Counted whiles | Python with Pets

Reminder

A while loop has the form:

while condition:
action

condition is either true or false, just like an if uses. Python will keep doing action while condition is true.

We saw loops using string variables in the condition:

best_pet = ''
while best_pet != 'dog':
best_pet = input('What is the best pet? ');
print('Best pet: ', best_pet)

Counting

We can also use numbers in loop conditions. For example:

count = 0
message = ''
while count < 3:
message += 'doggos '
count += 1
print(message)

Reflect

What's the output?

If you were logged in as a student, the lesson would pause here, and you'd be asked to type in a response. If you want to try that out, ask for an account on this site.

Adela

I got doggo three times: doggo doggo doggo

Right. So the program sticks together a word (doggo) the number of times in the while.

Remember

Doggos are the best things ever!

Let's track how it works with the debugger. Start the debugger, and it will stop before running line 1. Run the next coupla lines, so it stops on line 3. We're about to run the loop for the first time.

On line 3

Reflect

What happens next?

If you were logged in as a student, the lesson would pause here, and you'd be asked to type in a response. If you want to try that out, ask for an account on this site.

Adela

Python checks count. It's less than three, so Python goes to line 4.

Right!

You can check expressions in the console. Recall, an expression is some calculation.

Numeric expression: 2 + 3 * 4, evaluates to a number
String expression: 'Doggos' + ' ' + 'rock!', evaluates to 'Doggos rock!'
Logical expressions: count < 3, evaluates to True or False

You can type expressions in the console, hit Enter, and see the result. For example:

Evaluate expression in console

The debugger stopped before running line 3. I typed (actually, copy-and-pasted) a logical expression in the console, and it told me its value.

Now I'm gonna run the next three lines, 3, 4, and 5. Press Ctrl+F10 three times... there.

Because of line 4, Python slaps 'doggos ' on the end of message. The variable was MT, so now it's 'doggos '.

+= is "add to."

Line 5 adds one to count.

What next?

Reflect

After you run line 5, what's the next line Python will run?

If you were logged in as a student, the lesson would pause here, and you'd be asked to type in a response. If you want to try that out, ask for an account on this site.

Adela

Back to line 3, to continue the loop.

Stopped again

Right. We've run through the loop once. You can see that in count.

Reflect

What's the value of count < 3 at this point? What line will run next?

If you were logged in as a student, the lesson would pause here, and you'd be asked to type in a response. If you want to try that out, ask for an account on this site.

Adela

count < 3 is still True, so do the loop's code block again, starting at line 4.

Correct!

Run lines 4 and 5, then back to the while.

Stopped again

count < 3 is still True, so run the loop's block again.

Add to doggos, add to count, then back to line 3:

Loopy again

This time, count < 3 is False.

Reflect

What's the next line to run?

If you were logged in as a student, the lesson would pause here, and you'd be asked to type in a response. If you want to try that out, ask for an account on this site.

Ethan

The logical expression count < 3 is False, so skip the while's code block. Next line to run would be line 6.

That's right. Output message, and end.

A new data type

Ethan

I noticed something strange when running the code:

int?

What's that about?

Ah! Good eye for detail.

We've seen two data types so far: float for numbers, and string for text (actually, anything we can type). Now there's a third.

An integer is a whole number, like 7 and -13, but not 9.12 or -4.1.

Integers are primarily used for counting things. For example, if you're counting people, you have 23 people or 24 people, not 23.4 people. I wouldn't want 0.4 of a person at a party. Eeewwww.

Python uses integers where it can. So with the code...

count = 0

... Python sees the whole number (the zero), and makes count an integer.

Generally:

Use integers for counts
Use floats for other things

Adding

Another program:

total = 0
count = 0
while count < 5:
new_value = float(input('Value? '))
total += new_value
count += 1
print('Total: ' + str(total))

Reflect

What does the program do? If you don't know, try it.

If you were logged in as a student, the lesson would pause here, and you'd be asked to type in a response. If you want to try that out, ask for an account on this site.

Georgina

It asks you for 5 numbers, then tells you their total.

Right!

Ray

Hey... these lines:

count = 0
while count < 5:
...
count += 1

It looks to me count is what tracks how many times the loop is run, is that right?

Right. You could call it a loop control variable.

Ray

OK, so, count starts at 0, goes to 1 first time through the loop, then 2, then 3, then 4, then... No, 4 is the last value, because it loops while count < 5.

The program runs these lines 5 times:

new_value = float(input('Value? '))
total += new_value

The first one asks the user to type a number, the second one adds it to total. That sequence gets run 5 times.

So, the program adds up 5 numbers the user types.

Good! That's exactly what it does.

Adela

Wait, the program we had before, with the doggos, some of it is the same.

Program A	Program B
count = 0 message = '' while count < 3: message += 'doggos ' count += 1 print(message)	total = 0 count = 0 while count < 5: new_value = float(input('Value? ')) total += new_value count += 1 print('Total: ', total)

Program A

Program B

count = 0
message = ''
while count < 3:
message += 'doggos '
count += 1
print(message)

total = 0
count = 0
while count < 5:
new_value = float(input('Value? '))
total += new_value
count += 1
print('Total: ', total)

Right. They make a common pattern.

Reflect

What's a pattern?

If you were logged in as a student, the lesson would pause here, and you'd be asked to type in a response. If you want to try that out, ask for an account on this site.

Georgina

It's a common way of doing things, people find useful.

Yes, that's it.

Pattern

Count-controlled while loop

A while loop that runs a certain number of times.

Once you know a pattern, you can use it to write hundreds of different programs. When you're doing a task, don't start by asking yourself,

"Self, what Python statements do I use?"

Instead...

"Self, what patterns do I use?"

Ethan

Do these patterns work with other languages, or just Python?

Ooo, good question! They work with other languages, too. Here are versions of the totalling program in different languages.

Python	JavaScript	C#
total = 0 count = 0 while count < 5: new_value = float(input('Value? ')) total += new_value count += 1 print('Total: ', total)	let total = 0; let count = 0; while (count < 5) { let new_value = parseFloat(prompt('Value? ')); total += new_value; count += 1; } alert('Total: ' + total);	float total = 0; int count = 0; while (count < 5) { Console.Write("Value? "); float inputValue = (float)Console.ReadLine(); total += inputValue; count += 1; } Console.Write($"Total: {total}");

Python

JavaScript

total = 0
count = 0
while count < 5:
new_value = float(input('Value? '))
total += new_value
count += 1
print('Total: ', total)

let total = 0;
let count = 0;
while (count < 5) {
let new_value = parseFloat(prompt('Value? '));
total += new_value;
count += 1;
}
alert('Total: ' + total);

float total = 0;
int count = 0;
while (count < 5)
{
Console.Write("Value? ");
float inputValue = (float)Console.ReadLine();
total += inputValue;
count += 1;
}
Console.Write($"Total: {total}");

Ray

Wait. So when we're learning Python, we're learning other languages too? Kinda?

Yes. The first language is the hardest. The rest are easier because you already know some of the logic those languages use.

Summary

Loops can use numeric control variables (like while count < 5:), as well as strings (like while response != 'dog':).
A new data type: integer. Integers are whole numbers.
Learn one programming language, and others are easier to learn.