Python3 FlowControl

Python 3 : Flow Control - Loops, Conditionals, etc.

Back to Lists, dictionaries, sets, and tuples - [[Forward to Functions and Standard Modules |Python4-FunctionsAndStandardModules]]

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Presented By : Katy Huff

Based on Lecture Materials By: Milad Fatenejad

Conditionals

Conditionals (if statements) are also really easy to use in python. Take a look at the following examples:

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 sign

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 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"

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.

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 mult

Some 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 and aubergines garnished with... "

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. (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 script

Example : Infinite While Loop

Using the sequence example,

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 mult

Can you think of a sequence of numbers that would produce an infinite loop?

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. So 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:

sequence = ["one", 2, "three", 4, "five"]
for i in range(len(sequence)):
    print sequence[i]

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 the short file containing phone numbers TO YOUR DESKTOP. The file can be found in your PyTrieste repository and is called phonenums.txt. Do you recall how to use the find command? 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 answer

Example : Iteritems

Use 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.