14  Iterating values

This chapter is about how you repeat the same code for many different values – and the many reasons why this is useful.

The for-loop

Programs often need to do repetitive things. Consider this example below, where x is assigned a value that is then printed:

x = 1
print(x)
x = 5
print(x)
x = 3
print(x)
x = 7
print(x)

You can see that we do the same thing four times, with the only difference being that the variable x takes a new value each time. Now, carefully write the alternative version below and compare what is printed to what was printed in the above example.

for x in [1, 5, 3, 7]:
    print(x)    

It should be the same. What you just wrote is called a for-loop. It is called a for-loop because it does something for each of many values – in this case, for each value in our list.

The statements nested under the for-loop are run as many times as there are values in our list, and every time they are run, x is assigned a new value. The first time the statements are run, x is assigned the first value in the list. The second time they run x, the second value is assigned to the list. This continues until x has been assigned all the values in the list.

The semantics of a for-loop is as follows:

1. First, you write for.
2. Then, you write the name of the variable that will be assigned a new value for each iteration of the loop (x in the above case).
3. Then you write in.
4. The,n you write the name of an iterable or an expression that reduces to one. In the above case, it was the list [1, 5, 3, 7].
5. The statements nested under the for loop are indented with four spaces, just like with if-statements. These statements are executed once for every value in the iterable.

What is an iterable, you may ask? It is any Python value that knows how to serve one value at a time until there are none left. Only objects with an __iter__ method can do this. You will get an error if you try to iterate over a value that does not have an __iter__ method. Try the code below and see how Python complains that “‘int’ object is not iterable”:

for x in 4:
    print(x)

Try these variations of the for-loop above and notice how the rules 1-5 apply in each case:

for x in [1, 5, 3, 7]:
    print(x)    

list_of_numbers = [1, 5, 3, 7]
for x in list_of_numbers:
    print(x)    

for x in [1, 5] + [3, 7]:
    print(x)    

In each case, the expression after in reduces to the value [1, 5, 3, 7], which then serves as the iterable.

It is not only lists that are iterable. Strings are, too. Their ‘iter’ method of a string tells it that it should serve one character at a time. Try this:

for character in 'banana':
    print(character)

Neat, right?

In programming, you often need to iterate over integer values and sometimes quite a few (like the 250 million bases of the human chromosome one). It would be quite annoying if you had to manually make long lists of integers, so Python provides a built-in function called range that helps you out. It returns a special iterator value that lets you iterate over a specified range of numbers. Try the two examples below and compare what is printed:

total = 0
for number in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]:
    total += number
print(total)

total = 0
for number in range(10):
    total += number
print(total)

You can see that using range works just like using a list of numbers, but the cool thing about the range is that it does not return a list. It just serves one number at a time until it is done. This is also why you will not see a list if you try to print what range returns:

number_iterator = range(10)
print(number_iterator)

The range function needs three values to know which values to iterate over: “start”, “end” and “step”. It will assume sensible defaults if you do not give it all three arguments. Try this:

for i in range(0, 10, 1):
    print(i)

for i in range(0, 10):
    print(i)

for i in range(10):
    print(i)

You can see that the first and last arguments default to 0 and 1. If you give it two arguments, it will assume that they are “start” and “end”. If you only give it one argument, it will assume that it is the “end”.

Exercise 14-1

What do you think the third argument to range specifies? Try these variations and see if you can figure it out:

for i in range(0, 10, 1):
    print(i)

for i in range(0, 10, 2):
    print(i)

for i in range(0, 10, 3):
    print(i)

Check the documentation once you have decided.

Exercise 14-2

What will happen here:

for x in []:
    print(x)  

and here:

for x in range(0):
    print(x)  

and here:

for x in range(10, 10):
    print(x)  

Exercise 14-3

The two examples below print the same. Make sure you understand why. Write and experiment with the code on your own.

list_of_words = ['one', 'two', 'three']

# example 1
for word in list_of_words:
    print(word)

# example 2
list_length = len(list_of_words)
for index in range(list_length):
    print(list_of_words[index])

Exercise 14-4

Finish the code below so all the even numbers go into one list, and all the odd numbers go into the other (hint: remember the modulo operator?)

numbers = [4, 9, 6, 7, 4, 5, 3, 2, 6]
even = []
odd = []
for n in numbers:
    # your code here ...
    
    

Exercise 14-5

You can put any statements under the for loop. Here, it includes an if-statement that lets you generate a list of all the a characters in banana (in case you need that).

result = []
fruit = 'banana'
for character in fruit:
    if character == 'a':
        result.append(character)
print(result)

Now change the code so you instead get the indexes of the ‘a’ characters: [1, 3, 5]. Here are some hints:

1. You need a for-loop over a list of numbers.
2. range(len(fruit)) may be relevant numbers :-).
3. fruit[1] substitutes for 'a'.

Exercise 14-6

Imagine you want to throw a big party and have rented a place with space for 100 people. Now, you want to start inviting people. What kind of error do you get here, and why?

friends = ["Mogens", "Preben", "Berit"]
invited = []
for index in range(100):
    invited.append(friends[index])

Exercise 14-7

You can also put a for loop under another for loop, and the rules for each for loop are the same as those explained above. The statements nested under the for loop are indented with four spaces, just like with if statements. These statements are executed once for every value in the iterable. Think carefully about what you think is printed in the example below before you try it out.

for i in range(3):
    for j in range(3):
        print(i, j)

Make sure you understand i, j pairs are printed in the order they are.