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016 More control in Python: for

Introduction

Purpose

In this section we will learn how to add more control to our code by using loops. We will mainly be using the for statements for this. We will also learn about the use of assert statements to check our code is operating as intended.

Prerequisites

You will need some understanding of the following:

Looping with for

for ... in ...

Very commonly, we need to iterate or 'loop' over some set of items.

The basic stucture for doing this (in Python, and many other languages) is for item in group:, where item is the name of some variable and group is a set of values.

The loop is run so that item takes on the first value in group, then the second, etc. Notice in the code below that the expressions inside the loop use indentation to indicate the loop. As when we discussed indentation in if statements, be careful to align your statements or the code will fail.

'''
for loop
'''

group = [4,3,2,1]

for item in group:
    # print item in loop
    print(item)
    
print ('blast off!')
4
3
2
1
blast off!

The group in this example is the list of integer numbers [4,3,2,1]. A list is a group of comma-separated items contained in square brackets [] as we have seen before.

In Python, the statement(s) we run whilst looping (here print(item)) are indented.

The indent can be one or more spaces, the choice is up to the programmer. You can use <tab> but should probably avoid it. Whatever you use, it must be consistent. We suggest you use 4 spaces.

It is important to note the difference between the code above and:

'''
for loop
'''

group = [4,3,2,1]

for item in group:
    # print item in loop
    print(item)
    print ('blast off!')
4
blast off!
3
blast off!
2
blast off!
1
blast off!

In the second case, we have the print ('blast off!') statement inside the loop as it is indented. So it is executed each time we are in the loop. In the first case, it is outside the loop and is only run once the loop is completed.

Exercise 1

  • generate a list of strings called group with the names of (some of) the items in your pocket or bag (or make some up!)
  • set up a for loop with group, setting the variable item
  • within the loop, print each value of item in turn
  • at the end of the loop, print I'm done

Quite often, we want to keep track of the 'index' of the item in the loop (the 'item number').

One way to do this would be to use a variable (called count here).

Before we enter the loop, we initialise the count to zero. Then, within the loop, we would need to increment count b y one each time (i.e. add 1 to count):

'''
for loop with enumeration
'''

group = ['cat', 'fish', '🦄', 'house']

# Before we enter the loop, we initialise the `count` to zero.
count = 0

for item in group:
    # print the count value and item
    print(f'count: {count} : {item}')
    # increment count by 1
    count += 1
count: 0 : cat
count: 1 : fish
count: 2 : 🦄
count: 3 : house

Exercise 2

  • copy the code above
  • check to see if the value of count at the end of the loop is the same as the length of the list.
  • Why should this be so?

range()

If we want to use in index to count explicitly, we can use the range() function. The arguments of this are (stop), (start,stop) or (start,stop,step). If not sepcified, the default values os start is 0, and step, 1, so range(10) is equivalent to range(0,10,1).

The function returns an object similar to a list type, but known as an iterator. An iterator can be thought of as a list that returns a single item at a time. We generally use them in a for loop or similar structure. The iterator returns integers starting at start, up to (but not including) stop, in steps of step.

For example:

# (0,6,1) -> 0 to 6 (but not 6) in steps of 1
for i in range(6):
    print(i)
0
1
2
3
4
5

# (2,10,2) -> 0 to 10 (but not 10) in steps of 2
for i in range(2,10,2):
    print(i)
2
4
6
8

Exercise 3

  • use range() to print numbers counting down from 10 to 1 (inclusive)
  • include comments to explain your answer

enumerate()

Since counting in loops is a common task, we can use the built in method enumerate() to achieve the same thing as above.

The syntax to achieve the same as the code above is then:

'''
for loop with enumerate()
'''
group = ['hat','dog','keys']

for count,item in enumerate(group):
    # print counter in loop
    print(f'item {count} is {item}')
item 0 is hat
item 1 is dog
item 2 is keys

Exercise 4

  • copy the code above
  • as in the previous exercise, check to see if the value of count at the end of the loop is the same as the length of the list.
  • Explain why you get the result you do

looping over dictionaries, and assert

Let's set up a dictionary with the names of the months as keys, and the n umber of days in. each month as the item.

