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Difference between revisions of "Solving problems with Python"

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(Functions)
 
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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.
  
  
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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
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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
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.
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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):
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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' }
  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
  
 +
However, dictionaries can be interated for both key and value by looking at the full list of items.
  
The readline() function for a file iterates line by line through the file.   
+
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
 +
 
 +
myfile = open('catalog.dat')
 +
  while True:
 +
  myline = myfile.readline() 
 +
  if not myline:
 +
    break
 +
  print myline
  
mydata = open('catalog.dat')
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prints lines from ''catalog.dat''. The simpler
  print mydata.readline()
 
  
prints a line from ''catalog.dat''.  Subsequent readline()'s go through the file one line at a time. While
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for myline in open('catalog.dat'):
 +
  print myline
  
  for line in open('catalog.dat')
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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.
  print line
 
  
prints lines from the file one by one without reading it as a block.
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myfile = open('catalog.dat')
 +
mydata = myfile.readlines()
 +
myfile.close()
  
 
== Examples ==
 
== Examples ==

Latest revision as of 19:07, 20 January 2015

Now with many useful tools in hand from our short course on using Python in Physics and Astronomy, let us see how to make them work together to solve problems.


Flow control

The if statement is fundamental to making decisions within a program. It works simply

x=0.1
y=10.
z=0.
if x > 0.:
  y = 1./x
elif x < -1.:
  pass
elif x == 0:
  print 'Cannot divide by zero.'
  exit()
else:
  y = 1./x
z = y

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. The program

flag = True
x = 0.
while flag:
  x = x + 1.
  if x > 10.:
    flag = False
print x

increases x until it is 11. and then prints the value.

Loops such as this may include a try block. This enables handling an exception, such as in this program to calculate x2 with input from keyboard.

while True:
  try:
    x = int(raw_input("Please enter a number: "))
    break
  except ValueError:
    print "Oops!  That was no valid number.  Try again..."
y=x**2
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.

Functions

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):
  if (angle < 0.0):   
    n = int(angle / 360.0) - 1
    return (angle - float(n) * 360.0)
  
  elif (angle >= 360.0):  
    n = int(angle / 360.0)
    return (angle - float(n) * 360.0)
  
  else:   
    return (angle)


Functions may have any number of objects as arguments, of any data type. Once defined, you may use a function anywhere in a program.

Iteration

The while loop makes repeated passes through a block of code as long as a test condition is satisfied. This allows us to make sequential changes to achieve a desired outcome, such as evaluating a series until the error is acceptable, or fitting data until the fitting errors are minimized. Python also has several built-in ways to manage interation, and the most useful is the for loop.

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

generates this output

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[-1.0, 0.0]
[-1.0, 0.0, 1.0]
[-1.0, 0.0, 1.0, 4.0]
[-1.0, 0.0, 1.0, 4.0, 9.0]
[-1.0, 0.0, 1.0, 4.0, 9.0, 16.0]
[-1.0, 0.0, 1.0, 4.0, 9.0, 16.0, 25.0]
[-1.0, 0.0, 1.0, 4.0, 9.0, 16.0, 25.0, 36.0]
[-1.0, 0.0, 1.0, 4.0, 9.0, 16.0, 25.0, 36.0, 49.0]
[-1.0, 0.0, 1.0, 4.0, 9.0, 16.0, 25.0, 36.0, 49.0, 64.0]
[-1.0, 0.0, 1.0, 4.0, 9.0, 16.0, 25.0, 36.0, 49.0, 64.0, 81.0]


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 x2.0 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 through a list of its keys would be

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

which will print

51 spiral galaxy

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' }
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

myfile = open('catalog.dat')
while True:
  myline = myfile.readline()   
  if not myline:
    break
  print myline

prints lines from catalog.dat. The simpler

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

myfile = open('catalog.dat')
mydata = myfile.readlines()
myfile.close()

Examples

For examples of Python illustrating flow control, functions, and iteration, see the examples section.

Assignments

For the assigned homework to use these ideas, see the assignments section.