Solving problems with Python - Revision history

WikiSysop at 06:12, 21 February 2018

2018-02-21T06:12:16Z

← Older revision		Revision as of 06:12, 21 February 2018
Line 14:		Line 14:
	pass		pass
	elif x == 0:		elif x == 0:
	print 'Cannot divide by zero.'		print ('Cannot divide by zero.')
	exit()		exit()
	else:		else:
Line 30:		Line 30:
	if x > 10.:		if x > 10.:
	flag = False		flag = False
	print x		print (x)

	increases x until it is 11. and then prints the value.		increases x until it is 11. and then prints the value.
Line 41:		Line 41:
	break		break
	except ValueError:		except ValueError:
	print "Oops! That was no valid number. Try again..."		print ("Oops! That was no valid number. Try again...")
	y=x**2		y=x**2
	print y		print (y)

	Here a ''break'' exits the loop from the ''try'' block unless an exception is thrown. A while statement can also test for something that is changed in the loop.		Here a ''break'' exits the loop from the ''try'' block unless an exception is thrown. A while statement can also test for something that is changed in the loop.
Line 72:		Line 72:

	idata = range(10)		idata = range(10)
	print idata		print (idata)
	fdata = [-1.]		fdata = [-1.]
	for x in idata:		for x in idata:
	f = float(x)**2.0		f = float(x)**2.0
	fdata.append(f)		fdata.append(f)
	print fdata		print (fdata)

	generates this output		generates this output
Line 101:		Line 101:
	for key in messier.keys():		for key in messier.keys():
	if messier[key] == 'spiral galaxy':		if messier[key] == 'spiral galaxy':
	print key, messier[key]		print (key, messier[key])

	which will print		which will print
Line 112:		Line 112:
	for key, value in messier.items():		for key, value in messier.items():
	if value == 'spiral galaxy':		if value == 'spiral galaxy':
	print key, value		print (key, value)


Line 122:		Line 122:
	if not myline:		if not myline:
	break		break
	print myline		print (myline)

	prints lines from ''catalog.dat''. The simpler		prints lines from ''catalog.dat''. The simpler

	for myline in open('catalog.dat'):		for myline in open('catalog.dat'):
	print myline		print (myline)

	uses the iterative for loop to print lines from the file one by one as read. You can also open a file and read all the lines as a list and the iteration is hidden.		uses the iterative for loop to print lines from the file one by one as read. You can also open a file and read all the lines as a list and the iteration is hidden.

WikiSysop at 06:09, 21 February 2018

2018-02-21T06:09:45Z

← Older revision		Revision as of 06:09, 21 February 2018
Line 150:		Line 150:
	3		3

	Here "mylist" is iterable. Everything you can use ~~a statment~~ such "for... in..." on is an iterable. This includes lists, strings, and files. ~~When~~ you use a technique called "list comprehension" you can create a list in one line		Here "mylist" is iterable. Everything you can use with statement such "for... in..." is an iterable. This includes lists, strings, and files. Similarly, when you use a technique called "list comprehension" you can create a list in one line with an iterable

	mylist = [x*x for x in range(4)]		mylist = [x*x for x in range(4)]
Line 164:		Line 164:
	9		9

	This also shows how the "range(4)" function starts at 0 and ends at 3 in steps of 1.		This also shows how the "range(4)" function starts at 0 and ends at 3 in steps of 1.

	Iterables store their values in memory, and if you have a lot of values you might not want that. Generators are an ~~interator~~ that generates values as you go ~~this way~~:		Iterables store their values in memory, and if you have a lot of values you might not want that. ''Generators'' are an iterator that generates values as you go:

	mygenerator = (x*x for x in range(3))		mygenerator = (x*x for x in range(3))
Line 176:		Line 176:
	4		4

	The difference in syntax is the use if ( rather than [. ~~However~~ you cannot ask "for i in mygenerator" more than once.		The difference in syntax is subtle, the use if ( rather than [. The results are similar, except you cannot ask "for i in mygenerator" more than once.

