1. Python for Science and Engg:
Plotting experimental data
FOSSEE
Department of Aerospace Engineering
IIT Bombay
12 February, 2010
Day 1, Session 2
FOSSEE (IIT Bombay) Plotting with Python 1 / 34

2. Outline
1 Plotting Points
2 Lists
3 Simple Pendulum
4 Strings
5 Summary
FOSSEE (IIT Bombay) Plotting with Python 2 / 34

3. Plotting Points
Outline
1 Plotting Points
2 Lists
3 Simple Pendulum
4 Strings
5 Summary
FOSSEE (IIT Bombay) Plotting with Python 3 / 34

4. Plotting Points
Why would I plot f(x)?
Do we plot analytical functions or experimental data?
In []: x = [0, 1, 2, 3]
In []: y = [7, 11, 15, 19]
In []: plot(x, y)
Out[]: [<matplotlib.lines.Line2D object at 0xa73a
In []: xlabel(’X’)
Out[]: <matplotlib.text.Text object at 0x986e9ac>
In []: ylabel(’Y’)
Out[]: <matplotlib.text.Text object at 0x98746ec>
FOSSEE (IIT Bombay) Plotting with Python 4 / 34

5. Plotting Points
Is this what you have?
FOSSEE (IIT Bombay) Plotting with Python 5 / 34

6. Plotting Points
Plotting points
What if we want to plot the points!
In []: clf()
In []: plot(x, y, ’o’)
Out[]: [<matplotlib.lines.Line2D object
In []: clf()
In []: plot(x, y, ’.’)
Out[]: [<matplotlib.lines.Line2D object
FOSSEE (IIT Bombay) Plotting with Python 6 / 34

7. Plotting Points
FOSSEE (IIT Bombay) Plotting with Python 7 / 34

8. Plotting Points
Additional Plotting Attributes
’o’ - Filled circles
’.’ - Small Dots
’-’ - Lines
’- -’ - Dashed lines
FOSSEE (IIT Bombay) Plotting with Python 8 / 34

9. Lists
Outline
1 Plotting Points
2 Lists
3 Simple Pendulum
4 Strings
5 Summary
FOSSEE (IIT Bombay) Plotting with Python 9 / 34

10. Lists
Lists: Introduction
In []: x = [0, 1, 2, 3]
In []: y = [7, 11, 15, 19]
What are x and y?
lists!!
FOSSEE (IIT Bombay) Plotting with Python 10 / 34

11. Lists
Lists: Initializing & accessing elements
In []: mtlist = []
Empty List
In []: p = [ 2, 3, 5, 7]
In []: p[0]+p[1]+p[-1]
Out[]: 12
FOSSEE (IIT Bombay) Plotting with Python 11 / 34

12. Lists
List: Slicing
Remember. . .
In []: p = [ 2, 3, 5, 7]
In []: p[1:3]
Out[]: [3, 5]
A slice
In []: p[0:-1]
Out[]: [2, 3, 5]
In []: p[::2]
Out[]: [2, 5]
list[initial:final:step]
FOSSEE (IIT Bombay) Plotting with Python 12 / 34

13. Lists
List operations
In []: b = [ 11, 13, 17]
In []: c = p + b
In []: c
Out[]: [2, 3, 5, 7, 11, 13, 17]
In []: p.append(11)
In []: p
Out[]: [ 2, 3, 5, 7, 11]
FOSSEE (IIT Bombay) Plotting with Python 13 / 34

14. Simple Pendulum
Outline
1 Plotting Points
2 Lists
3 Simple Pendulum
4 Strings
5 Summary
FOSSEE (IIT Bombay) Plotting with Python 14 / 34

15. Simple Pendulum
Simple Pendulum - L and T
Let us look at the Simple Pendulum experiment.
L T T2
0.1 0.69
0.2 0.90
0.3 1.19
0.4 1.30
0.5 1.47
0.6 1.58
0.7 1.77
0.8 1.83
0.9 1.94
LαT 2
FOSSEE (IIT Bombay) Plotting with Python 15 / 34

16. Simple Pendulum
Lets use lists
In []: l = [0.1, 0.2, 0.3, 0.4, 0.5,
0.6, 0.7, 0.8, 0.9]
In []: t = [0.69, 0.90, 1.19,
1.30, 1.47, 1.58,
1.77, 1.83, 1.94]
FOSSEE (IIT Bombay) Plotting with Python 16 / 34

