GCSE · Computer Science · Edexcel · Spec 1CP2

Strings

To get the first letter of 'Python', you ask for letter number 0. That one odd idea, counting from zero, unlocks almost everything a program does with text.

Computer Science · Strings

Number the boxes, then read them off

Two short strings and nine lines of Python. Each time you press Next, one line runs and the table shows which positions it used. The column headings are the positions.

Before you start: what do you think word[3] gives for 'Python', t or h? Commit to one, then step through and see.

 1  word = 'Python'
 2  print(len(word))
 3  print(word[0])
 4  print(word[3])
 5  print(word[len(word) - 1])
 6  print(word[-1])
 7  word = 'HELLO'
 8  print(word[0:2])
 9  print(word[1:4])

[0]	[1]	[2]	[3]	[4]	[5]	result
P	y	t	h	o	n

Output

Step 1: Here's the trick: before you do anything with a string, number its characters. 'Python' is six characters in a row, and the numbering starts at 0, not 1. P is at position 0 and n is at position 5.

1 / 9

Use Next and Back to run one line at a time. Keep your eye on which positions each line uses.

Step 1 of 9: Here's the trick: before you do anything with a string, number its characters. 'Python' is six characters in a row, and the numbering starts at 0, not 1. P is at position 0 and n is at position 5..

Watch out: A slice stops BEFORE its stop index. word[1:4] uses positions 1, 2 and 3, never position 4.

What do you think?

One step past the end

Same string as before: word = 'Python'. This time the program runs print(word[6]).

What happens? Pick the idea closest to what you think right now.

It prints n. 'Python' has six letters, so number 6 is the last one.It prints nothing, just a blank, because there's no letter there.It prints P, because it loops back round to the start.It gives an error, because there is no index 6.

How sure are you?

I'm sureI think soI'm guessing

It prints n. 'Python' has six letters, so number 6 is the last one.

This is a common misconception

That's counting from 1. Positions start at 0, so n, the sixth character, is at index 5. Index 6 would be a seventh character, and 'Python' doesn't have one.

It prints nothing, just a blank, because there's no letter there.

This is a common misconception

Python doesn't quietly hand back a blank. Asking for a position the string doesn't have is an error: an index out of range error.

It prints P, because it loops back round to the start.

This is a common misconception

Positions don't loop round. -1 is a special way to reach the last character, but counting forwards past the end doesn't bring you back to P. There's nothing at index 6, so it's an error.

It gives an error, because there is no index 6.

This matches the evidence

Exactly. The last index is the length minus one: 6 - 1 = 5. Any index greater than that causes an index out of range error. And len counts every character, spaces and punctuation included: 'Computer Science!' has length 17, so its last index is 16, and index 17 would be out of range too.

Last index = len(word) - 1. Anything past it is an index out of range error.

Your turn to build

Build a new string, one character at a time

Complete the program so it builds a new string in which every letter of word appears twice. 'hop' should become 'hhoopp'.

word = 'hop'One new tool first: with strings, + joins them end to end to make a new string. That's called concatenation, and it's how you build a string up.
missing step

Which line is step 2?

new_word = wordnew_word = ''new_word = ' '

Which job is it doing?

Sort the string handling

Pick a line of code, then choose the job it does.

Still to sort

Length (0)

How many characters are in the string?

Joining (concatenation) (0)

Adding two or more strings together.

Substring (slicing) (0)

Extracting part of a string.

Where the line is: You get a piece of the string back, not a yes-or-no answer about it.

Case conversion (0)

Changing the text to uppercase or lowercase.

Searching (0)

Finding a letter or word inside a string.

Where the line is: You're asking whether a piece is in there, not cutting it out.

10 of 10 still to sort.

Programs do five everyday jobs with text. Each line of code below does exactly one of them.

Watch out: A slice and a search both involve part of a string, but they do different jobs. A slice takes a piece OUT of the string; in asks whether a piece is IN it.

ord() and chr(): mirror images

ord()vschr()

Every character also has a number behind it. Memory trick: chr() hands you back a CHaRacter.

Focus

Direction

ord()

Character → number

chr()

Number → character

The insight

This is the whole difference between them, and it's exactly the part people mix up. Decide which way you're going first.

What goes in the brackets

ord()

One character, e.g. 'a'

chr()

A whole number, e.g. 97

What comes back

ord()

Its ASCII value: ord('a') is 97

chr()

The character with that value: chr(97) is 'a'

Capital A

ord()

ord('A') is 65

chr()

chr(65) is 'A'

Put it all together

Write it: a username generator

Tools you'll need: a slice, word[start:stop]; + to join strings; and random.choice(text), which picks one random character from text, as long as the program has run import random first.

first_name = 'Priya' and surname = 'Shah'. Write Python code that creates a username made of the first three letters of first_name, then the first three letters of surname, then one random character chosen from the string '0123456789'. Display the username. [4 marks]

Your answer

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

The mark scheme

import random comes before random.choice is used. (1 mark)
Both slices are right: first_name[0:3] and surname[0:3], giving 'Pri' and 'Sha'. (1 mark)
random.choice('0123456789') picks the random character. (1 mark)
The three parts are joined with + into one string, and it is displayed with print. (1 mark)

Model answer

import random first_name = 'Priya' surname = 'Shah' username = first_name[0:3] + surname[0:3] + random.choice('0123456789') print(username) One possible output is PriSha7. The last character is chosen at random, so it can be different each time the program runs.

