GCSE · Computer Science · AQA · Spec 8525

Integer / real to string conversion

In Python, "Score: " + 2048 stops your program dead. The number has to become text first — and exactly when you convert it changes the answer.

Python · watch the types

Same digits, new job

Step through the program. Next to every value is its data type — keep your eye on that, not just the digits.

Before you press Next: the digits of score never change in this program. So what does?

 1  score = 2048
 2  time = 12.5
 3  score_text = str(score)
 4  time_text = str(time)
 5  message = "Score " + score_text + " in " + time_text + " seconds"
 6  print(message)
 7  digits = len(score_text)
 8  print(digits)

Variables

score=2048 · integer

Output

Step 1: score holds a whole number, so its type is integer. It can do maths: score + 1 would give 2049.

1 / 8

Press Next and watch the type after each value. The digits stay put; the type is what changes.

Step 1 of 8: score holds a whole number, so its type is integer. It can do maths: score + 1 would give 2049..

Watch out: Look at the quote marks. 2048 is a number; "2048" is text made of the characters 2, 0, 4 and 8. They look almost the same on screen, but a program treats them completely differently.

Predict, then check

Two lines, same numbers, same str(). The only difference is the moment the conversion happens.

Line A, print("Total: " + str(3 + 4)), prints Total: 7. What does line B, print("Total: " + str(3) + str(4)), print?

Your call

Convert it, or leave it?

Put each job in the right column.

Still to sort

Convert to a string first (0)

The number has to behave like text: joined into a message, or treated as characters.

Where the line is: It's the JOB that decides, not how the number looks. Text jobs need a string.

Use the number as it is (0)

The job is maths, or comparing sizes.

Where the line is: Arithmetic and comparisons need a number. Turning it into text first would stop them working.

6 of 6 still to sort.

Each card is a job a program needs to do with a number. Decide before you read why.

Spot the bug

Why did the program stop?

This Python program should print three lines about a run. It prints Laps run: 6 and then stops with a TypeError. Which line is to blame?

A program with one bug — which line goes wrong?

Your turn to write it

Write it, then check it

Answered 12 questions, average 4.5 seconds

A quiz app stores how many questions a player answered in the integer variable answered, and their average time per question, in seconds, in the real variable avg_time. Write code (pseudo-code or a programming language you have studied) that outputs ONE message in the form shown. [3 marks]

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

UK note

In AQA pseudo-code the two operations are written IntToString(…) and RealToString(…), and the message is displayed with OUTPUT.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

  • A string conversion operation turns a value into a string. Integer to string: 42 becomes "42". Real to string: 12.5 becomes "12.5".
  • The digits stay the same; the type changes. A string is text, so you can join it to other text and count its characters, but you can't do arithmetic with it.
  • Convert a number when it has to be joined into text, or when you need its digits as characters.
  • In Python, str() does both conversions. Python won't join a string to a number, so "Score: " + 10 stops the program with a TypeError.
  • Conversion makes a new value. The original variable is still a number.

The big picture

Converting a number to a string keeps its digits but changes its type — and the type decides what you can do with it. Integer to string turns 42 into "42"; real to string turns 12.5 into "12.5", with the decimal point as one of the characters. You convert when a number has to join other text, or when you want to work with its digits as characters, such as counting them. Once values are strings, + joins them instead of adding, so do any maths first and convert the answer. In Python, str() does both conversions.

Key points

1Do the maths first, then convert: "Total: " + str(3 + 4) gives Total: 7, but "Total: " + str(3) + str(4) gives Total: 34.
2When both sides are strings, + joins them end to end and adds no space: "3" + "4" is "34".
3Real to string keeps every digit and the decimal point. It doesn't round, and it doesn't add money-style zeros: str(7.5) is "7.5", not "7.50".
4In Python, a real that happens to be a whole number keeps its .0: str(15.0) is "15.0".
5A number's digits become countable characters only once it's a string: len(str(507)) is 3, while len(507) is an error.
6Converting to a string is not converting to binary. You get the digits you'd see on screen, as characters.
7Going the other way — from a string back to an integer or a real — is a separate operation.

Worked example

Problem

A shop program stores quantity = 3 (an integer) and price = 1.25 (a real). Write one line of Python that prints the message: You owe 3.75 for 3 items

⚠ Watch out

Joining a number straight onto text, such as print("Lives: " + lives). In Python that stops the program with a TypeError. Convert it first: print("Lives: " + str(lives)).

🧠

Memory hook

Same digits, new job. Numbers do maths; strings do sentences. Convert at the moment the number joins the sentence — after the maths is done.

✓

Check yourself

Without looking back: if n = 305, what is str(n)? What is len(str(n))? And after those lines have run, what type is n?

Flashcards

(9)
What does integer-to-string conversion do?
It turns a whole number into text made of the same digits: 42 becomes "42".
What does real-to-string conversion do?
It turns a number with a decimal part into text. The decimal point becomes one of the characters: 3.5 becomes "3.5".
Which Python function converts an integer or a real to a string?
str(). For example, str(42) gives "42" and str(3.5) gives "3.5".
Give two reasons to convert a number to a string.
To join it into a message with other text, and to treat its digits as characters — for example, to count them.
What does + do with two numbers, and with two strings?
With numbers it adds: 3 + 4 is 7. With strings it joins: "3" + "4" is "34".
What happens in Python if you run "Score: " + 10?
A TypeError — the program stops, because Python won't join text to a number. Fix it with "Score: " + str(10).
After s = str(n), what type is n?
Still a number. str() makes a new string value; it doesn't change the original variable.
Can you do arithmetic with the string "12.5"?
No. It's text. To calculate, use the number itself.
Why must you convert before using len() to count a number's digits?
len() counts the characters in a string. An integer has no length, so len(2048) is an error, but len(str(2048)) is 4.

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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