1. Teaching London Computing
1.1 Numbers and Strings
Margaret Derrington KCL Easter 2014

2. Remember the two PYTHON windows
The shell
Where the program runs
The IDLE
Where you write and edit a program
You can also OPEN an existing program
From ANY FILE menu
It will open in its own IDLE window where you can edit it and click RUN
And it will run in the shell window.
If you close the SHELL, it will open when you RUN something
If you close the IDLE you can open a new one from the SHELL
If you close them both, you will have to open Python all over again

3. The Shell The shell is a bit like a calculator
If you write a line of code in there
Next to the prompt >>>
When you click RETURN/ENTER/SEND
It will run and print… there, where it is, in the shell
But you cannot write programs here
Because every time you click ENTER
It runs what you have just typed
But one line scripts will run here…
So it is useful for testing and checking things
If you are writing a complicated program in the IDLE, you can check or test bits of it as you go.

4. So let’s test some stuff in the shell
Type a string in the shell “Hello World”
And it will print it back to you
Type a sum * or
It will give you the answers…
So, what happens if you add a string to a number?
What happens if you multiply a string by a number?
What happens if you add two strings? Or three?
Interesting?
It’s called concatenation

5. ARITHMETIC
Insert numbers for x and y and work out what these OPERATORS do:
x + y
x – y
x * y
x ** y
x / y
x // y -x +y
pow(x,y)
You can test them all in the python shell……
……………………..they are one-liners!!!
Check what happens with
10 – 5 – 2
10 – (5-2)
TWO kinds of division
10/4
10//4
Well THREE actually 10%4
And 2 * how does this compare with a calculator?

6. Indexing and slicing strings
INDEXING returns a numbered character
"william"[0] – gives ‘w’
"william"[1] – gives ‘i’
"william"[6] – gives ‘m’
and “william”.index(“i”) gives 1
…the index or position of the first i
SLICING gives a range
"william"[1:4]– gives ‘ill’
The numbering starts with ZERO!!!!!
(not all languages do this, but many do)
INcluding character number 1
EXcluding character number 4

7. String Length
len() is a function that returns the LENGTH of a string; the number of characters
len(“hello world”) gives (including the space)
How would you make python return “world?
Use slicing and THINK about the numbers
You should be able to do this in FOUR different ways!!!!!!
Hint:: use len and index

8. String Length
len() is a function that returns the LENGTH of a string; the number of characters
len(“hello world”) gives (including the space)
How would you make python return “world?
Use slicing and THINK about the numbers
“hello world” [6:10]
Or “hello world” [6:len(“hello world”)]
Or “hello world” [“hello world”.index(“ ”):10] or
“hello world” [“hello world”.index(“ ”): 6:len(“hello world”)]

9. Errors Python has fewer errors than other languages (e.g. Java)
This has both pros and cons
Not everything we write makes sense
Syntax error:
123abc
1 ! 3
“hello
“I can’t understand what you are asking”
Wassup???
Wosswrong????

10. Errors Python has fewer errors than other languages (e.g. Java)
This has both pros and cons
Not everything we write makes sense
Syntax error:
123abc – not a number
1 ! – not an operator
“hello – a string with no end
“I can’t understand what you are asking”

11. Why do these not make sense???
Evaluation Errors
“The text looks ok but when I try to calculate, it makes no sense”
42 + “hello”
42 / 0
“hello”[17]
Why do these not make sense???
What’s wrong?

12. Evaluation Errors
“The text looks ok but when I try to calculate, it makes no sense”
42 + “hello” – can’t add a number to a string
42 / – can’t divide by zero
“hello”[17] – can’t index beyond the end
“hello”[5] is this correct ?????

13. We have speeded up and learnt a LOT more!!!
IDLE, SHELL, open save run
ARITHMETIC including some new operators
** // and % and a function pow (n,m)
Strings: indexing, slicing, concatenating, the function len() and the method “string”.index()
And a little bit about errors.
We have been doing these in the shell
Next we are going to use what we have learnt to write some PROGRAMS…… where?

14. Back in the IDLE window Write a program that will print your name
print the number of letters in your name
Slice your name and print it letter by letter each on a different line
Add the number on letters in your first name to the number in your second name and print the answer.
Save your program in your named file in my documents… call it something suitable.
THINK…. Are you going to do this all at once or are you going to try it out bit by bit, adding each next bit when you have got the rest to work?
……… just wondered…. 