Programming basics · ⁨编程基础⁩

The number that did not fit

• On 4 June 1996 the first Ariane 5 rocket veered off course and destroyed itself 37 seconds after launch, with four satellites on board.
• The cause was one conversion: a 64-bit decimal value describing the rocket's sideways speed was copied into a 16-bit integer. Ariane 5 was faster than Ariane 4, the number was too big for the box, and the guidance computer shut down.
• Every variable has a data type 数据类型, and the type decides what fits. Choosing it is the first job in turning a design into code.
• This lesson is the basics: constants, variables, types, expressions, input and output, and the ready-made routines you may call.

From design to code

• A design arrives as a flowchart 流程图 or as structured English 结构化英语. You turn it into pseudocode 伪代码, and later into a real language.
• Find the variables 变量 and their types. Input and output boxes become INPUT and OUTPUT; decision diamonds become IF … ELSE … ENDIF or CASE; loop arrows become WHILE, REPEAT … UNTIL or FOR; process boxes become assignments.
• Check by tracing a small input through your pseudocode before you call it finished.

Each flowchart symbol becomes one kind of pseudocode statement

Constants and variables

• A constant 常量 holds a value that never changes while the program runs: CONSTANT Pi = 3.14159. A variable holds one that may change, and is declared with a type: DECLARE Radius : REAL.
• Use a constant for a fixed value that appears in several places, such as a tax rate or a maximum score.
• The benefits the mark scheme lists: it is set once and cannot be changed accidentally; a change is made in one place and reaches every statement that uses it; the identifier 标识符 gives the value a meaningful name.

A variable can be reassigned; a constant cannot

The six data types

• INTEGER for whole numbers (a count, an index, a loop counter). REAL for numbers with a fractional part (a price, an average).
• CHAR for one character in single quotes, 'A'. STRING for text in double quotes, "Hello".
• BOOLEAN for TRUE or FALSE (a flag such as Found). DATE for a calendar date.
• Read the type off how the variable is used: a decimal point means REAL, TRUE means BOOLEAN, single quotes mean CHAR, an array index or DIV and MOD mean INTEGER. Write the type in capitals as the guide spells it.

Worked example: state the appropriate data type

Assignment and expressions

• Assignment 赋值 uses the arrow: Total ← Total + 1, Average ← Sum / Count.
• Arithmetic operators 运算符 are + - * /, plus DIV for whole-number division and MOD for the remainder: 7 DIV 2 = 3 and 7 MOD 2 = 1.
• Comparisons are =, <>, <, >, <=, >=; logic is AND, OR, NOT.
• Precedence 优先级, highest first: NOT, then * / DIV MOD, then + -, then comparisons, then AND, then OR. Use brackets when unsure.

Input and output

• OUTPUT "Enter your name:" prompts; INPUT Name reads the keyboard into the variable; OUTPUT "Hello, ", Name prints several items separated by commas.
• Strings are joined with &, called concatenation 拼接: "A" & "BC" is "ABC".
• Keep the quotes straight: "7" is a string, 7 is a number, and '7' is a single character.

Built-in functions and library routines

• A program library 程序库 holds routines that are already written, compiled and tested; calling one is faster and safer than writing your own. The Paper 2 insert 附页 lists the library routines 库例程 you may use, with their exact names.
• Strings: LENGTH(s), LEFT(s, n), RIGHT(s, n), MID(s, start, n) with positions counted from 1, TO_UPPER(s), TO_LOWER(s). Conversion: NUM_TO_STR(x), STR_TO_NUM(s), IS_NUM(s), ASC(c), CHR(n).
• Numbers and dates: INT(x) cuts to the whole part; RAND(n) gives a real from 0 up to but not including n, so INT(RAND(6)) + 1 is a dice roll; DAY, MONTH, YEAR, DAYINDEX, SETDATE, TODAY; EOF(f) for files.

The common string routines on s = "COMPUTER", positions 1 to 8

Worked example: evaluate each expression

Worked example: find the error

Marks that slip away

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