The following
while
loop prints all the items in the
list.
But we’re about to see simpler ways to do the same thing.
monkey rooster dog pig rat ox tiger hare dragon snake horse sheep
for
loop
to loop through the numbers in a
range.
We can do this because the return value of the
range
function is
iterable.
for i in range(n): #could also say range(0, n) or range(0, n, 1) print(animals[i])
monkey rooster dog pig rat ox tiger hare dragon snake horse sheep
list.
We can do this because a
list
is
iterable.
for animal in animals: print(animal)
monkey rooster dog pig rat ox tiger hare dragon snake horse sheepSo far, we have seen that a
for
loop can loop through
range
stringstr.)
while
loop:
i = 0 while i < n: print(i, animals[i]) i += 1
0 monkey 1 rooster 2 dog 3 pig 4 rat 5 ox 6 tiger 7 hare 8 dragon 9 snake 10 horse 11 sheepbut it’s simpler to use a
for
loop and a
range.
for i in range(n): print(i, animals[i])
0 monkey 1 rooster 2 dog 3 pig 4 rat 5 ox 6 tiger 7 hare 8 dragon 9 snake 10 horse 11 sheep
n.
We saw
enumerate
here.
for i, animal in enumerate(animals): print(i, animal)
0 monkey 1 rooster 2 dog 3 pig 4 rat 5 ox 6 tiger 7 hare 8 dragon 9 snake 10 horse 11 sheep
Let’s align the columns and start the numbers at 1.
for i, animal in enumerate(animals, start = 1): print(f"{i:2} {animal}")
1 monkey 2 rooster 3 dog 4 pig 5 rat 6 ox 7 tiger 8 hare 9 dragon 10 snake 11 horse 12 sheep
for animal in animals[:3]: print(animal)
monkey rooster dogPrint all but the first item in the list:
for animal in animals[1:]: print(animal)
rooster dog pig rat ox tiger hare dragon snake horse sheepPrint the items in reverse order:
for animal in reversed(animals): #or for animal in animals[-1::-1]: print(animal)
sheep horse snake dragon hare tiger ox rat pig dog rooster monkey
""" italy.py Draw the Italian flag on a tkinter Canvas widget. """ import tkinter #Colors of the vertical bars, from left to right. colors = [ "#008C45", #green "#F4F5F0", #white "#CD212A" #red ] stripeWidth = 200 #measured in pixels width = len(colors) * stripeWidth height = width * 2 // 3 #Make sure the height is an integer. root = tkinter.Tk() root.title("Italian Flag") root.geometry(f"{width}x{height}") canvas = tkinter.Canvas(root, highlightthickness = 0) for i, color in enumerate(colors): canvas.create_rectangle(i * stripeWidth, 0, (i + 1) * stripeWidth, height, width = 0, fill = color) canvas.pack(expand = tkinter.YES, fill = "both") root.mainloop()
pip3 install playsound pip3 install pyobjc pip3 install AppKit
""" Play a list of audio files. """ import sys import time import playsound audioFiles = [ #puck guitar string "/Library/Frameworks/Python.framework/Versions/3.8" \ "/lib/python3.8/test/audiodata/pluck-pcm24.au", #Chinese sound effect "http://oit2.scps.nyu.edu/~meretzkm/swift/button/chinese.mp3", #"Stand By Me" "http://plg208.free.fr/sauvegardes%20PC%20portable%20mes%20doc" \ "/MUSIQUE/Ben%20E.%20King%20-%20Stand%20By%20Me.mp3", #You'll Never Walk Alone" "http://www.fcsongs.com/uploads/audio" \ "/Liverpool%20FC%20-%20You%20Will%20Never%20Walk%20Alone.mp3" ] for audioFile in audioFiles: try: playsound.playsound(audioFile, True) except OSError as error: print(error, file = sys.stderr) sys.exit(1) time.sleep(1) sys.exit(0)Make it print “Now playing track 1…”, etc.
list
of numbers.
months = [
31, #January
28, #February
31, #March
30, #April
31, #May
30, #June
31, #July
31, #August
30, #September
31, #October
30, #November
31 #December
]
summation = 0
for month in months:
summation += month #means summation = summation + month
average = summation / len(months)
print(f"A year has {summation} days.")
print(f"The average month has {average} days (approx. {average:.4f}).")
A year has 365 days. The average month has 30.416666666666668 days (approx. 30.4167).
It’s easier to use the
sum
built-in
function.
months = [
31, #January
28, #February
31, #March
30, #April
31, #May
30, #June
31, #July
31, #August
30, #September
31, #October
30, #November
31 #December
]
summation = sum(months) #or summation = functools.reduce(operator.add, months, 0)
average = summation / len(months)
print(f"A year has {summation} days.")
print(f"The average month has {average} days (approx. {average:.4f}).")
A year has 365 days. The average month has 30.416666666666668 days (approx. 30.4167).
If all you want is the average,
it’s even easier to use the
mean
function from the
Python Standard Library.
import statistics months = [ 31, #January 28, #February 31, #March 30, #April 31, #May 30, #June 31, #July 31, #August 30, #September 31, #October 30, #November 31 #December ] try: average = statistics.mean(months) except StatisticsError as error: print(error, file = sys.stderr) sys.exit(1) print(f"The average month has {average} days (approx. {average:.4f}).")
The average month has 30.416666666666668 days (approx. 30.4167).
sum,
could you use
min
to find the length of the shortest month?
The length of the shortest month is 28 days.
print.
