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Python3 FlowControl
Back to Lists, dictionaries, sets, and tuples - [[Forward to Functions and Standard Modules |Python4-FunctionsAndStandardModules]]
Presented By : Katy Huff
Based on Lecture Materials By: Milad Fatenejad
This part of the lesson includes a lot of text, but it will be useful to run it yourself in iPython.
To paste text from another application (i.e. these lecture notes) into iPython :
- select text from the wiki
- copy with ctrl+c
- in iPython, type %paste
The code should paste and execute in iPython.
If you also type %autocall to turn autocall OFF, you may be able to paste with ctrl+v though this won't work with all ipython builds.
A conditional (if statement) is some statement that in general says : "When some boolean is true, do the following. Elsewise, do this other thing."
Many equivalence test statements exist in Python that are similar in other languages:
i=1
j=2
i==j # i is equal to j : FALSE
i<j # i is less than j
i<=j # i is less than or equal to j : TRUE
i>j # i is greater than j
i>=j # i is greater than or equal to j : FALSE
i!=j # i is not equal to j : TRUEHowever, python has other equivalence test statements that are fairly unique to python. To check whether an object is contained in a list :
beatle="John"
beatles=["George", "Ringo","John", "Paul"]
print beatle in beatles # is John one of the beatles? : TRUE
print "Katy" not in beatles # this is also TRUE. Conditionals (if statements) are also really easy to use in python. Take a look at the following example:
i = 4
sign = "zero"
if i < 0:
sign = "negative"
elif i > 0:
sign = "positive"
else:
print "Sign must be zero"
print "Have a nice day"
print signThe behavior of this code snippet should be pretty clear, but there is something peculiar. How does Python know where the if-statement ends? Other languages, like FORTRAN, MatLab, and C/C++ all have some way of delimiting blocks of code. For example, in MatLab you begin an if statement with the word "if" and you end it with "end if". In C/C++ you delimit blocks with curly braces. Python uses ''indentation'' to delimit code blocks. The indentation above is NOT just to make things look pretty - it tells Python what the body of the if-statement is. This is true when ever we create any code blocks, such as the bodies of loops, functions or classes.
Aside: Compact if-statement:
Python has an easy to use if-syntax for setting the value of a variable. Try entering this into IPython:
i = 5
sign = "positive" if i > 0 else "negative"Lets start by looking at while loops since they function like while loops in many other languages. The example below takes a list of integers and computes the product of each number in the list up to the -1 element.
A while loop will repeat the instructions within itself until the conditional that defines it is no longer true.
mult = 1
sequence = [1, 5, 7, 9, 3, -1, 5, 3]
while sequence[0] is not -1:
mult = mult * sequence[0]
del sequence[0]
print multSome new syntax has been introduced in this example.
- On line 3 We begin the while loop. Notice that instead of using the not-equals symbol, !=, we can simply enter "is not" which is easier to read. This while loop will execute until sequence[0]= -1 . That is, until deletes all of the entries of the sequence that come before -1.
- On line 4, we compute the product of the elements just to make this more interesting.
- On line 5, we use the del keyword to remove the first element of the list, shifting every element down one.
Watch Out
Since a while loop will continue until its conditional is no longer true, a poorly formed while loop might repeat forever. For example :
i=1
print "Well, there's egg and bacon, egg and spam, egg bacon and"
while i is 1:
print "spam "
print "or Lobster Thermidor a Crevette with a mornay sauce served in a Provencale manner with shallots..." Since the variable i never changes within the while loop, we can expect that the conditional, i=1 will remain true forever and the while loop will just go round and round, as if this restaurant offered nothing but spam. (If you try this at home, please note that one way to interrupt a non-terminating process is ctrl+c or ctrl+z.
To create nested if loops, the indentation (preferably two or four spaces) should increase for each looping level.
weapons=["surprise","fear","ruthless efficiency","an almost fanatical devotion..."]
tries=0
script=""
while tries < len(weapons) :
i=0
while i<tries :
script += weapons[i]
script += " and "
i+=1
script += weapons[tries]
script += ". "
if tries == len(weapons) - 1 :
script += " and nice red uniforms. Oh damn!"
tries +=1
print scriptFor loops in python operate a little differently from other languages. Lets start with a simple example which prints all of the numbers from 0 to 9:
for i in range(10):
print iYou may be wondering how this works. Start by using help(range) to see what the range function does.
Help on built-in function range in module __builtin__:
range(...)
range([start,] stop[, step]) -> list of integers
Return a list containing an arithmetic progression of integers.
range(i, j) returns [i, i+1, i+2, ..., j-1]; start (!) defaults to 0.
