Lesson 17
input() - reading a number
After this lesson: You'll read a number, turn it from text into a number and calculate with it.
Video
The video for this lesson is coming soon.
Explanation
In the previous lesson we met three magic machines: int(), float() and str(). Remember? "5" is text, and 5 is a number - they look similar, but you can only calculate with the number. Today we'll join the int() machine with input() - and calculate with numbers typed on the keyboard! 🪄
Watch out, a trap! input() always gives text - even when someone types a number. See what happens when we try to double it...
Text times 2
Type a number 🔢 (e.g. 21) and press Enter ⏎. See what the program prints and what type x has - input() always gives text, even when you type a number!
Type a number on the screen - let's see what times does with it.
1x = input()2print(x * 2)
▍Why was the number repeated?
The number you typed wasn't doubled - it was written twice. For example, when you type 21, input() gives the text "21" - two characters: a two and a one. Like in lesson 16: the text "21" looks like a number, but it isn't one. And text times 2 is the text repeated twice: "2121", not 42. 7 gives 77, and 100 gives 100100. Python doesn't know you meant a number!
Adding is even worse: "21" + 1 is a TypeError - text and a number can't be added. And it doesn't matter which number you type - it's always text.
You already know the fix: the int() machine!
Text plus a number
Type any number 🔢 (e.g. 99) and press Enter ⏎. The program stops with a TypeError - look at what's in the box x, and below we explain why it happened.
Type a number on the screen - let's see what the plus does with it.
1x = input()2print(x + 1)
▍What happened? Step by step
Type, for example, 99:
input()puts the text'99'🔤 into the box x - you can tell by the quotes in the box.- Python reaches
print(x + 1). On the left of the plus there's text'99'🔤, on the right a number1🔢. - The plus can do two different things:
- it adds numbers:
99 + 1is100, - it joins text:
"99" + "1"is"991".
- it adds numbers:
- Here there's text and a number at once. Add and give
100? Or join and give991? Python doesn't guess 🤷 - it stops the program with a TypeError (wrong type).
Let's read the message piece by piece:
| Piece | What it means |
|---|---|
TypeError | wrong type - something isn't the type it should be |
can only concatenate str | I can only join text... |
(not "int") | ...not with a number... |
to str | ...only with other text |
Together: "I can only join text with text, not with a number".
How to fix it? First turn the text into a number with the int() machine - that's exactly what we'll do next!
Without int() and with int()
Type a number 🔢 (e.g. 21) and press Enter ⏎ - the same typed line gives two different results: without int() and with int().
Type a number on the screen - you'll see two ways: without int() and with int().
1number_str = input()2print(number_str + number_str)3number = int(number_str)4print(number + number)
▍int() turns text into a number
In lesson 16 we did it in two steps: first the text in one box (price_text = "3.5"), then the machine made a number out of it in a second box (price_float = float(price_text)). With input() it's exactly the same:
number_str = input()
number = int(number_str)
input()reads text, e.g."21", and puts it into the box number_str.int(number_str)makes a number out of it (from"21"- the number21) and puts it into the box number.
In this lesson we always write it on two lines: first input(), then int(). That way you can see every step.
Two steps: text and a number
Type a number 🔢 (e.g. 21) and press Enter ⏎. Watch the text from the box text go through the int() machine and come out as a real number in the box number. Then see what * 2 does to the text, and what it does to the number.
Type a number on the screen and press Enter - input() gives it as text.
1number_str = input()2number = int(number_str)3print("Text times 2:", number_str * 2)4print("Number times 2:", number * 2)
▍A times table for any number ✖️
Type any number 🔢 (e.g. 8) and press Enter ⏎ - the program prints its times table straight away.
Type a number on the screen and press Enter - you'll see its times table in pictures.
1n_str = input()2n = int(n_str)3print(n, "x 1 =", n * 1)4print(n, "x 2 =", n * 2)5print(n, "x 3 =", n * 3)6print(n, "x 4 =", n * 4)7print(n, "x 5 =", n * 5)
▍Countdown to Christmas 🎄
How many days are left until Christmas? Type the number of days 📅 (e.g. 78) and press Enter ⏎ - the program turns it into weeks and hours.
Type a number of days on the screen and press Enter - you'll see them on a calendar.
1days_str = input()2days = int(days_str)3print("Christmas is in", days, "days")4print("That is", days // 7, "weeks and", days % 7, "days")5print("Or", days * 24, "hours!")
▍Two numbers on two lines
When the input has two numbers, each on its own line, we do the two steps for each number: input() reads one line, and int() makes a number out of it:
first_str = input()
first_number = int(first_str)
second_str = input()
second_number = int(second_str)
The sum of two numbers
Type two numbers 🔢, each on its own line (e.g. 6 and 7). Press Enter ⏎ after each number.