The last two statements do the same thing.
print(months) print(*months) print(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
[31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31] 31 28 31 30 31 30 31 31 30 31 30 31 31 28 31 30 31 30 31 31 30 31 30 31
list:
import sys months = [ "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" ] m = "June" for i, month in enumerate(months): if month == m: break else: print(f"The for loop did not find {m}.") sys.exit(1) print(f"The for loop found {m} at index {i}.") try: i = months.index(m) except ValueError as error: print(error) sys.exit(1) else: print(f"The index method found {m} at index {i}.") if m in months: print(f"{m} is a month.") sys.exit(0) else: print(f"{m} is not a month.") sys.exit(1)
The for loop found June at index 5. The index method found June at index 5. June is a month.
""" Output the lyrics to "The Twelve Days of Christmas". """ import sys gifts = [ None, #so that "partridge" is index number 1 "partridge in a pear tree.", # 1 "turtle doves", # 2 "french hens", # 3 "calling birds", # 4 "gold rings", # 5 "geese a-laying", # 6 "swans a-swimming", # 7 "maids a-milking", # 8 "ladies dancing", # 9 "lords a-leaping", #10 "pipers piping", #11 "drummers drumming" #12 ] n = len(gifts) for day in range(1, n): if day == 1: suffix = "st" elif day == 2: suffix = "nd" elif day == 3: suffix = "rd" else: suffix = "th" print(f"On the {day}{suffix} day of Christmas my true love gave to me") for d in range(day, 0, -1): if d > 1: prefix = d elif day == 1: prefix = "A" else: prefix = "And a" print(f"\t{prefix} {gifts[d]}") #\t is a tab character print() sys.exit(0)
On the 1st day of Christmas my true love gave to me A partridge in a pear tree. On the 2nd day of Christmas my true love gave to me 2 turtle doves And a partridge in a pear tree. On the 3rd day of Christmas my true love gave to me 3 french hens 2 turtle doves And a partridge in a pear tree. On the 4th day of Christmas my true love gave to me 4 calling birds 3 french hens 2 turtle doves And a partridge in a pear tree. On the 5th day of Christmas my true love gave to me 5 gold rings 4 calling birds 3 french hens 2 turtle doves And a partridge in a pear tree. On the 6th day of Christmas my true love gave to me 6 geese a-laying 5 gold rings 4 calling birds 3 french hens 2 turtle doves And a partridge in a pear tree. On the 7th day of Christmas my true love gave to me 7 swans a-swimming 6 geese a-laying 5 gold rings 4 calling birds 3 french hens 2 turtle doves And a partridge in a pear tree. On the 8th day of Christmas my true love gave to me 8 maids a-milking 7 swans a-swimming 6 geese a-laying 5 gold rings 4 calling birds 3 french hens 2 turtle doves And a partridge in a pear tree. On the 9th day of Christmas my true love gave to me 9 ladies dancing 8 maids a-milking 7 swans a-swimming 6 geese a-laying 5 gold rings 4 calling birds 3 french hens 2 turtle doves And a partridge in a pear tree. On the 10th day of Christmas my true love gave to me 10 lords a-leaping 9 ladies dancing 8 maids a-milking 7 swans a-swimming 6 geese a-laying 5 gold rings 4 calling birds 3 french hens 2 turtle doves And a partridge in a pear tree. On the 11th day of Christmas my true love gave to me 11 pipers piping 10 lords a-leaping 9 ladies dancing 8 maids a-milking 7 swans a-swimming 6 geese a-laying 5 gold rings 4 calling birds 3 french hens 2 turtle doves And a partridge in a pear tree. On the 12th day of Christmas my true love gave to me 12 drummers drumming 11 pipers piping 10 lords a-leaping 9 ladies dancing 8 maids a-milking 7 swans a-swimming 6 geese a-laying 5 gold rings 4 calling birds 3 french hens 2 turtle doves And a partridge in a pear tree.
Now output the lyrics to Green Grow the Rushes, O.
mylist = [
4,
8,
6,
2,
1,
0,
7,
5,
3,
9
]
n = len(mylist)
for i in range(0, n, 2):
mylist[i] = 0
for i in mylist:
print(i)
0 8 0 2 0 0 0 5 0 9
n // 2 * [0]
is a
list
of five
0’s.
mylist[0::2]
is a
slice
of five items in the
list.
mylist = [
4,
8,
6,
2,
1,
0,
7,
5,
3,
9
]
n = len(mylist)
mylist[0::2] = n // 2 * [0]
for i in mylist:
print(i)
0 8 0 2 0 0 0 5 0 9
list
using a
slice
that has a stride.
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""" eratosthenes.py Print the prime numbers from 2 to n inclusive using the Sieve of Eratosthenes. l is the number of multiples of prime in the range from 2*prime to n inclusive. For example, there are 3 multiples of 23 in the range from 46 to 100 inclusive. These multiples are 46, 69, 92. """ import sys n = 100 #Begin by assuming that every integer from 2 to n inclusive is a prime number. #The numbers 0 and 1 are not prime numbers. numbers = (n + 1) * [True] numbers[0] = numbers[1] = False prime = 2 while prime <= n: print(f"{prime:2}") l = len(numbers[2 * prime::prime]) numbers[2 * prime::prime] = l * [False] #The multiples of prime are not prime numbers. try: prime = numbers.index(True, prime + 1) except ValueError: break #The surviving Trues are at the prime indices. print() print([i for i in range(n + 1) if numbers[i]]) print(*[i for i in range(n + 1) if numbers[i]]) sys.exit(0)
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97] 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97