Introduction to ProgrammingWorkshop 2

PHYS1101 Discovery Skills in Physics

Dr. Nigel DipperRoom 125dn.a.dipper@durham.ac.uk

Part 1

• Sequences – arrays, lists etc

• ‘for’ loops – repeating code in a loop

• Indexing – into sequences

• Exercises

Sequences - 1

• We need to hold sequences of numbers• The main sequences in Python are:• Strings EG theName = ‘Monty Python’• Lists EG [1, 2.34, ‘fred’]• numpy arrays EG (1.1, 2.2, 3.3) – These can be n-dimensional

Type Index? Modify? Add elements?

Mixed types?

Tuple Yes No No Yes

String Yes Yes Yes No

List Yes Yes Yes Yes

Array Yes Yes No No

Dictionary No Yes Yes Yes

Examples of sequences

>>> myList = [1,2,3,4,5]

>>> print myList[2]

[1,2,3,4]

>>> myArray = numpy.array(myList)

>>> print myArray

[1 2 3 4]

>>> myTuple = (1,2,3)

>>> print myTuple

(1,2,3)

>>> myString = “Hello world”

>>> print myString

Hello world

‘for’ loops - 1

• If we want to do something many times, we write the code once and put it in a loop. There are two types of loop:

• ‘while’ loops terminate on a condition.• ‘for’ loops are told how many times to loop.• A very common way to build a for loop is to use the built-in ‘range()’

function. • Try this out at the python prompt. Type:

>>> range(10)

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]• It returns a list of integers starting at zero and ending at 10-1.• The parameter sent to the range function is one more than the max.• range() has other parameters; use help(range) to understand it.• Exercise: Make a list starting at 1, ending at 9 in steps of 2. It should

contain: [1,3,5,7,9].

‘for’ loops – 2

• A for loop needs a list or other sequence to iterate over. • We can use the built-in function range() to make a list.• The for loop will execute a block of code so remember to start with a

colon and indent all of the block• Try this from the python prompt:

>>> for i in range(10):

… print i

…• It should print the integers from 0 to 9. (Not 0 to 10!)• If you want to loop over every element in an array, use the length of

the array (len() is a built-in function) as the parameter to range():

>>> a = numpy.array([2,3,4,5])

>>> for i in range(len(a)):

… print a[i]

…

Indexing - 1

• Before we do an example with a ‘for’ loop…• How do we put data into and get data out of an array?• This process is called indexing. Try this:• Create a small 1-d array called myData with 5 real numbers in it• Change one of the values then print the array to see the change:

>>> myData[2] = 123.4

>>> print myData• Copy one of the values to a variable and print it:

>>> x = myData[3]

>>> print x• Note that indexing always uses square brackets [ ]• Later we will see how to read or write larger array parts or ‘slices’.

Indexing - 2

• Indexing is very similar for arrays, lists and strings.• Try it with a string:

>>> theName = ‘Monty Python’

>>> print theName[2]

>>> n• Remember that indexing starts at zero!• The data in any type of sequence are like items in pigeon holes.• The index is the label on the pigeon hole:

M o n t y P y t h o n

0 1 2 3 4 5 6 7 8 9 10 11

theName:

Index:

Exercises

• Do the following exercises.• The details are in the course notes.• Solutions will be on DUO after the workshop

• Exercise 6.3 – Array operations

• Exercise 6.4 – Find mean of a real array

• Exercise 7.1 – Use a for loop to find squares and cubes• You must show your solution for exercise 7.1 to a demonstrator before you leave.

• Exercise 7.2 –Find squares and cubes using array operators

Part 2

• for loops without ‘range()’

• while loops

• More exercises…

‘for’ loops - 3

• We often use range() to generate the object for a for loop to iterate over.

• But we don’t have to… Try this:

>>> values = [1.1, 2.3, 7.1, 4.4]

>>> for value in values:

… print value,

1.1 2.3 7.1 4.4

• values can actually be any ‘iterable object’!• Try it again where values is an array; arrays are iterable.• Note: the comma after value causes print to put the data all on one

line

While loops

• We use for loops when we know how many times we want to go round the loop.

• When we don’t know this, we use a while loop. • Exercise: Save this program to a file and run it:

x = range(10)

index = 0

while x[index] <= 5:

index = index +1

print ‘Now x is greater than 5’• This is a pointless example since we know beforehand when the

condition will be met; it just shows the syntax• Note that there is no automatic increment of a loop variable as there

is in a for loop; you must increment it yourself

Exercises

• Do the following exercises.• The details are in the course notes.• Solutions will be on DUO after the workshop

• Finish these first!!• Exercises: 6.3, 6.4, 7.1, 7.2

• Finish these now if you can or before the next workshop• Exercise 7.3 – Generate Fibonacci numbers

• Exercise 7.4 – Find the golden ratio