Head-first Python

Author: christophe@pallier.org

Can you guess what the following program does?

if 2 + 2 == 5:
    print("Oops")
else:
    print("Fine")
    print("2 + 2 = 4")

Remark: Python uses indentation rather than delimiters to regroup series of instructions into a block

Can you guess what the following program does?

name = raw_input("What is your name? ")
print("Hello " + name + "!")

Remarks:

• '+' as the string concatenation operator
• (in Python3: raw_input -> input)

Can you guess what the following program does?

x = raw_input("Enter a number: ")
if int(x) % 2 == 0:  # '%' is the modulo operator
    print(x + " is even")
else:
    print (x + " is odd")

Can you guess what the following program does?

while True:
    x = raw_input('Enter a number ("q" to stop)')
    if x == 'q':
        break  # interupts the while loop
    if int(x) % 2 == 0:  # '%' is the modulo operator
        print(x + " is even")
    else:
        print (x + " is odd")

Can you guess what the following programs do?

x = 1
while x <= 10:
    print(x)
    x = x + 1

reg1, reg2 = 5, 2
while reg1 > 0:
    reg2 += 1
    reg1 -= 1
print(reg1, reg2)

Using a 'while' loop, write a program which computes the sum of the first 'n' integers.

...

A few remarks about python lists

# the elements do not have to be of the same type
a = [3, 4, 'dog']  
print(a[0]) # change the index 0 into 1, then 2

# you can even embbed lists inside lists
sentence = [['the', 'dog'], [ 'bites', [['the', 'cat']]]]
sentence[1][1]

a = [1, 2, 3]
b = a
a[1] = 666
b

This is because a and b point to the same memory area!

a = [1, 2, 3]
b = a[:]  # makes a copy in memory
a[1] = 666
b

a = [1, 2, 3]
b = a
a = [4, 5, 6]
b

Can you guess what the following program does?

sum = 0
for i in [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]:
	sum = sum + i
print(sum)

Remark: we could have used the expression 'for x in range(11)'

Write a program that prints the multiplication tables from 1 to 10

...

Remarks:

# python's for has some nifty features
for n, item in enumerate(['alpha', 'beta', 'gamma']):
    print n, item
    
for _ in range(10):
    print('All work and no play make Jack a dull boy!')

Defining functions in python with 'def'

def somme(n):
    sum = 0
    for i in range(n + 1):
        sum = sum + i
    return sum

print(somme(10))
print(somme(20))

Write a function that returns the average of a list of numbers

...

Given the following function, write a program that prints the prime numbers below 100

def is_factor(a, b):
    return b % a == 0

print(is_factor(3, 27))
print(is_factor(3, 28))

        

True
False

Guess what the following program does

def prod(x):
	p = 1
	for e in x:
		p = p * e
	return p
	
prod([4, 5, 6])

Modify the following program to draw regular polygon of 'n' sides

import turtle
turtle.forward(100)
turtle.left(120)
turtle.forward(100)
turtle.left(120)
turtle.forward(100)
turtle.done()

Functions that call themselves are called 'recursive'

def fact(n):
    if n == 0:
        return 1
    else:
        return n * fact(n - 1)
    
print(fact(10))

Write a function that returns the n^{th} Fibonnaci number

F(0)=1; F(1)=1; F(n)=F(n-1)+F(n-2)

Study the following program

import random   # module importation
n = random.randint(1, 99)
guess = input('Guess a number between 1 and 99 ? ')

while guess != n:
	if guess > n:
		print('Too large!')
	if guess < n:
		print('Too Small!')
	guess = input('New guess?')

print("Correct!")

Write a program where you think about a number and the computer tries to find it

...

Write a program which, given a string of '0' and '1' representing a number in binary format, prints the value of this number

...

Write a program which, given a number in decimal, prints its binary representation

...

Write a simulator of a Register Machine a la Rodrego

The input is a a list of instructions, e.g. [['inc',1,1], ['deb',1,0,3], ['end']] (no need for line number)