	When defining a function to create a generator, you would use "yield" rather than "return"
			When defining a function to create a generator if you want to make many of them, you would use "yield" rather than "return" in the definition

	def createGenerator():		def createGenerator():
Line 195:		Line 196:
	4		4

	This example could be modified to return a huge set of values, but only once. The "mygenerator" code returns the generator object, and it is run each time "for" uses the generator. The first time the "for" calls the generator object created from your function~~, it~~ will run the code in your function from the beginning until it hits yield, then it will return the first value of the loop. ~~Then, each other~~ call will run the loop you have written in the function one more time, and return the next value, until there is no value to return. The generator is considered empty once the function runs but does not hit yield anymore. This can happen if the loop comes to an end, or if it does not ~~you do not~~ satisfy another condition.		This example could be modified to return a huge set of values, but only once. The "mygenerator" code returns the generator object, and it is run each time "for" uses the generator. The first time the "for" calls the generator, an object is created from your function. It will run the code in your function from the beginning until it hits yield, then it will return the first value of the loop. Each subsequent call will run the loop you have written in the function one more time, and return the next value, until there is no value to return. The generator is considered empty once the function runs but does not hit yield anymore. This can happen if the loop comes to an end, or if it does not satisfy another condition.

WikiSysop at 06:02, 21 February 2018

2018-02-21T06:02:23Z

@@ Line 134: / Line 134: @@
   mydata = myfile.readlines()
   myfile.close()
 == Examples ==
 For examples of Python illustrating flow control, functions, and iteration, see the [http://prancer.physics.louisville.edu/astrowiki/index.php/Python_examples examples] section.
 == Assignments ==
 For the assigned homework to use these ideas,  see the [http://prancer.physics.louisville.edu/astrowiki/index.php/Python_assignments assignments] section.

WikiSysop: /* Functions */

2015-01-20T19:07:11Z

Functions

← Older revision		Revision as of 19:07, 20 January 2015
Line 49:		Line 49:
	== Functions ==		== Functions ==

	Within a Python program you can define your own functions. Here's one to take an angle in degrees and reduce it an angle between 0 and 360.		Within a Python program you can define your own functions. Here's one to take an angle in degrees and reduce it to an angle between 0 and 360.

	def map360(angle):		def map360(angle):

WikiSysop: /* Flow control */

2015-01-20T19:06:46Z

Flow control

← Older revision		Revision as of 19:06, 20 January 2015
Line 22:		Line 22:
	Notice that indentation (by any fixed number of spaces) is used to separate the functions within the statement, and that each branch is defined by a '':''. The end of a branch occurs when the indentation goes back to the previous level. Each decision is based on a logical ''boolean'' value such as (x > 0.), which is True when x is greater than 0. and False otherwise. Within the if processing, a ''pass'' is a way to do nothing, and an exit() leaves the entire program.		Notice that indentation (by any fixed number of spaces) is used to separate the functions within the statement, and that each branch is defined by a '':''. The end of a branch occurs when the indentation goes back to the previous level. Each decision is based on a logical ''boolean'' value such as (x > 0.), which is True when x is greater than 0. and False otherwise. Within the if processing, a ''pass'' is a way to do nothing, and an exit() leaves the entire program.

	A ''while'' statement tests whether its argument is true, and sets up a loop that continues as long as it is. ~~Program~~		A ''while'' statement tests whether its argument is true, and sets up a loop that continues as long as it is. The program

	flag = True		flag = True
Line 41:		Line 41:
	break		break
	except ValueError:		except ValueError:
	print "Oops! That was no valid number. Try again..."		print "Oops! That was no valid number. Try again..."
	y=x**2		y=x**2
	print y		print y

WikiSysop at 21:08, 14 February 2013

2013-02-14T21:08:01Z

← Older revision		Revision as of 21:08, 14 February 2013
Line 1:		Line 1:
	Now with many useful tools in hand, let us see how to make them work together to solve problems.		Now with many useful tools in hand from our [http://prancer.physics.louisville.edu/astrowiki/index.php/Python_for_Physics_and_Astronomy short course on using Python in Physics and Astronomy], let us see how to make them work together to solve problems.

