2_Flow_Control.md

Python 2 : Flow Control - Loops, Conditionals, etc

Based on Lecture Materials By: Milad Fatenejad and Katy Huff

Modified by Karin Lagesen

Conditionals

A conditional (if statement) is some statement that in general says : "When some boolean is true, do the following. Otherwise, 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 : True
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 : TRUE

However, 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 "Tom" 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"

What do you expect the value of sign to be after it has run?

Now type it in, and run it, and see if the value of sign is what you expected it to be. Note, you have to press enter twice after "Have a nice day", then you can show the value of sign by printing it.

The 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 whenever we create any code blocks, such as the bodies of loops, functions or classes. This is also why we had to print enter twice after the if statement - after the first time, python is offering you to type in more things that would be executed in the else, so you press enter to tell python that you are done, and that it should evaluate the if.

Short exercise

Repeat the if statement above, and test out what the value if i has to be in order to be able to trigger the else clause.

While Loops

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.

counter = 0
sequence = [1, 2, 3, 4, 5, 6, 7, 8]
while counter < len(sequence):
  print sequence[counter] * sequence[counter]
  counter += 1

Some new syntax has been introduced in this example.

• On line 1, we define a counter.

• On line 3 we begin the while loop. The conditional of the while loop is in this case whether the counter is less than the length of the sequence we are operating on. Since the last element in the list will always be one less than the length of the list, this will become false

• On line 4, we compute the product of the elements just to make this more interesting.

• On the last line we increment the counter, using the += syntax, this means that whatever is after the += sign will be added to the value that the variable already has.

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=0
mystring="Well, there's egg and bacon, egg and spam, egg bacon and"
while i < len(mystring):
  fields = mystring.split()
  print fields[i]


print "When will this be printed?!" 

Tip: if you run this, you can interrupt it with Ctrl-C!

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 and continue forever.

Combining while and if

To create nested while and if loops, the indentation (preferably two or four spaces) should increase for each looping level. Let's put the following source code in a file and save it as "script.py".

words =	["first", "second#", "third", "f#ourth", "fifth"]
counter = 0
while counter < len(words):
    if "#" not in words[counter]:
       print words[counter]
    counter += 1

For Loops

For 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 i

You 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.

For Loops with Lists and Dictionaries

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 c

this 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 

break and continue

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 n

Something 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 n

What is the difference between the output of these two?

Final Example

We'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 is in your repository is called phonenums.txt.

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 answer

Short exercise

• Modify the script so that it prints the results nicely, with first the area code, then a tab, and then the number of times it occurs.

• Modify this script so that it completely ignores phone numbers which contain the number 4.

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