GCSE · Computer Science · Edexcel · Spec 1CP2

Primitive data types

Type 1, then 2, ask the program to add them, and out comes 12. What does the computer think it is looking at?

Computer Science · Data types

Why does 1 + 2 make 12?

Same two inputs, run twice. You type 1, then 2. Step through and watch what each variable is holding.

Before you step on: you type 1 and then 2. What do you think Version A prints?

 1  # Version A: no casting
 2  num1 = input()
 3  num2 = input()
 4  print(num1 + num2)
 5  # Version B: cast with int
 6  num1 = int(input())
 7  num2 = int(input())
 8  print(num1 + num2)

num1	num2
"1" · string

Output

Step 1: You typed a number, but input() hands it back as a string. So num1 holds the text 1, not the number 1.

1 / 6

Compare the type written beside each value in Version A and Version B.

Step 1 of 6: You typed a number, but input() hands it back as a string. So num1 holds the text 1, not the number 1..

Watch out: The computer isn't broken. It's doing exactly what the data type tells it to do.

Computer Science · Data types

Which data type is it?

Pick a value, then choose the data type you think it belongs to. Watch the tricky ones.

Still to sort

Integer (0)

Whole numbers.

Where the line is: No decimal part. If it has a decimal part, it is real.

Real (floating point) (0)

Decimal numbers.

Where the line is: A decimal number stays real even when the decimal part is .0.

Boolean (0)

True or False.

String (0)

Text, and it can also hold numbers, characters and symbols.

Where the line is: Several characters, or a number that was typed into input(), go here.

Char (character) (0)

A single character.

Where the line is: One character only. Text of several characters is a string.

7 of 7 still to sort.

Computer Science · Casting

Finish the program

A shop program reads a whole-number quantity and a price with pence, multiplies them, then shows the total as text. Choose each missing casting step.

print("Enter quantity, then price")
missing step

Which line is step 2?

quantity = input()quantity = float(input())quantity = int(input())

Predict, then check

A learner wanted whole numbers but used float().

The program has num1 = float(input()) and num2 = float(input()), then prints num1 + num2. You type 3, then 3. What prints?

Computer Science · Casting

Does casting fix everything?

A learner has changed their program to num1 = int(input()). It works perfectly when they type 1 and 2.

Which of these is closest to what you think right now?

That's it: once the input is cast with int, the program can't crash.int() turns whatever the user types into a number.If the user types a word such as four, the cast fails while the program is running and it crashes.

How sure are you?

I'm sureI think soI'm guessing

That's it: once the input is cast with int, the program can't crash.

This is a common misconception

Casting only changes the type if the input can be converted. Type a word such as four and the cast fails: an error occurs and the program crashes when it is run.

int() turns whatever the user types into a number.

This is a common misconception

int() can only convert what it is given. A number typed as text becomes an integer, but a word such as four can't be turned into one, so the program crashes.

If the user types a word such as four, the cast fails while the program is running and it crashes.

This matches the evidence

Yes. Casting fixed the 12-or-3 problem, but it can't make a word into a number. An error occurs when the program runs and the program crashes.

Casting changes a value's type. It does not guarantee the user typed something that can be converted.

Computer Science · Explain it

Say it in your own words

A program asks for two numbers and adds them. A learner types 1 and 2, but it prints 12. Explain why this happens, and say how the program should be changed so it prints 3. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

input() returns the typed values as strings, even though numbers were typed. (1 mark)
Adding two strings with + joins them (concatenation), so "1" and "2" make 12. (1 mark)
Cast each input to an integer, for example num1 = int(input()), so + adds the numbers. (1 mark)

Model answer

The inputs are strings, because input() returns a string even when a number is typed. The + joins the two strings, so 1 and 2 become 12. Casting each input with int, for example num1 = int(input()), makes them integers so + adds them and gives 3.

Name the type, say what + did because of it, then give the cast that fixes it.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

A data type is the kind of data a computer can store, plus the actions you can perform on it.
Programs use data types so data is stored and processed correctly. For example, you can't do calculations with strings.
Integer stores whole numbers, like 3. Real (floating point) stores decimal numbers, like 5.3.
Have a goA till stores a price of 7.25. Does that fit integer or real (floating point)?
Real (floating point).
It has a decimal part, and integers store whole numbers only.
Boolean holds True or False. String holds text, and can also hold numbers, characters and symbols.
Char (character) is a single character, like a. It's a general data type: Python holds a single character as a string.
Have a goPriya insists 'banana' is a char because it's made of letters. Which data type fits better, and why is she wrong?
String. A char is one character and 'banana' is six.
A char holds a single character, like a. Text of several characters is a string.
input() returns a string in Python, even when the user types a number. So a typed 15 is a string.
Have a goYou type 15 into a program that uses input(). Can it do calculations with that 15 exactly as it arrives?
No. It arrives as a string, not a number.
input() returns a string even when you type a number, and you cannot do calculations with strings.
Casting converts a value from one data type to another: str() gives a string, int() an integer, float() a real number.
Add two uncast inputs and the strings are joined: 1 and 2 gives 12. Cast each with int() and you get 3.
Using float() when you wanted a whole number gives a decimal: entering 5 outputs 5.0, and adding 3 and 3 outputs 6.0.
Even with int(), typing a word such as four causes an error that crashes the program when it runs.

The big picture

Every value has a data type, and the type decides what the computer can do with it. input() always hands back a string, so adding two uncast inputs joins them instead of adding them. Casting with int(), float() or str() changes the type, but it won't rescue an input that can't be converted.

Key points

1A data type is the kind of data a value is, and it decides what you can do with it.

2The common data types are integer, real (floating point), Boolean, string and char.

3input() always returns a string in Python, so cast it before calculating with it.

4str(), int() and float() are the Python casting functions for string, integer and real.

5The wrong type gives the wrong result (joined strings, an unwanted decimal) or a crash at run time.

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Worked example

Problem

A game asks for a whole-number score and then needs to add 10 to it. The user types 15. Work out what input() gives back, what to change, and what the program then calculates.

⚠ Watch out

Treating a number typed into a program as already being a number. It arrives as a string, so it has to be cast before it can be used in a calculation.

🧠

Memory hook

input() hands you text. Cast it before you count with it.

✓

Check yourself

A user types 20 into num = input(). What data type is num, and what one change would let you add 5 to it?

Flashcards

(14)

What is a data type?

The kind of data a computer can store, and the actions you can perform on it.

Why do programs use data types?

So data is stored and processed correctly. For example, you can't do calculations with strings.

Integer

A data type that stores whole numbers, for example 3.

Real (floating point)

A data type that stores decimal numbers, for example 5.3.

Boolean

A data type that holds True or False.

String

A data type that holds text, and can also hold numbers, characters and symbols.

Char (character)

A single character, for example a. A general data type: Python holds a single character as a string.

What data type does input() return in Python?

A string, even if the user types a number.

What is casting?

Converting a value from one data type to another.

Which Python functions cast to a string, an integer and a real number?

str(), int() and float().

What does num1 = int(input()) do?

It casts the user's input string into an integer that can be used for calculations.

What happens if you add two uncast inputs?

The two strings are concatenated (joined). Entering 1 and 2 outputs 12 instead of 3.

What do you get if you use float() when you wanted a whole number?

A decimal (floating point) number: entering 5 outputs 5.0, which might not be what you intended.

Can an int() cast still go wrong?

Yes. If the user types a word such as four, an error occurs and the program crashes when it runs.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Keep me posted

Why does 1 + 2 make 12?

Which data type is it?

Finish the program

Does casting fix everything?

Say it in your own words

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