Variables and data types
- A variable is a named store for data that can change: score = 0. A constant is a value that doesn't change.
- Data types: integer (a whole number, such as 7), float or real (a decimal, such as 2.5), string (text, in quotes, such as "Sam") and Boolean (True or False).
- Input and output in Python: name = input("What is your name? ") and print("Hello", name).
- input() always gives a string, so use int() to turn it into a whole number: age = int(input("How old are you? ")).
Selection and iteration
- Selection: if score >= 50: … elif … else: … In Python, indentation shows which lines belong inside the if statement.
- A FOR loop repeats a set number of times: for i in range(5): repeats 5 times, with i going from 0 to 4.
- A WHILE loop repeats while a condition is true, for example until the user guesses the right answer.
- Block-based languages such as Scratch use the same ideas, with blocks instead of typed code.
Operators and lists
- Arithmetic: + − * /, // (whole-number division), % (remainder) and ** (power). 17 // 5 is 3 and 17 % 5 is 2.
- Comparison: == (equal to), != (not equal to), <, >, <= and >=. Logic: and, or, not.
- A list stores several items: fruits = ["apple", "pear"]. Positions (indexes) start at 0, so fruits[0] is "apple".
Errors and testing
- A syntax error breaks the rules of the language, such as a missing bracket, so the program won't run.
- A logic error lets the program run, but it gives the wrong result.
- Test with normal data (typical values), boundary data (at the edge of what is allowed) and invalid data (values that should be rejected).
Key terms
- Variable
- A named store for data that can change.
- Constant
- A value that doesn't change while a program runs.
- Integer
- A whole number.
- Float
- A number with a decimal point (also called real).
- String
- Text, written inside quotation marks.
- Boolean
- A value that is either True or False.
- FOR loop
- A loop that repeats a set number of times.
- WHILE loop
- A loop that repeats while a condition is true.
- List
- A data structure that stores several items in order.
- Index
- The position of an item in a list, starting at 0.
- Syntax error
- A mistake that breaks the rules of the programming language.
- Logic error
- A mistake that makes a program give the wrong result.