CS8String Handling and Random Numbers

A string is a sequence of characters: letters, digits, spaces and symbols.

• Each character has a position, its index, counting from 0, just like an array.
• Length is the number of characters, including spaces and punctuation. In pseudo-code: LEN('St Ives') is 7. In Python: len("St Ives").
• The last character is at index LEN − 1.
• The empty string, '' (Python ""), has length 0. It is useful as the starting value when you build a new string.
• In Python:

  city = "St Ives"
  print(len(city))                 # 7
  print(city[0])                   # S
  print(city[len(city) - 1])       # s, the last character

• len() works on lists too: see Arrays and records.

String

A sequence of characters.

Length

The number of characters in a string, including spaces: LEN in pseudo-code, len() in Python.

Empty string

A string with no characters: '' in pseudo-code, "" in Python. Its length is 0.

• One character: write the index in square brackets. word[0] is the first character.
• Position of a character: POSITION(word, 'e') in pseudo-code, word.find("e") in Python. It gives the index of the first match.
• If the character isn't in the string, the result is −1, which can never be a real position.
• Matching is case-sensitive: "Pinecone".find("p") is −1, because the string has a capital P.
• Pseudo-code:

  word ← 'Pinecone'
  OUTPUT word[0]                  # P
  OUTPUT POSITION(word, 'e')      # 3, the first e
  OUTPUT POSITION(word, 'z')      # -1, not found

• Python:

  word = "Pinecone"
  print(word[0])            # P
  print(word.find("e"))     # 3
  print(word.find("z"))     # -1

• In Python you can read word[2] but you can't change it with word[2] = "x". To change a string, build a new one.

Characters are numbered from 0. POSITION finds the first match.

POSITION / .find()

Returns the index of the first occurrence of a character in a string, or −1 if it isn't there.

A substring is a run of characters next to each other inside a string. AQA's pseudo-code and Python count the end differently.

• AQA pseudo-code: SUBSTRING(start, end, s). Both the start and the end positions are included. SUBSTRING(1, 4, 'Pinecone') is 'inec'.
• Python: a slice s[start:end] stops one before the end number. "Pinecone"[1:4] is "ine".
• So to turn SUBSTRING(a, b, s) into Python, write s[a:b + 1].
• Leave out the start to begin at 0: s[:3] is the first three characters. Leave out the end to run to the last character: s[4:].
• Notice the order: AQA's SUBSTRING puts the string last.
• Python:

  title = "Pinecone"
  print(title[1:4])     # ine
  print(title[:4])      # Pine
  print(title[4:])      # cone

Same start and end numbers, different results.

Substring

Part of a string made of characters that are next to each other.

SUBSTRING(start, end, s)

AQA's pseudo-code substring: from start to end, both included.

Slice

Python's s[start:end]: from start up to, but not including, end.

• Concatenation joins strings end to end with +: 'sun' + 'flower' is 'sunflower'.
• No space is added. If you want one, concatenate it: first + " " + last.
• Both sides must be strings. Convert a number first with str(), or Python stops with a TypeError.

  first = "Ada"
  last = "Lovelace"
  full = first + " " + last
  print(full)               # Ada Lovelace
  age = 36
  message = full + " was " + str(age)
  print(message)            # Ada Lovelace was 36

• A comma is not concatenation. message = full, " was " makes a tuple (a pair of values), not one string.
• Inside print(), commas are fine: print(full, "was", age) prints the items with a space between each.
• An f-string is another way to build output: print(f"{full} was {age}"). It converts the number for you.

Concatenation

Joining two strings end to end to make a new string, using +.

Every character has a number, its character code. AQA uses ASCII codes, which Unicode shares.

• Character to code: CHAR_TO_CODE('a') is 97. In Python: ord("a").
• Code to character: CODE_TO_CHAR(97) is 'a'. In Python: chr(97).
• Codes come in groups, in order: 'A' to 'Z' are 65 to 90, 'a' to 'z' are 97 to 122, and '0' to '9' are 48 to 57.
• So if you know 'A' is 65, then 'D' is 68. Each lower-case letter is 32 more than its capital.
• Is it a capital letter? ord(ch) >= 65 and ord(ch) <= 90. Don't write a chained comparison such as 65 <= ord(ch) <= 90.
• Moving along the alphabet: chr(ord("c") + 1) is "d".
• To wrap round at the end of a group, use MOD (%). This moves each digit on by one, so '9' becomes '0':

  digit = "9"
  newCode = (ord(digit) - 48 + 1) % 10 + 48
  print(chr(newCode))       # 0

• Changing case:

  letter = "g"
  code = ord(letter)        # 103
  upper = chr(code - 32)    # G
  print(code, upper)

Digits, capitals and lower-case letters each have codes in order.

