For Loops

Welcome to probably one of the most important lessons we can learn in the world of programming - the for loop

Why is it important? Let's suppose we want to display all the numbers from 0 to 10

We could do it like this

print("1")
print("2")
print("3")
print("4")
print("5")
print("6")
print("7")
print("8")
print("9")
print("10")

It's clear that we repeated the code, and that is completely forbidden. Why? Well, what happens if we want to display all the numbers from 1 to 1000? There's no way we want 1000 print() statements to modify manually

This is where the power of the for loop comes in

i = 1
for i in range(1, 1000):
    print(i)

This will print all the numbers from 1 to 999. Why doesn't it include 1000? range() is a function that already exists in Python and tells us where we want to start and where we want to stop - 1. That is, range(1, 1000) means from 1 to 999. If we want from 1 to 1000, we can say range(1, 1001)

i = 1
for i in range(1, 1001):
    print(i)

Now we see all the numbers from 1 to 1000

Let's explain this code

i = 1

for looks at i and checks the variable i from before. It knows i = 1, so it starts from there. Then for looks further and sees range(1, 1001). It knows this means all numbers from 1 to 1000 (1001 - 1), so it takes i and on each iteration increases it by 1, like i = i + 1

First, i = 1, then it enters the block and prints it, after which it automatically does i = i + 1

Now i = 2, it prints it, then again does i = i + 1

Now i = 3, it prints it, then does i = i + 1 again

... and so on :). Isn't it fascinating? We basically make the computer do this repetition for us

in is a keyword. It tells Python to take the variable i and check it against range()

More about range()

So far we've given range() two numbers - where it starts and where it stops - 1. But range() is more flexible than that

If we give it a single number, Python assumes we want to start from 0

for i in range(5):
    print(i)

This prints 0, 1, 2, 3, 4 - that is, 5 numbers starting from 0

We can also give a third number, called the step. It says by how much we jump from one value to the next. For example, if we want only the even numbers from 0 to 10

for i in range(0, 11, 2):
    print(i)

This prints 0, 2, 4, 6, 8, 10. Instead of i = i + 1, now for does i = i + 2 on each step

Combining for with variables

Remember variables? We can use them together with for to gather a result over the course of the loop

Let's suppose we want to add up all the numbers from 1 to 5. We need a variable to hold the total. We declare it before the loop, with the value 0, and increase it on each iteration

total = 0
for i in range(1, 6):
    total = total + i
print(total)

Let's follow what happens:

at the start total = 0
i = 1 → total = 0 + 1 = 1
i = 2 → total = 1 + 2 = 3
i = 3 → total = 3 + 3 = 6
i = 4 → total = 6 + 4 = 10
i = 5 → total = 10 + 5 = 15

At the end, print(total) prints 15

Very important: print(total) is outside the loop (it has no leading spaces). If we put it inside, we would see the total after every step, not just at the end

total = 0
for i in range(1, 6):
    total = total + i
    print(total)

Now we moved print(total) inside the loop, so it prints the total on every iteration

Combining for with if

We already know if. What happens if we put it inside a for loop? Then Python checks the condition on every iteration

For example, we want to print only the even numbers from 1 to 10

for i in range(1, 11):
    if i % 2 == 0:
        print(i)

On each value of i, we enter the loop and check if i % 2 == 0 (that is, "the remainder of the division by 2 is 0", so the number is even). If yes, we print it. If not, we skip it and move on

This prints 2, 4, 6, 8, 10

Notice the two levels of spacing: if is indented once (it's inside for), and print is indented twice (it's inside if)

Mission: The Energy Collector

The station gathers energy from a series of cells numbered from 1 to cells. Only the cells with an odd number work, the rest are broken.

Make a variable named cells for how many cells there are (pick any value you like), then write a program that:

if there are 0 cells → print No cells
if there is 1 cell → print A single cell
otherwise → loop with for over the numbers from 1 to cells, add up only the odd numbers into a variable total, and print total

Examples

With 6 cells, the odd ones are 1, 3, 5, so the program prints

With 0 cells it prints

No cells

With 1 cell it prints

A single cell