We will introduce a new term assert to test that the lengths of the lists are equal before we proceed. This takes the form:

asset statement

If statement is True, the assertion passes (the code flow continues). If it is false, the code execution will stop at that point. It is very useful for error checking.

'''
Using the months exercise from 014_Python_groups

first construct the dictionary we want: days_in_month
'''
months = ["January","February","March","April","May",\
         "June","July","August","September","October",\
         "November","December"]
# create a list called `ndays` with the number of days in each month (for this year)
ndays = [31,29,31,30,31,30,31,31,30,31,30,31]

# now use assert to test if the lengths are equal:
# we will do that by making the statement:
#    len(months) == len(ndays)
# which can be True or False

assert len(months) == len(ndays)

# Use these two lists to make a dictionary called `days_in_month` 
# with the key as month name and value as the number of days in that month.
days_in_month = dict(zip(months,ndays))

# what are the keys
print(f'the keys are: {days_in_month.keys()}')
the keys are: dict_keys(['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December'])

The for ... in ... structure we saw above applies to any group of items (or more formally, any iterable item). How could we apply this to looping over dictionaries for instance?

One straightforward way would be to simply loop over the dictionary keys:

'''
loop over the keys and print key and value
'''
for k in days_in_month.keys():
    d = days_in_month[k]
    print(f'Month {k} has {d} days')
Month January has 31 days
Month February has 29 days
Month March has 31 days
Month April has 30 days
Month May has 31 days
Month June has 30 days
Month July has 31 days
Month August has 31 days
Month September has 30 days
Month October has 31 days
Month November has 30 days
Month December has 31 days

This works fine, but we can simplify the structure by looping over the iterable object items() instead of `keys().

print(list(days_in_month.items()))
[('January', 31), ('February', 29), ('March', 31), ('April', 30), ('May', 31), ('June', 30), ('July', 31), ('August', 31), ('September', 30), ('October', 31), ('November', 30), ('December', 31)]

items() returns a set of tuples containing (key, value). So we can directly loop over that to have the much simpler code:

'''
use items
'''
for k,d in days_in_month.items():
    print(f'Month {k} has {d} days')
Month January has 31 days
Month February has 29 days
Month March has 31 days
Month April has 30 days
Month May has 31 days
Month June has 30 days
Month July has 31 days
Month August has 31 days
Month September has 30 days
Month October has 31 days
Month November has 30 days
Month December has 31 days

Exercise 5

  • set up list of numbers (years) from 2008 to 2019 inclusive,
  • set up a list of corresponding chinese zodiac names as the items (look online for this information).
  • check that the lists have the same length
  • form a dictionary from the two lists, using dict(zip()) as in the examples above
  • use .items() as above to loop over each year, and print the year name and the zodiac name with an f-string of the form: f'{y} is the year of the {z}', assuming y is the key and z the item.
  • Describe what you are doing at each step

list comprehensions

So far, we have dealt with explicit for loops of the form:

    for x in alist:
        ...

This is very flexible, allows us to put multiple items in the loop, nest conditional or other statements and is the standard looping structure in Python.

Quite often though, we want to gather the information processed in the loop into a list. One way to do this is:

    # initiate list called blist
    blist = []
    
    # loop over items in list alist
    for x in alist:
    
        # call function `a_function with argument b
        b = a_function(x)
        
        # append b to blist
        blist.append(b)

This is quite explicit in what is going on, and contains good comments on the process, but it is not at all an elegant piece of code, and is overly-complicated for what it achieves.

We have mentioned the word Pythonic previously, meaning taking advantage of the (elegant) features of the programming language to write beautiful code. It is perhaps worth introducing the following Python Easter egg at this point, from PEP20:

import this
The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

A more elegant way to write a simple loop code that results in a list, is to use a feature called list comprehensions.

