333333333333

Line 403:		Line 404:

	>> z = 11/9		>> z = 11/9
	>> print z		>> print (z)
	1.2222222222222223		1.2222222222222223
	>> z = int(z)		>> z = int(z)
	>> print z		>> print (z)
	1		1

Line 465:		Line 466:

	>> mystring ='My kingdom for a horse (Richard III).'		>> mystring ='My kingdom for a horse (Richard III).'
	>> print mystring		>> print (mystring)
	'My kingdom for a horse (Richard III).'		'My kingdom for a horse (Richard III).'

WikiSysop at 06:14, 27 February 2018

2018-02-27T06:14:48Z

← Older revision		Revision as of 06:14, 27 February 2018
Line 114:		Line 114:
	will generate an error.		will generate an error.

	How much to indent is your choice. The [ https://www.python.org/dev/peps/pep-0008/ style guide] recommends 4 spaces. My preference is for just 2. Some programmers like 8, the standard spacing of a tab indentation. You'll find that if you have several levels of indentation, the loss of workspace and shifting content to the right can be very annoying and hard to read. Using 2 or 4 spaces makes a readable program where your eye can follow the indentation levels easily.		How much to indent is your choice. The [https://www.python.org/dev/peps/pep-0008/ style guide] recommends 4 spaces. My preference is for just 2. Some programmers like 8, the standard spacing of a tab indentation. You'll find that if you have several levels of indentation, the loss of workspace and shifting content to the right can be very annoying and hard to read. Using 2 or 4 spaces makes a readable program where your eye can follow the indentation levels easily.

	Lastly, add abundant comments as you work. A comment line is one that begins with a "#" symbol Comment lines should be indented just like the program content they apply to. You can add a comment after a program element on the same line, though this is sometimes hard to read. However you do it, leaving comments will help you later when you return to an old program and have forgotten what it was doing, or why.		Lastly, add abundant comments as you work. A comment line is one that begins with a "#" symbol Comment lines should be indented just like the program content they apply to. You can add a comment after a program element on the same line, though this is sometimes hard to read. However you do it, leaving comments will help you later when you return to an old program and have forgotten what it was doing, or why.