Character code

The number that represents a character, such as 65 for 'A'.

CHAR_TO_CODE / ord()

Converts a character to its character code.

CODE_TO_CHAR / chr()

Converts a character code to its character.

• You need four conversions: string to integer, string to real, integer to string and real to string.
• "7" + "8" is the string "78", but 7 + 8 is 15. The data type decides what + does.
• input() always returns a string, so convert numbers before doing arithmetic with them.
• int("4.5") stops with an error: the string isn't a whole number. Use float() for it.
• Python:

  a = "4"
  b = "5"
  print(a + b)              # 45
  print(int(a) + int(b))    # 9
  price = float("2.75")
  print(price * 2)          # 5.5
  label = "Total: " + str(price * 2)
  print(label)              # Total: 5.5

Casting

Converting a value from one data type to another, such as int("16").

• FOR-IN gives each character in turn: FOR ch IN word in pseudo-code, for ch in word: in Python.
• Counting characters: start a counter at 0 and add 1 when a character matches. Each comparison needs both sides.

  word = input("Word: ")
  vowels = 0
  for ch in word:
      if ch == "a" or ch == "e" or ch == "i" or ch == "o" or ch == "u":
          vowels = vowels + 1
  print(vowels)

• Building a new string: start with the empty string and concatenate one character at a time. This reverses a word:

  word ← 'stressed'
  backwards ← ''
  FOR i ← LEN(word) - 1 TO 0 STEP -1
      backwards ← backwards + word[i]
  ENDFOR
  OUTPUT backwards

• The same in Python:

  word = "stressed"
  backwards = ""
  for i in range(len(word) - 1, -1, -1):
      backwards = backwards + word[i]
  print(backwards)          # desserts

• String operations are often used to check input, such as its length or its first character. How to write a full validation check is covered in Validation, authentication, testing and errors.

Iterating over a string

Using a loop to take each character of a string in turn.

Programs use random numbers for games, simulations and picking items fairly.

• AQA pseudo-code: RANDOM_INT(1, 6) gives a random integer from 1 to 6, with both ends included.
• Python (AQA's form): from random import randrange, then randrange(1, 7). The second number is excluded, so this gives 1 to 6.
• So for Python, write the second number as one more than the largest value you want.
• A dice roll:

  from random import randrange
  roll = randrange(1, 7)
  print(roll)

• AQA's papers may also show import random with random.randrange(1, 7). It does the same thing.
• Python's randint(1, 6) includes both ends, like RANDOM_INT. It is correct Python, but AQA's papers use randrange, so don't mix the two up.
• Multiples: 5 * randrange(1, 11) gives 5, 10, 15 … 50.
• You don't need to know how a computer makes its random numbers.

Same values, different second number.

Random number generation

The use of algorithms to produce sequences of numbers that appear unpredictable.

RANDOM_INT(a, b)

AQA pseudo-code: a random integer from a to b, both included.

randrange(a, b)

Python: a random integer from a up to b − 1 (b excluded).

• Two outcomes (a coin):

  from random import randrange
  flip = randrange(0, 2)
  if flip == 0:
      print("Heads")
  else:
      print("Tails")

• Picking from an array: generate a random index from 0 to LEN − 1. In pseudo-code: colours[RANDOM_INT(0, LEN(colours) − 1)]. In Python: colours[randrange(0, len(colours))].
• Simulations: repeat many times and count the results.

  from random import randrange
  sixes = 0
  for throw in range(600):
      if randrange(1, 7) == 6:
          sixes = sixes + 1
  print(sixes)              # about 100, different every run

• Each call gives a new number. If you need the same number twice (to show it and then check it), store it in a variable first.
• Generate the number inside the loop if every turn needs a new one, and before the loop if it must stay the same all game.
• Don't replace the random code with a fixed number to make your answer easier: examiners don't give the marks.

Random index

A random integer from 0 to LEN − 1, used to pick an element of an array at random.