The basic syntax to replace what we saw above is:

        [a_function(x) for x in alist]

which returns a list with the function a_function(x) applied to each element in the list alist. Some examples:

# x^2 for integers 0,...,9
# First, using full for loop
blist = []
for  x in range(10):
    blist.append(x*x)
    
blist
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# x^2 for integers 0,...,9
# list comprehension
[x*x for x in range(10)]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

We can apply conditional statements in a list comprehension. The syntax is:

        [a_function(x) for x in alist if condition]

or, if an else is required

        [a_function(x) if condition else b_function(x) for x in alist]

adict = {"first": "gold","second": "silver","third": "bronze"}
alist = ["gold","silver","bronze"]

# given a list of items of strings and integers, interpret
# position as a list of Olympic medal types
# the positions may be integers 1,2,3 or strings
# "first", "second", "third"

# an example input list
items = ["first",1,3,"third",2,"second"]
# first, as a full for / if set
blist = []
for v in items:
    if type(v) == str:
        # look it up in the dictionary
        blist.append(adict[v])
    else:
        # get from list
        # index for list is v-1
        blist.append(alist[v-1])
        
print(f'{items} ->\n{blist}')
['first', 1, 3, 'third', 2, 'second'] ->
['gold', 'gold', 'bronze', 'bronze', 'silver', 'silver']

# now using list comprehension
[adict[v] if (type(v) == str) else alist[v-1] for v in items]
['gold', 'gold', 'bronze', 'bronze', 'silver', 'silver']

This is such a common type of expression that you will find lots of cases you will use a list comprehension. Don't make them too complicated however, as you may defeat the purpose of making it elegant, and instead make it simply unreadable.

Consider using them whenever you have a simple expression in a list. You can nest these expressions (i.e. put one inside the other) but again, and that can be an elegant solution, but be wary of making the code an unreadable mess.

Exercise 6

  • Use a list comprehension to generate a list of squared numbers from $0^2$ to $10^2$

Summary

We should now know how to use for statements in Python to control program flow. This is a common feature of all coding languages, but it is important here that you get used to doing this in Python.

We know that conditions inside for statements use indentation in Python, and we know to be careful in our use of this. We have learnt about enumerate() and range().

We have also seen the use of assert to do some checking that our code is correct.

There are additional notes in docs.python.org you can follow up to deepen your understanding of these topics. You can get more practice with assert at w3schools.

Command Comment Example Result
for item in list: loop, setting item to each value in list sequentially see Example 1
for key,value in list_of_tuples: loop, setting a,b to each value in list of tuples See Example 2: list({"a":7,"b":3}.items()) [('a', 7), ('b', 3)]
range(start,stop,step) index iterator from start to stop in steps of step list(range(1,6,2)) [1, 3, 5]
enumerate(list) provide index of list list(enumerate(['a','b','c'])) [(0, 'a'), (1, 'b'), (2, 'c')]
assert condition test that condition is True, exit otherwise assert True
[f(x) for x in alist] list comprehension, applying f(x) to each item x in alist [i*i for i in range(3)] [0, 1, 4]
[af(x) for x in alist if ca(x)] list comprehension with conditional statement, applying af(x) to each element x in alist if ca(x) is True [i for i in range(11) if i%2] [1, 3, 5, 7, 9]
[af(x) if ca(x) else bf(x) for x in alist] list comprehension with conditional statement, applying af(x) to each element x in alist if ca(x) is True, otherwise applying bf(x) [i if i%2 else 0 for i in range(4)] [0, 1, 0, 3]

Example 1:

    for item in [1,2,3]:
        # print item in loop
        print(item)

Result:

    1
    2
    3

Example 2:

    for key,value in {"a":7,"b":3}.items():
        print(f'key is {key}, value is {value}')

Result:

    key is a, value is 7
    key is b, value is 3

Example 3:

    for i,(key,value) in enumerate({"a":7,"b":3}.items()):
        print(f'key {i} is {key}, value is {value}')

Result:

    key 0 is a, value is 7
    key 1 is b, value is 3