Type number 1 on the screen and press Enter - input() gives it as text.
1first_str = input()2first_number = int(first_str)3second_str = input()4second_number = int(second_str)5print(first_number + second_number)6print(first_number * second_number)
▍A real task: candies
Type the number of candies 🍬 (e.g. 23), then the number of children 🧒 (e.g. 4). Press Enter ⏎ after each number. The program shares the candies for any numbers - only the children can't be 0, because you can't divide by zero (ZeroDivisionError).
Type number 1 on the screen and press Enter - input() gives it as text.
1candies_str = input()2candies = int(candies_str)3children_str = input()4children = int(children_str)5print(candies // children)6print(candies % children)
▍A garden 🌻
Type the garden's length 📏 (e.g. 6), then its width (e.g. 4). Press Enter ⏎ after each number. The program works out the area and the fence around it.
Type number 1 on the screen and press Enter - input() gives it as text.
1length_str = input()2length = int(length_str)3width_str = input()4width = int(width_str)5print("Area:", length * width)6print("Fence:", 2 * (length + width))
▍Pocket money 💰
Type how much you get every week 💰 (e.g. 15), then for how many weeks you save 📅 (e.g. 8). Press Enter ⏎ after each number.
Type number 1 on the screen and press Enter - input() gives it as text.
1pocket_money_str = input()2weekly_pocket_money = int(pocket_money_str)3weeks_str = input()4number_of_weeks = int(weeks_str)5print(weekly_pocket_money * number_of_weeks)
▍And numbers with a dot? float(input())
From lesson 16 you also know the float() machine. When the input may be a number with a dot (e.g. a price 2.5), in the second step we use float() instead of int():
price_str = input()
price = float(price_str)
Remember: int("2.5") is a ValueError, but float("2.5") works. In AlgoBaza tasks the input is almost always whole numbers, so you'll mostly use int().
Ice cream at any price 🍦
Type the price of one ice cream 🍦 - it can have a dot, e.g. 3.5 - and press Enter ⏎. float() makes a number out of it, and the program works out two ice creams.
Type a number on the screen and press Enter - input() gives it as text.
1print("How much is one ice cream?")2price_str = input()3price = float(price_str)4print("Two ice creams:", price * 2)
▍Cinema tickets 🎟️
Type the price of one ticket 🎟️ - e.g. 12.5 - and press Enter ⏎. float() makes a number with a dot out of it, and the program works out the tickets for three people.
Type a number on the screen and press Enter - input() gives it as text.
1print("How much is one ticket?")2price_str = input()3price = float(price_str)4print("Three tickets:", price * 3)
▍How heavy is the backpack? 🎒
Type two weights in kilograms ⚖️, each on its own line - e.g. 2.5 (books 📚) and 1.5 (pencil case and lunch 🥪). Press Enter ⏎ after each one. Every weight goes through float(), then the program adds them.
Type number 1 on the screen and press Enter - input() gives it as text.
1print("How heavy are the books?")2books_str = input()3books = float(books_str)4print("How heavy is the rest?")5rest_str = input()6rest = float(rest_str)7print("The backpack weighs:", books + rest, "kg")
▍The input has two lines: 3 and 4. What does the program print?
a = input()b = input()print(a + b)
The input is 5. What does the program print?
n_str = input()n = int(n_str)n = n * nprint(n + 1)
Common mistakes
- You forget the second line with
int()- then you calculate with text: strange results (2121) or a TypeError. - The line
int(number_str)on its own does nothing - the result must go into a box:number = int(number_str). - You calculate with the wrong box:
number_str * 2instead ofnumber * 2. number_str holds text, number holds a number.
Text or number? A comparison
Plus joins text but adds numbers. That's why int() is so important.
The same with int()
How old will you be?
Type your age 🎂 and press Enter ⏎ - the program works out how old you'll be in 5 and in 10 years. 🎂
Type a number on the screen and press Enter - input() gives it as text.
1age_str = input()2age = int(age_str)3print("In 5 years:", age + 5)4print("In 10 years:", age + 10)5print("Days alive (about):", age * 365)
▍The input is 5. What does the program print?
n = input()print(n * 3)
The input is 5. What does the program print?
n_str = input()n = int(n_str)print(n * 3)
Remember
input()always gives text - even when the input is a number.- Read a number in two steps:
number_str = input(), thennumber = int(number_str). - Each number on its own line is a separate
input()and a separateint(). - A number with a dot:
float(number_str)instead ofint(number_str). - Without
int()plus joins and times repeats text.
Remember: first input(), then int() - and only then calculate. In the tasks the examples show what the input will be.
Tasks from the video
We solve them together in the video - pause it and try first on your own.
Practice tasks
Try to solve them without the video. Stuck? Every task has hints and an editorial.
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