Control Structures II

Python Basics: For Loops

Questions:

“How can I do the same operations on many different values?”

Objectives:

“Explain what a for loop does.”
“Correctly write for loops to repeat simple calculations.”
“Trace changes to a loop variable as the loop runs.”
“Trace changes to other variables as they are updated by a for loop.”

It is very good coding practice to reuse as much code as posible as generally, the fewer lines of code there are, the fewer bugs you’ll have. Loops are a very good way of doing this where a block of code is repeated a number of times or on each element in a collection. An example task that we might want to repeat is printing each character in a word on a line of its own.

word = "lead"

We can access a character in a string using its index. For example, we can get the first character of the word "lead", by using word[0]. One way to print each character is to use four print statements:

print(word[0])
print(word[1])
print(word[2])
print(word[3])

l
e
a
d

This is a bad approach for two reasons:

It doesn’t scale. If we want to print the characters in a string that’s hundreds of letters long, we’d be better off just typing them in.
It’s fragile. If we give it a longer string, it only prints part of the data,and if we give it a shorter one, it produces an error because we’re asking for characters that don’t exist.

word = 'tin'
print(word[0])
print(word[1])
print(word[2])
print(word[3])

t
i
n

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-3-7974b6cdaf14> in <module>()
      3 print(word[1])
      4 print(word[2])
----> 5 print(word[3])

IndexError: string index out of range

Here’s a better approach:

word = 'lead'
for char in word:
    print(char)

l
e
a
d

This is shorter — certainly shorter than something that prints every character in a hundred-letter string — and more robust as well. The improved version uses a for loop to repeat an operation — in this case, printing — once for each thing in a sequence.

Python Concepts: Iteration and Iterator

Two very important concepts in Python are Iteration and Iterators.

The Iteration is a process of repeating the same piece of code several times, this is mostly used in a for or while loops.

The Iterator is an object that allows you to iterate over collections of data, such as lists, tuples, dictionaries, and sets. For this lesson we have only discussed list and dictionaries.

Iterators take responsibility for two main actions:

Returning the data from a stream or container one item at a time
Keeping track of the current and visited items

The general form of a loop is:

for item in group of items:
    # do things

We can call the loop variable anything we like, but there must be a colon at the end of the line starting the loop. Also, we must indent anything we want to run inside the loop.

Updating variables with a `for` loop

Here’s another loop that repeatedly updates a variable:

length = 0 #Counter
vowels = 'aeiou'

for char in vowels:
    length += 1
print('There are', length, 'vowels')

There are 5 vowels

It’s worth tracing the execution of this little program step by step.

Since there are five characters in 'aeiou', the statement on line 3 will be executed five times.

The first time around, length is zero (the value assigned to it on line 1) and vowel is 'a'. The statement adds 1 to the old value of length, producing 1, and updates length to refer to that new value.
The next time around, vowel is 'e' and length is 1, so length is updated to be 2.
After three more updates, length is 5; since there is nothing left in 'aeiou' for Python to process, the loop finishes and the print statement on line 4 tells us our final answer.

Note that a loop variable is just a variable that’s being used to record progress in a loop. It still exists after the loop is over, and we can re-use variables previously defined as loop variables as well:

letter = 'z'
for letter in 'abc':
    print(letter)
print('after the loop, letter is', letter)

a
b
c
after the loop, letter is c

Note also that finding the length of a string is such a common operation that Python actually has a built-in function to do it called len:

print(len('aeiou'))

len is much faster than any function we could write ourselves, and much easier to read than a two-line loop; it will also give us the length of many other things that we haven’t met yet, so we should always use it when we can.

From 1 to N

Python has a built-in function called range that creates a sequence of numbers range can accept 1, 2, or 3 parameters.

If one parameter is given, range creates an array of that length, starting at zero and incrementing by 1.

For example, range(3) produces the numbers 0, 1, 2.

If two parameters are given, range starts at the first and ends just before the second, incrementing by one. For example, range(2, 5) produces 2, 3, 4.
If range is given 3 parameters, it starts at the first one, ends just before the second one, and increments by the third one. For exmaple range(3, 10, 2) produces 3, 5, 7, 9.

for num in range(2,16,3):
    print(num)

You can also use range with the length of the list to iterate through a list.

cat_names = ["Tomasina", "Cometa", "Betsy", "Fiona"]
for index in range(0,len(cat_names),2):
    print(f"{index}){cat_names[index]}")

0)Tomasina
2)Betsy

Count Occurences of Letters in a String

Count the number of occurences of each letter in a string. Use a dictionary to store the number of times each letter comes up and the loop variable as the key to that dictionary.

You will need to check if the letter has been encountered before, i.e. is it in the dictionary? and if not, create it.

my_str = "A long string that contains lots of different letters"
letter_freq = {} # Dictionary as table of frequency

for char in my_str:
    char = char.lower() # Do not distinguish upper and lower case
    if char not in letter_freq.keys():
        letter_freq[char] = 0 # Initial frequency of 0
    letter_freq[char] += 1
print("Letter frequency:  ", letter_freq)

Letter frequency:   {'a': 3, ' ': 8, 'l': 3, 'o': 4, 'n': 5, 'g': 2, 's': 4, 't': 8, 'r': 3, 'i': 3, 'h': 1, 'c': 1, 'f': 3, 'd': 1, 'e': 4}

Looping Over Lists

The easiest way to loop over list is to follow the same pattern we have used for strings.

guests = ["Mario", "Luigi", "Peach"]

for guest in guests:
    print(f"Welcome {guest}!")

Welcome Mario!
Welcome Luigi!
Welcome Peach!

In general any iterable object can be loop through using this pattern. Iterable is an object which can be looped over or iterated over with the help of a for loop.

Looping Over Dictionaries

Dictionaries are iterable objects, but when you iterate you are provided with keys.

cat = {"name": "Cometa", "color":"black","breed":"Russian-Blue"}

for item in cat:
    print(item)

name
color
breed

You can verbosely request the keys, values or both of them by making use of the keys, values and items methods. The items method in particular provides both the keys and values at the same time.

Getting the Keys:

cat = {"name": "Cometa", "color":"black","breed":"Russian-Blue"}

for key in cat.keys():
    print(key)

name
color
breed

Getting the values:

cat = {"name": "Cometa", "color":"black","breed":"Russian-Blue"}

for value in cat.values():
    print(value)

Cometa
black
Russian-Blue

Getting the both keys and values:

cat = {"name": "Cometa", "color":"black","breed":"Russian-Blue"}

for key, value in cat.items():
    print(f"{key}={value}")

name=Cometa
color=black
breed=Russian-Blue

While loops do exist!

while is another example of iteration, and what it means is that while the condition still true, the code will be repeating itself until is no longer true.

Example of while loops:

times = 0 #counter
while times < 3:
    print("Hello!")
    times += 1 #update counter
    print(times)

Hello!
1
Hello!
2
Hello!
3

Keypoints:

“Use for variable in sequence to process the elements of a sequence one at a time.”
“The body of a for loop must be indented.”
“Use len(thing) to determine the length of something that contains other values.”

Python Basics: For Loops

Python Concepts: Iteration and Iterator

Updating variables with a for loop

From 1 to N

Count Occurences of Letters in a String

Looping Over Lists

Looping Over Dictionaries

While loops do exist!

Updating variables with a `for` loop