Intermezzo

• Review together what has been learned so far, and compare with the lecture’objectives

• Clarify Python syntax

• Consolidate some practical skills and some notions of Python with simple exercices

Objectives of the AIP

At the end, you should be able to write simple Python scripts to:

- compute basic statistics from files containaing numeric data
- compute the number of occurrences of words in a text file
- play the "guess a number" game

Then, you should be able profit more from the other Ateliers (Atelier d’Experimentation Humaine, Atelier de Statistiques, …)

Python syntax

We have hardly mentioned Python’s syntax yet (because it is boring…)

Let us go to the lecture 04_Python_in_a_nutshell

A peculiarity of Python: the use of indentation do delimit blocs of instructions.

Let us check the script AIP2015/resources/python-scripts/human-guess-a-number.py and verify that it comprises only the elements just described.

Exercices

Expressions

1. How much is worth 350 euros in USD if the exchange rate if 1.17 USD = 1 euro?

. . .

350 * 1.17

You can use Python’s command line as a calculator.

. . .

Loops and conditional statements

2. What does the following code do?

if (2 + 2) == 5:
    print("Sorry?")
else:
    print("Bonjour!")

. . .

Answer: It prints ‘Bonjour!’

. . .

3. What does the following code do?

while True:
    print('!')

. . .

Answer: It prints ‘!’ forever. Press Ctrl-C to interrupt it. Or use the task manager of your operating system.

4. What does the following program do?

answer = ""
while answer != 'You are the best!':
    answer = raw_input('Who is the best?')
print('Sure!'

Run it!

. . .

5. What does the following code do?

for x in ('a', 'b', 'c', 'd', 'e'):
    print x=='c'

. . .

Run it!

Remark: strings are sequences, therefore the following code works in the same way:

for x in 'abcde':
    print x=='c'

6. How to print a string of 80 ‘#’?

. . .

s = ""
for _ in range(80):
    s = s + "#"
print(s)

Remark: the operator ’’ on strings can also be used “#” 80

. . .

7. What does the following code do?

reg0 = 10
reg1 = 5
while reg0 > 0:
    reg0 = reg0 - 1
    reg1 = reg1 + 1
print(reg0)
print(reg1)

. . .

Remember the register machine rodrego?

. . .

8. How to improve the following program:

print "2 * 1 = 2",
print "2 * 2 = 4"
print "2 * 3 = 6"
print "2 * 4 = 8"
...
print "2 * 10= 20"

. . .

for i in range(1, 11):
    print "2 * " + str(i) + " = " + str(2*i)

. . .

9. How to print all multiplication tables from 1 to 10?

for a in range(1, 11):
    print('---')
    for b in range(1, 11):
        print str(a) + ' * ' + str(b) + ' = ' + str(a*b)

Defining functions

10. Try to guess what the following script does and run it:

import turtle

def square(n):
  turtle.fd(n)
  turtle.right(90)
  turtle.fd(n)
  turtle.right(90)
  turtle.fd(n)
  turtle.right(90)
  turtle.fd(n)
  turtle.right(90)

square(50)
a = raw_input('stop?')

Define the following function and execute it

def spirale():
    for n in (50, 60, 70, 80, 90, 100):
        square(n)
        turtle.left(30)

How would you improve this code?

11. What does the following code do?

nombres = []
while True:
        answer = raw_input('data point (Press just Enter to finish)?')
    if answer == '':
                break
    nombres.append(float(answer))

sum, sumsq = 0.0, 0.0
for x in nombres:
    sum = sum + x
    sumsq = sum + x*x

print(sum/len(nombres))
print(sumsq/len(nombres))

12. define a function f that takes ‘x’ and ‘y’ as parameters and returns 0.5*(x+y/x).

. . .

def f(x, y):
    return 0.5 * ( x + (y / x))

Compute f(10, 2), f(f(10,2),2), f(f(f(10,2),2),2), and so on.

Compute f(10, 9), f(f(10,9),9), f(f(f(10,9),9),9) and so on…

Compute f(10, 16), f(f(10,16),16), f(f(10,16),16,16),…

Computer f(10, 25), f(f(10,25),25), and so on…

Can you form a conjecture on what happens?

. . .

13. write a script that plays computer-guess-a-number, that is, you think about a number and the computer makes a succession of guess, and you tell it whether it is too large, to low or correct.