When step is given, it specifies the increment (or decrement).
For example, range(4) returns [0, 1, 2, 3]. The end point is omitted!
These are exactly the valid indices for a list of 4 elements.
Range is a function that returns a list containing a sequence of integers. So, range(10) returns the list [0,1,2,3,4,5,6,7,8,9]. The for loop then simply iterates over that list, setting i to each value.
With range, we learned that for loops in python are really used to iterate over sequences of things (they can be used for much more, but for now this definition will do). Try entering the following to see what happens:
for c in ["one", 2, "three", 4, "five"]:
print cthis is equivalent to:
c = ["one", 2, "three", 4, "five"]
for i in range(len(c)):
print c[i]With a list, then, it's clear that we can use the in keyword to indicate a list of things. What about a nested loops around a list of lists?
italy = ["Rome", "Pisa", "Florence", "Venice", "Trieste"]
argentina = ["Mendoza", "Buenos Aires", "Patagonia"]
india = ["Ahmedabad","Kolkata", "Chennai", "Jaipur", "Surat"]
us = ["Chicago", "Austin", "New York", "San Fran"]
nations = [italy, argentina, india, us]
nationnames = ["italy","argentina", "india", "us"]
for nation in nations :
print nationnames[nations.index(nation)] + ": "
for city in nation :
print " " + city Of course, this information is better stored in a dictionary, isn't it? The data makes more sense if the keys were the nation names and the values were lists of cities. Importantly, python has given us a tool specifically for dictionary looping.
The syntax for looping through the keys and values of a dictionary is :
for key, value in dictionary.iteritems():
Importantly, you don't have to use the words key and value. That's just what will fill those variables. Here, we rewrite the previous loop using this clever syntax.
italy = ["Rome", "Pisa", "Florence", "Venice", "Trieste"]
argentina = ["Mendoza", "Buenos Aires", "Patagonia"]
india = ["Ahmedabad","Kolkata", "Chennai", "Jaipur", "Surat"]
us = ["Chicago", "Austin", "New York", "San Fran"]
nations = {"italy":italy, "argentina":argentina, "india":india, "us":us}
for nation, cities in nations.iteritems() :
print nation + " : "
for city in cities :
print " " + city A break statement cuts off a loop from within an inner loop. It helps avoid infinite loops by cutting off loops when they're clearly going nowhere.
reasonable = 10
for n in range(1,2000):
if n == reasonable :
break
print nSomething you might want to do instead of breaking is to continue to the next iteration of a loop, giving up on the current one..
reasonable = 10
for n in range(1,2000):
if n == reasonable :
continue
print nWhat is the difference between the output of these two?
Importantly, Python allows you to use an else statement in a for loop.
That is :
knights={"Sir Belvedere":"the Wise", "Sir Lancelot":"the Brave", \
"Sir Galahad":"the Pure", "Sir Robin":"the Brave", "The Black Knight":"John Clease"}
favorites=knights.keys()
favorites.remove("Sir Robin")
for name, title in knights.iteritems() :
string = name + ", "
for fav in favorites :
if fav == name :
string += title
break
else:
string += title + ", but not quite so brave as Sir Lancelot."
print stringWe've seen a lot so far. Lets work through a slightly lengthier example together. I'll use some of the concepts we already saw and introduce a few new concepts. To run the example, you'll need to locate a short file containing phone numbers. The file can be found in your PyTrieste repository within the phonenums directory and is called phonenums.txt. Now we have to move ipython to that directory so it can find the phonenums.txt file. You navigate within ipython in the same way that you navigate in the shell, by entering "cd [path]" .
This example opens a text file containing a list of phone numbers. The phone numbers are in the format ###-###-####, one to a line. The example code loops through each line in the file and counts the number of times each area code appears. The answer is stored in a dictionary, where the area code is the key and the number of times it occurs is the value.
areacodes = {} # Create an empty dictionary
f = open("phonenums.txt") # Open the text file
for line in f: # iterate through the text file, one line at a time (think of the file as a list of lines)
ac = line.split('-')[0] # Split phone number, first element is the area code
if not ac in areacodes: # Check if it is already in the dictionary
areacodes[ac] = 1 # If not, add it to the dictionary
else:
areacodes[ac] += 1 # Add one to the dictionary entry
print areacodes # Print the answerUse the iteritems dictionary method in combination with a for loop to print the keys/values of the areacodes dictionary one to a line. In other words, the goal is to write a loop that prints:
203 4 800 4 608 8 773 3
This example is a little tricky to figure out, but give it a shot.