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Python basics
–
lists & tuples
–
range
–
for-in loop
–
functions
–
exercise
Data type → list
Ordered collection of items
●
●
●
●
●
items can be referenced by [index] (starting from 0)
items can be of any type (including other lists)
can contain items of various types
can be modified (grow, shrink , item changes)
can be joined, sliced, compared
x = [item, item, item, …]
aa == [1,
[1, 2,
2, 3]
3]
bb == ['apple','orange']
['apple','orange']
cc == [[3,'red'],[5,'blue']]
[[3,'red'],[5,'blue']]
print
print a[0],
a[0], b[1],
b[1], c[1],
c[1], c[1][0]
c[1][0]
list cont.
Some operation on lists:
–
–
–
–
–
–
–
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
[1,
[1,
x[index] = newitem → change existing item
x.insert(index, item) → insert item at index position
x.append(item) →add item at the end
del x[index] → remove item at index position
x.remove(item) → remove item (first occurrence)
len(x) → return list length
item in x → reports existence of item in list (True of False)
aa == [1,
[1, 2,
2, 3]
3]
a.append(5)
a.append(5)
a.insert(3,4)
a.insert(3,4)
print
print aa
2,
2, 3,
3, 4,
4, 5]
5]
>>>
>>> del
del a[1]
a[1]
>>>
>>> a.remove(4)
a.remove(4)
>>>
>>> 11 in
in aa
True
True
>>>
>>> print
print aa
[1,
[1, 3,
3, 5]
5]
Data type →tuple
Tuple → a fixed list
●
●
●
tuples can story any types of objects
access to elements is identical to lists
no modifications are allowed
x = (item, item, item, …)
>>>
>>> aa == (1,
(1, [2])
[2])
>>>
>>> print
print a[0]
a[0]
11
>>>
>>> a[0]=0
a[0]=0
TypeError:
TypeError: 'tuple'
'tuple' does
does not
not support
support item
item assignment
assignment
>>>
>>> a[1][0]=0
a[1][0]=0
>>>
>>> print
print aa
(1,
(1, [0])
[0])
List of integers → range()
List of integers with arithmetic progression
range(stop)
–
–
range(3) → [0, 1, 2]
range(10) → [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
range(start, stop [,step])
–
–
–
range(1, 3) → [1, 2]
range(-1, 2) → [-1, 0, 1]
range(1, 11, 2) → [1, 3, 5, 7, 9]
>>>
>>> aa == range(1000)
range(1000)
>>>
>>> len(a)
len(a)
1000
1000
>>>
>>> aa == [range(1000)]
[range(1000)]
>>>
>>> >>>
>>> print
print len(a),
len(a), len(a[0])
len(a[0])
11 1000
1000
Definite loop → for-in
Definite loop
●
●
●
repeat the body defined number of times
iterate over elements of list or tuple (also non-numerical)
break and continue are applicable
for item in list:
instr
for
for nn in
in range(10):
range(10):
print
print n**2
n**2
for
for nn in
in [4,
[4, 2,-1]:
2,-1]:
print
print n**2
n**2
I=('pretty','smart','modest')
I=('pretty','smart','modest')
for
for ss in
in I:
I:
print
print "I'm
"I'm "+s
"+s
Functions
Function – sequence of instructions executed
as single instruction
–
–
–
function can take parameters and return value(s)
names created inside function are local
names existing in outer scope are accessible
def function( parameters ):
instructions
return [value(s)]
def
def fib(n):
fib(n):
if
if n<=0:
n<=0: return
return 00
a,
a, bb == 0,
0, 11
for
for ii in
in range(n-1):
range(n-1):
a,
a, bb == b,
b, a+b
a+b
return
return bb
...
...
print
print fib(3)
fib(3)
def
def fibseq(n):
fibseq(n):
seq
seq == []
[]
for
for ii in
in range(n):
range(n):
seq.append(fib(i))
seq.append(fib(i))
return
return seq
seq
...
...
cc == fibseq(10)
fibseq(10) #c
#c is
is aa list
list
This is implementation of Fibonacci sequence has poor
performance and is here just for demonstration purpose
Function documentation
Describes functionality, parameters
and return value
–
used for built-in documentation system
def
def fib(n):
fib(n):
"""
""" Fibonacci
Fibonacci number"""
number"""
if
if n<=0:
n<=0: return
return 00
a,
a, bb == 0,
0, 11
for
for ii in
in range(n-1):
range(n-1):
a,
a, bb == b,
b, a+b
a+b
return
return bb
...
...
print
print fib(3)
fib(3)
def
def fibseq(n):
fibseq(n):
"""
"""
Returns
Returns the
the sequence
sequence
of
of n-Fibonacci
n-Fibonacci
numbers
numbers
"""
"""
seq
seq == []
[]
for
for ii in
in range(n):
range(n):
seq.append(fib(i))
seq.append(fib(i))
return
return seq
seq
print
print fibseq(10)
fibseq(10)
Exercise
Problem: Hailstone numbers sequence
–
cn+1 = ½∙cn
if cn is even
–
cn+1 = 3∙cn+1
if cn is odd
–
stop when reaching 1
–
(polish: liczby gradowe)
example: c=3 → 10, 5, 16, 8, 4, 2, 1
def
def next_hsn(c):
next_hsn(c):
if
if cc %% 22 ==
== 0:
0:
return
return c//2
c//2
else:
else:
return
return 3*c+1
3*c+1
def
def hsn(c):
hsn(c):
sequence
sequence == [c]
[c]
while
while c>1:
c>1:
cc == next_hsn(c)
next_hsn(c)
sequence.append(c)
sequence.append(c)
return
return sequence
sequence
print
print hsn(27)
hsn(27)
Exercise
Investigation:
1) compute and store all hsn sequences (up to 100)
2) find the longest sequence
3) find the highest value reached
● optionally: find the sequence with biggest sum of elements
● optionally: find the most common sequence length
● optionally: find the longest common subsequence
## solution
solution to
to 11
all_hsn
all_hsn == []
[]
for
for cc in
in range(101):
range(101):
all_hsn.append(hsn(c))
all_hsn.append(hsn(c))
print
print all_hsn
all_hsn
hsn(0) →[0]
hsn(1) →[1]
hsn(2) →[2, 1]
...