WikiSysop: /* Iteration */

2013-02-14T06:55:40Z

Iteration

← Older revision		Revision as of 06:55, 14 February 2013
Line 107:		Line 107:
	51 spiral galaxy		51 spiral galaxy

	However, dictionaries can be interated for both key and value ~~this way~~		However, dictionaries can be interated for both key and value by looking at the full list of items.

	messier = {'1' : 'planetary nebula', '2' : 'globular cluster', '51' : 'spiral galaxy' }		messier = {'1' : 'planetary nebula', '2' : 'globular cluster', '51' : 'spiral galaxy' }
Line 113:		Line 113:
	if value == 'spiral galaxy':		if value == 'spiral galaxy':
	print key, value		print key, value


	The readline() function for a file iterates line by line through the file, and		The readline() function for a file iterates line by line through the file, and

WikiSysop: /* Iteration */

2013-02-14T06:52:18Z

Iteration

← Older revision		Revision as of 06:52, 14 February 2013
Line 96:		Line 96:
	In the first line, range(10) creates a list with values from 0 to 9 which is the first line of the output. We also made a floating point list with one value of -1. to start things off. The ''for'' loop interated the idata list, and for each element (now labeled ''x'') we calculated x<sup>2.0</sup> and appended it to the fdata floating point list. Inside the iterative loop we printed the list as it grew.		In the first line, range(10) creates a list with values from 0 to 9 which is the first line of the output. We also made a floating point list with one value of -1. to start things off. The ''for'' loop interated the idata list, and for each element (now labeled ''x'') we calculated x<sup>2.0</sup> and appended it to the fdata floating point list. Inside the iterative loop we printed the list as it grew.

	Lists and strings are ''iterable'', and so are dictionaries and even files. An example of iterating a dictionary would be		Lists and strings are ''iterable'', and so are dictionaries and even files. An example of iterating a dictionary through a list of its keys would be

	messier = {'1' : 'planetary nebula', '2' : 'globular cluster', '51' : 'spiral galaxy' }		messier = {'1' : 'planetary nebula', '2' : 'globular cluster', '51' : 'spiral galaxy' }
Line 107:		Line 107:
	51 spiral galaxy		51 spiral galaxy

			However, dictionaries can be interated for both key and value this way

			messier = {'1' : 'planetary nebula', '2' : 'globular cluster', '51' : 'spiral galaxy' }
			for key, value in messier.items():
			if value == 'spiral galaxy':
			print key, value

	The readline() function for a file iterates line by line through the file, and		The readline() function for a file iterates line by line through the file, and

WikiSysop: /* Iteration */

2013-02-14T06:41:00Z

Iteration

← Older revision		Revision as of 06:41, 14 February 2013
Line 99:		Line 99:

	messier = {'1' : 'planetary nebula', '2' : 'globular cluster', '51' : 'spiral galaxy' }		messier = {'1' : 'planetary nebula', '2' : 'globular cluster', '51' : 'spiral galaxy' }
	for key in messier:		for key in messier.keys():
	if messier[key] == 'spiral galaxy':		if messier[key] == 'spiral galaxy':
	print key		print key, messier[key]

	which will print		which will print

	51		51 spiral galaxy

WikiSysop: /* Iteration */

2013-02-14T06:36:43Z

Iteration

← Older revision		Revision as of 06:36, 14 February 2013
Line 119:		Line 119:
	prints lines from ''catalog.dat''. The simpler		prints lines from ''catalog.dat''. The simpler

	for myline in open('catalog.dat')		for myline in open('catalog.dat'):
	print myline		print myline

	uses the iterative for loop to print lines from the file one by one as read. You can also open a file and read all the lines as a list and the iteration is hidden		uses the iterative for loop to print lines from the file one by one as read. You can also open a file and read all the lines as a list and the iteration is hidden.

	myfile = open('catalog.dat')		myfile = open('catalog.dat')