exercise11a.md

Exercise 11 a (For Loop)

There comes a need to repeat certain processes for a particular number of times or for as long as a particular condition holds as we write our codes. So, say we have,

print("hello world!")

and we want to repeat this line three times, we would do,

print("hello world!")
print("hello world!")
print("hello world!")

or we can do,

print("Hello world!\n" * 3)

Say, we want a user to enter a positive number then maybe perform some computation on it, how do we keep prompting the user that we need a positive number?

# assume in all cases, the user enters a number
num = int(input("Enter a positive number: "))

if num > 0:
    print(f"Great, {num} + 2 = {num + 2}")

In this case, the user would have to run the program again, every time they enter a non-positive number.

What if we have to do it like 100 times, 1000 times? We will need a loop.

Here we would look into loops ( also known as iterations). There are two types of loops in Python. These are the:

• for loop
• while loop

Range

A new keyword, not that strange, is range. range takes three arguments: start, stop and jump. stop can be called end and jump, step.

range(0, 5, 1) # start=0, end=5, step=1
range(-5, 2, 2) # start=5, end=2, step=2

range produces a sequence of numbers from start to end - 1, increasing start step times. So range(0, 5, 1) will produce: 0, 1, 2, 3, 4, starting from 0, then 0 + 1, which is the step. range(-5, 2, 2) will produce: -5, -3, -1, 1. This continues as far as the number is less than end. range can take start and end and by default, step is 1. As such range(0, 5) is the same as range(0, 5, 1). range can take end and by default, start is 0 and step is 1. range(0, 5, 1) is the same as range(5). range(3) = 0, 1, 2. One thing is that, the number are produced as far as start is less than end.

For loop

Usually, a for loop is used when we know the number of times we need to loop. Something like when the loop should end - looping for a particular number of times.

Structure of a for loop

for element in some_structure:
    # do something

The element is a name we have used to refer to the current object (element) in the some_structure. index or i is mostly used. some_structure is an object which can be iterated upon. Some example of iterable are set , list , dict , str , range, etc.

Range looping

Let's print numbers from 1 to 10, using the range function.

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

This will print a vertical list of numbers from 0 to 9. We wanted to print from 1 to 10. We can add 1 to number then print it or we can start from 1 and end on 11.

for number in range(10):
    print(number + 1)

for number in range(1, 11):
    print(number)

We can print even numbers from 2 to 20 excluded.

for number in range(2, 20, 2):
    print(number)

You should modify the above code to print odd numbers.

Let's look at another snippet that prints the string, <number> is even if the number is even else it prints the number.

for number in range(20):
    if number % 2 == 0:
        print(f"{number} is even")
    else:
        print(number)

20 will be excluded. Modify the code to include 20.

At this moment, we should be able to print a lot of Hello world!s without rewriting the print statement.

# print hello world three times
for i in range(3):
    print("hello world!")

Structure Looping

We have discussed strings already. Do you recall string indexing, where we pass some number into a square bracket to get a character of the string at that position?

name = "John Doe"

# looping through `name` using index
end = len(name)
for i in range(end):
    print(name[i])

# looping through `name` without using index
# this is looping through the structure itself
for ch in name:
    print(ch)

Concept of iterating - lamely

Say we have a string and it is an iterable (this permits the looping on the string).

lang = "Python"

for ch in lang:
    print(ch)

There is a pointer, should we assume. This pointer here is ch, which points to the first element in the iterable (here, a string - lang) if there is any. The first character is P so P is printed out. The pointer then checks if there is another element. If there is, the pointer, ch is assigned that element. This process continues until there is no next element.

Example 1

A program the accepts five integer inputs from the user and prints the sum and average.

s = 0
for i in range(5):
    n = int(input("Enter a positive number: "))

    s = s + n
    # s += n

avg = s / 5

print(f"Average: {avg}")

With sample input and output

Enter a positive number: 2
Enter a positive number: 3
Enter a positive number: 4
Enter a positive number: 7
Enter a positive number: 8
Average: 4.8

Example 2

In this snippet, we will take a long space-separated string and print out the words and their corresponding number of characters.

# we will not be using any function
text = "looping through name without index"

size = 0
word = ""

for ch in text:
    if ch != " ":
        size += 1
        word += ch
    else:
        print(f"{word} = {size}")
        size = 0
        word = ""

There above snippet can be reduced to just three lines.

text = "looping through name without index"

for word in text.split():
    print(f"{word} = {len(word)}")