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Python

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By: Ben Blake, Andrew Dzambo, Paul Flanagan

- Spacing
- Comments
- Header
- Consistency with variables – keep it simple
- Set all variables equal to zero initially
- Notes on changes to code – version control
- Good formatting example: http://www.personal.psu.edu/amd5554/resources/documented_code.pdf

- Declaring variables – don't need to create variable initially
- Indenting in loops – no end statement
- Capitalization matters – Temp, temp, tEmp, TEMP are all different variables

- Mathematical expressions are the same
- + Addition
- - Subtraction
- * Multiplication
- / Division
- ** Exponentiation
- 9.8E-8 = 9.8 * (10 ** (-8))

- Built-in functions
- float, int, max, min, abs

- Imported functions
- sin, cos, tan, asin, acos, atan, log (natural log), log10 (base 10 log), exp, sqrt, pi, e

- Trigonometric functions work exclusively in radians
- k = m.cos(a * m.pi / 180.0)
- degrad = m.pi / 180.0
- k = m.cos(a * degrad)

- Some commands/functions need to be imported in order to be used
- Some libraries that can be imported: math, numpy, pylab
- Different ways to import
- from math import cos, sin, acos, pi
- import math
- k = math.cos(a * m.pi / 180.0)

- import math as m
- k = m.cos(a * m.pi / 180.0)

- Linecount += 1 ==>linecount= linecount + 1
- Average /= linecount==> average = average / linecount
- Balance -= payment ==> balance = balance – payment
- Population *= growth ==> population = population * growth

- Need to distinguish between read-only (input) files and writeable (output) files
- “r” = read-only, “w” = writeable
- infile = open(“weather.csv”, “r”)
- outfile = open(“pressure.txt”, “w”)

- Reading input files
- vap_list = infile.readlines()
- for vaporpressure in vap_list:

- Print statements
- Print >> outfile, x, y, vaporpressure
- If a number immediately follows the %, it is the width (in spaces) of the field in which the object will be written
- Print ‘%4f’ % x, ‘%4f’ % y this will print x and y as floating point numbers over 4 spaces

- Types: for, if, while loops
- Indenting denotes code is in loop
- To close loop, unindent the next line
- Example of a simple loop - counts # of x's in xlist
for x in xlist:

y += 1

print y

- Determinant loop – use when you know how long you want the program to run
- Similar to the “do loop” in ForTran and C++
- Two examples of for loops – can use either an input file or an array
for station in stations:

for k in range(n):

- Used to make logical decisions
- Can be imbedded inside for loops
if logical_expression_1:

# do this block when logical_expression_1 is true

elif logical_expression_2:

# do this block when logical_expression_2 is true

else:

# do this block when neither logical expression above is true

- Comparisons of one variable to another or to a constant using comparison operators
- == equals
- < less than
- <= less than or equal to
- != not equals
- > greater than
- >= greater than or equal to

- Indeterminant loop – use when duration of loop is unknown
- Can be imbedded inside for loops
- General while loop structure
while logical_expression:

# statements to run as long as logical_expression stays true

- Can use to terminate a loop or part of a specific loop if a statement becomes true
- Example of how break statement is used
x = 0

for x in xlist:

if x >= 40:

x += 1

break

else:

x += 1

- Collection of strings, floating point numbers, or integers listed in some order
- Arrays are special form of list in which all elements are of same data type
- Numeric Python module (numpy) is used to work with arrays

- List – create a defined list-type object
- x = list([4.0, ‘Hypsometric’, 34])

- Range – returns list of integers in specified range – important in for loops
- range(4) returns [0, 1, 2, 3]
- range (2,4) returns [2, 3]

- Len – counts how many numbers are in a list
- len(range(2,4)) produces a value of 2

- Sum – adds up the numbers in a list

- Input files consists of strings which can be split into component strings and then converted into numbers
- Split method is used to break strings into more useful components
- Default separator is a blank space, but separators can be anything, such as , : ; / -
- Line splitting most useful when done inside a loop
- Line = “32, 32.4, 36.8, Freezing Points”
- q = float(line.split(“,”)[2]) = 36.8