17. Simple Pendulum
Plotting L vs T 2
We must square each of the values in t
How to do it?
We use a for loop to iterate over t
FOSSEE (IIT Bombay) Plotting with Python 17 / 34

18. Simple Pendulum
Plotting L vs T 2
In []: tsq = []
In []: len(l)
Out[]: 9
In []: len(t)
Out[]: 9
In []: for time in t:
....: tsq.append(time*time)
....:
....:
FOSSEE (IIT Bombay) Plotting with Python 18 / 34

19. Simple Pendulum
How to come out of the for loop?
Hitting the “ENTER” key twice returns the cursor to the
previous indentation level
In []: for time in t:
....: tsq.append(time*time)
....:
....:
In []: print tsq, len(tsq)
FOSSEE (IIT Bombay) Plotting with Python 19 / 34

20. Simple Pendulum
FOSSEE (IIT Bombay) Plotting with Python 20 / 34

21. Simple Pendulum
What about larger data sets?
Data is usually present in a file!
Lets look at the pendulum.txt file.
$ cat pendulum.txt
1.0000e-01 6.9004e-01
1.1000e-01 6.9497e-01
1.2000e-01 7.4252e-01
1.3000e-01 7.5360e-01
1.4000e-01 8.3568e-01
1.5000e-01 8.6789e-01
...
Windows users:
C:> type pendulum.txt
FOSSEE (IIT Bombay) Plotting with Python 21 / 34

22. Simple Pendulum
Reading pendulum.txt
Let us generate a plot from the data file
File contains L vs. T values
L - Column1; T - Column2
FOSSEE (IIT Bombay) Plotting with Python 22 / 34

23. Simple Pendulum
Plotting from pendulum.txt
Open a new script and type the following:
l = []
t = []
for line in open(’pendulum.txt’):
point = line.split()
l.append(float(point[0]))
t.append(float(point[1]))
tsq = []
for time in t:
tsq.append(time*time)
plot(l, tsq, ’.’)
FOSSEE (IIT Bombay) Plotting with Python 23 / 34

24. Simple Pendulum
Save and run
Save as pendulum_plot.py.
Run using %run -i pendulum_plot.py
FOSSEE (IIT Bombay) Plotting with Python 24 / 34

25. Simple Pendulum
FOSSEE (IIT Bombay) Plotting with Python 25 / 34

26. Simple Pendulum
Reading files . . .
for line in open(’pendulum.txt’):
opening file ‘pendulum.txt’
reading the file line by line
line is a string
FOSSEE (IIT Bombay) Plotting with Python 26 / 34

27. Strings
Outline
1 Plotting Points
2 Lists
3 Simple Pendulum
4 Strings
5 Summary
FOSSEE (IIT Bombay) Plotting with Python 27 / 34

28. Strings
Strings
Anything within “quotes” is a string!
’ This is a string ’
" This too! "
""" This one too! """
’’’ And one more! ’’’
FOSSEE (IIT Bombay) Plotting with Python 28 / 34

29. Strings
Strings and split()
In []: greet = ’hello world’
In []: greet.split()
Out[]: [’hello’, ’world’]
This is what happens with line
In []: line = ’1.20 7.42’
In []: point = line.split()
In []: point
Out[]: [’1.20’, ’7.42’]
FOSSEE (IIT Bombay) Plotting with Python 29 / 34

30. Strings
Getting floats from strings
In []: type(point[0])
Out[]: <type ’str’>
But, we need floating point numbers
In []: t = float(point[0])
In []: type(t)
Out[]: <type ’float’>
FOSSEE (IIT Bombay) Plotting with Python 30 / 34

31. Strings
Let’s review the code
l = []
t = []
for line in open(’pendulum.txt’):
point = line.split()
l.append(float(point[0]))
t.append(float(point[1]))
tsq = []
for time in t:
tsq.append(time*time)
plot(l, tsq, ’.’)
FOSSEE (IIT Bombay) Plotting with Python 31 / 34

32. Strings
FOSSEE (IIT Bombay) Plotting with Python 32 / 34

33. Summary
Outline
1 Plotting Points
2 Lists
3 Simple Pendulum
4 Strings
5 Summary
FOSSEE (IIT Bombay) Plotting with Python 33 / 34

34. Summary
What did we learn?
Plotting points
Plot attributes
Lists
for
Reading files
Tokenizing
Strings
FOSSEE (IIT Bombay) Plotting with Python 34 / 34