Python For Loops | Tutorials, Examples & Notes

Imagine buying new shoes, wearing them once, then putting them in your closet and never touching them again. Instead of wearing them again, you buy another pair. What happens if you do this every day?

Very soon, your closet will be full of shoes, and you will waste a LOT of money!

When we write code, we don’t want to spend extra time writing the same things over and over again. If we already have code in our program that prints “Hi”, we should avoid writing another line that also prints “Hi.”

Loops are one way to reuse pieces of code. Let’s learn about for loops.

Python Loops (For) 1

Run the example:

Change the value of

smart

to 1, then run it again.

The output is the same, but the code making that output is different.

When

smart

is set to 0, Python runs five different print statements.

When

smart

is set to 1, Python uses a for loop to print “Hi” 5 times.

Imagine how much time this would save if you had to print something 1,000 times!

This is a beautiful thing called iteration.

Python Loops (For) 2

For tells Python that you want to make a for loop. But what does x do?

Change:

for x in range(1, 6):
     print("Hi")

for x in range(1, 6):
     print(x)

Make sure

smart

is still set to 1, then run it.

Question: Based on the output, what is the value of
```
x
```
?
Answer: The value of
```
x
```
changes each time the for loop runs. The first time the loop runs, it is 0. The next time, it’s one. Then two, then three, and so on.

Note:

only works inside the for loop!

Python Loops (For) 3

Question: Why does the loop run 5 times?
Answer: In this case, the
```
range
```
function tells the for loop how many times to run. We gave the function two numbers: a bottom (1), and a top (6). The loop starts on the bottom number and stops right before the top number. If you want to go to 20, your top should be 21. If you want to go to 100, your top should be 101.

Python Loops (For) 4

Activity:

We’ll make a game where two cars race. Link

import random
import time

car = "ō͡≡o"
space = " "
roadside = "-----"
distance1 = 0
distance2 = 0
speed = 3

for x in range(1, 11):
   # Get a random number between 1 and the value of speed.
   vel1 = random.randint(1, speed)

# Whatever distance is right now, add vel1 to it.
   distance1 = distance1 + vel1
   # How much distance does the car have? That's how many spaces we want behind it.
   print((space * distance1) + car)

vel2 = random.randint(1, speed)
   distance2 = distance2 + vel2
   print((space * distance2) + car)

# What "round" are we on?
   print(x)
   print(roadside)
   # Wait 1/3 of a second before we keep going.
   time.sleep(0.3)

Python Loops (For) 5

Explanation:

import random

tells Python that we want to use the tools Python has for working with random numbers. import time tells Python that we want to use the tools it has for working with time.

Then, we define some key variables at the top so they’re easier to find and change later.

Our for loop will run 10 times. Each time, it does these things:

Makes a new variable called vel1 which is a random number between 1 and the value of speed.
Adds vel1 to whatever distance is right now. If on the first run vel1 is 3, then this part of the code will mean distance1 = 0 + 3. If it is 2 on the next run, the code will mean distance1 = 3 + 2. In this way, the distance1 variable gets bigger each time the loop runs.
Makes space behind the car. How much space? It depends on the distance that car has traveled. It’s actually possible to multiply strings! If we want to print “yo” three times, we can do so like this:
```
print("yo" * 3) 
```
Repeats steps 1 – 3 for the second car.
Prints out the round number, which is stored in the variable x.
Prints some dashes that look like the side of a road to make the output easier to understand.
Tells Python to wait for about ⅓ of a second before it continues running the program. This way we can see our cars moving!