Get It Interpreter Scripts Arrays. Basic Python. K. Cooper 1. 1 Department of Mathematics. Washington State University. Basics

Transcription

1 Basic Python K. 1 1 Department of Mathematics 2018

2 Python Guido van Rossum 1994 Original Python was developed to version continues to receive maintenance New Python 3.x 2008 The 3.x version will supersede the 2.x versions Currently on 3.5 We will be agnostic about Python version

3 Why Python? We have Matlab, R, Octave, Maple, Mathematica... Why would we learn yet another language? It is open source It is free Used in many disciplines Data science Web development Math/stat modeling

4 Getting It Many different versions it is everywhere I suggest using Enthought Canopy Comes with Python 2.7 default Installs Numpy, Scipy, Matplotlib, Sympy Nice IDE (interactive development environment) simple, but useful Others... Anaconda PyCharm

5 Python Interpreter We have noted that Python is an interpreted language. We can run the virtual machine directly: just type python or type in the command line window of your development tool. At that point, we at least have a nice calculator. >>> 3+4 >>> 3.1/4.2 >>> 72.1*.0001 >>> 7**2.1 Note that exponentiation is done using **, not.

6 Operator Precedence Precedence: ** first, * or / second, left to right, and + or last, left to right. Use parentheses to force precedence they way you want it. >>> 3** >>> 3**(2+1) 27

7 Special Functions >>> cos(1) Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name cos is not defined Python does not know special functions natively. The power of Python is that it has many modules. In particular, there is a math module. >>> import math >>> math.cos(1)

8 Modules The statement to get a module is import. Here are examples. 1. import math imports the module using names starting with math. 2. import math as m imports the module using names starting with m. 3. from math import cos imports the cos() function from math, no prefix 4. from math import cos as mycos imports the cos() function from math module, but calls it mycos() 5. from math import * imports the whole math module, with no prefixes We will not use the Math module. We ll use Scipy instead.

9 Data Types One of the big differences between Python 2.x and 3.x concerns data types. Python 2.x is somewhat more strongly typed than 3.x: integer arithmetic is different from floating point. >>> >>> >>> Python 2.x understands the difference between integers and floating point numbers. As soon as one floating point number appears, any computation is converted to floating point.

10 Integer Arithmetic The problem with this is that the result of any purely integer computation must be an integer. >>> 3/4 0 The non-integer part of the result is just truncated. If we do not want this to happen we must take care to pollute any computation with a floating point number. >>> 3/ In Python 3.x both of these calculations result in 0.75.

11 Integer Arithmetic It is best just to express all floating point numbers in floating point form. In Python 2.7: >>> 1/2 + 3./ >>> 1./2. + 3/4 0.5 >>> 1./ / Of course, it is even better simply to type 0.5 when we want 1/2. We can also use a function to convert numbers or variables to float: >>> float(3)/float(4) 0.75

12 Variables We want to be able to save results of computations for future use. We use variables for this. >>> a = 2.**3.1 >>> a The equals sign represents assignment: evaluate the expression on the right side, and put it in the variable on the left. >>> a = a+1 >>> a

13 Python Scripts A script is just a file containing a number of Python commands to be executed in sequence: in Linux... #!/usr/bin/python2.7 print( Hello, world! ) The first line tells the computer to use the python 2.7 interpreter on the commands in the file. The second line invokes the print() function to print out the character string Hello, world! We type these two lines into a file, and name it e.g. hello.py. Change the permissions using a command chmod ugo+x hello.py, to make it executable. Then just type the name hello.py. We could also just type python hello.py

14 Arrays Arrays are computer variables that contain a number of entries of the same type. Think of them as like vectors or matrices. They do not always behave the way we might think.

15 Lists The built-in array for Python is called a list. It is demarcated using brackets. This is perhaps the simplest example of a data structure. #!/usr/bin/python2.7 list1 = [1.,2,3] list2 = [0,-1,-2] print(list1+list2) print(2*list1) The result is [1.0, 2, 3, 0, -1, -2] [1.0, 2, 3, 1.0, 2, 3] You see that binary operations on lists are all about concatenation. We don t like that in general.

16 Enlarging Lists We can add elements to a list using e.g..append(). #!/usr/bin/python2.7 list1 = [1.,2,3] list1.append(4.0) print(list1) This results in [1.0, 2, 3, 4.0]

17 range We can create larger lists that follow an arithmetic progression using range(). #!/usr/bin/python2.7 list1=range(4) list2=range(1,4) list3=range(0,4,2) print(list1,list2,list3) The result is ([0, 1, 2, 3], [1, 2, 3], [0, 2])

18 Ranges range(n) range(a,n) range(a,n,s) If only one argument is given, then it represents the value where the count stops (greater than the last element). To emphasize,the list never includes this number n! By default, the list starts with entry 0. If there are two arguments, the first is the value of the first element. The third argument, if any, is the step taken between successive entries of the list.

19 Lists... can be created as a comma-separated collection of numbers enclosed in brackets.... can be created using range().... can contain entries of different data types.... can be extended using methods such as append().... can be concatenated using the + operator.... can be repeated using the * operator.... are of very limited use to mathematicians. We can t really do arithmetic with them. They are slow.