Leave out import random and the program fails with NameError: name 'random' is not defined. And check each slice against numbered positions: [0:3] means positions 0, 1 and 2.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

A string is any run of characters inside quote marks: letters, spaces, digits, punctuation, anything. Give each character a numbered position and nearly every string job becomes easy.

What you need to know

What a string is, and why digits inside one are treated as text
How len() and zero-based indexing work, including the last character and the index out of range error
How to slice a substring, and why the stop index is left out
How to loop over a string, and how to build a new string with concatenation
The five common string handling jobs: length, joining, substring, case conversion and searching
What ord() and chr() do, and how to pick a random character with random.choice()

The big picture

A string is a collection of characters inside quote marks, and digits in a string are text, not numbers. Each character has an index, starting at 0. len() counts the characters, the last one is at len() - 1 (or -1), and any index beyond that is out of range. A slice runs from its start index up to, but not including, its stop index. + joins strings, upper() and lower() convert case, in searches for a substring, and a for loop visits each character. ord() and chr() convert between characters and ASCII values, and random.choice() picks a random character.

Key points

1A string is a collection of characters inside quote marks: letters, spaces, punctuation, digits and any other symbols. Digits in a string are handled as text characters, not numbers.

2Each character has an index, its position in the string. Indexing starts at 0, so for word = 'Python', word[0] is 'P' and word[3] is 'h'.

3len() returns the number of characters, including spaces and punctuation. The last character is at len(word) - 1, or word[-1]. An index greater than len(word) - 1 causes an index out of range error.

4A slice word[start:stop] includes the start index and stops before the stop index, so for word = 'HELLO', word[1:4] is 'ELL'. A substring is just another string, so == and in work on it too.

5+ joins strings (concatenation) and adds no space. To build a string in a loop, set it to '' before the loop, then concatenate onto its end inside the loop.

6for character in word visits each character in turn. for i in range(len(word)) visits each index, and word[i] gives the character at that position.

7upper() and lower() convert all the letters in a string to uppercase or lowercase. in checks whether a substring is contained in a string.

8ord() converts a character into its ASCII value and chr() does the reverse: ord('a') is 97 and chr(97) is 'a'. After import random, random.choice(word) picks a random character from word.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

word = 'robot' for i in range(0, 5, 2): print(word[i]) What does this program display?

⚠ Watch out

Counting positions from 1. The first character is at index 0, so the last one is at len(word) - 1, not at len(word), which is out of range. The same slip makes people include the stop index in a slice, but word[1:4] stops before position 4.

🧠

Memory hook

Zero is where you start, length minus one is where you end, and a slice always stops one box short.

✓

Check yourself

code = 'X7-Blue'. Without running it, work out len(code), code[-1] and code[1:3]. Then say what happens with code[7], and why.

Flashcards

(14)

What is a string?

A collection of characters inside quote marks: letters, spaces, punctuation, digits or any other symbols. Digits in a string are text, not numbers.

What does len() return?

The number of characters in a string, counting spaces and punctuation. len('Computer Science!') is 17.

What is an index?

A character's numerical position in a string. Python starts counting at 0, so the first character is word[0].

Where is the last character of a string?

At index len(word) - 1. Python also lets you use word[-1].

When do you get an index out of range error?

When the index you give is greater than the length of the string minus one.

What does word[start:stop] give you?

A substring (a slice): from the start index up to, but not including, the stop index.

What is concatenation?

Joining strings together with + to make a new string. No space is added, so cherry + drops gives cherrydrops.

How do you build up a new string with a loop?

Set a variable to a blank string, '', before the loop. Inside the loop, concatenate the new characters onto its end.

What are the two ways to loop over a string?

for character in word gives you each character in turn. for i in range(len(word)) gives you each index, and word[i] is the character there.

What do upper() and lower() do?

upper() converts all the letters in a string to uppercase; lower() converts them all to lowercase.

What does in do with two strings?

It checks whether the first one (a substring) is contained in the second, e.g. '@' in email.

What are substrings used for?

Jobs like validation (does an email address contain the right parts?), creating usernames and identifying file extensions.

What do ord() and chr() do?

ord() converts a character to its ASCII value; chr() converts a number back into a character. ASCII gives every character a unique number, with A at 65.

How do you pick a random character from a string?

import random, then random.choice(my_string). Forget the import and you get NameError: name 'random' is not defined.

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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Number the boxes, then read them off

One step past the end

Build a new string, one character at a time

Sort the string handling

Write it: a username generator

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