程序设计

A variable 变量 is a named store that holds a value which can change while the program runs. A constant 常量 is a named store whose value is fixed and does not change.

You should declare 声明 them (say their name and type) before use:

DECLARE Score : INTEGER
DECLARE Name : STRING
CONSTANT Pi ← 3.142

Use a constant for a value that never changes (like Pi), so it is set in one place and is easy to read.

A data type 数据类型 says what kind of value a variable holds. You must know five basic types.

Input 输入 reads a value from the user. Output 输出 shows a value on the screen.

OUTPUT "What is your name?"
INPUT Name
OUTPUT "Hello ", Name

Every program is built from three control structures.

Sequence

Sequence 顺序 means the steps run one after another, in order, from top to bottom.

INPUT Length
INPUT Width
Area ← Length * Width
OUTPUT Area

Selection

Selection 选择 chooses which steps to run, based on a condition. Use an IF statement, or a CASE statement when there are many choices.

IF Score >= 50
  THEN
    OUTPUT "Pass"
  ELSE
    OUTPUT "Fail"
ENDIF

CASE OF Grade
    'A' : OUTPUT "Excellent"
    'B' : OUTPUT "Good"
    OTHERWISE OUTPUT "Keep trying"
ENDCASE

Iteration

Iteration 迭代 (a loop 循环) repeats steps. There are three kinds.

A count-controlled loop 计数循环 repeats a fixed number of times:

FOR I ← 1 TO 5
    OUTPUT "Hello"
NEXT I

A pre-condition loop 前测循环 checks the condition before each repeat, so it may run zero times:

WHILE Answer <> "stop" DO
    INPUT Answer
ENDWHILE

A post-condition loop 后测循环 checks the condition after each repeat, so it always runs at least once:

REPEAT
    INPUT Password
UNTIL Password = "secret"

• totalling 求和 — keep adding values to a running total: Total ← Total + Value.
• counting 计数 — add 1 to a counter each time: Count ← Count + 1.

Total ← 0
FOR I ← 1 TO 10
    INPUT Mark
    Total ← Total + Mark
NEXT I
OUTPUT Total

Worked example. A program must read 5 marks, then output the total and the average.

Total ← 0
FOR I ← 1 TO 5
    INPUT Mark
    Total ← Total + Mark
NEXT I
Average ← Total / 5
OUTPUT Total, Average

Two lines carry the marks. Total ← 0 must sit before the loop: put it inside and the total resets on every pass, so the program outputs only the last mark. And Average ← Total / 5 must sit after the loop, because the total is not complete until every mark has been added. Initialise before, calculate after - that ordering is what the question is really testing.

Arithmetic operators

For example, 17 MOD 5 is 2, and 17 DIV 5 is 3.

Relational operators

These compare two values and give a Boolean result: =, <, <=, >, >=, and <> (not equal to).

Logical operators

These join conditions: AND (both must be true), OR (at least one must be true), NOT (reverses true/false).

IF Age >= 13 AND Age <= 19
  THEN
    OUTPUT "Teenager"
ENDIF

A string is made of characters. Useful operations:

• length 长度 — the number of characters in the string;
• substring 子串 — a smaller part taken from the string;
• upper case 大写 — change letters to capitals;
• lower case 小写 — change letters to small letters.

name ← "Computer"
OUTPUT LENGTH(name)          // 8
OUTPUT SUBSTRING(name, 1, 4) // "Comp"
OUTPUT UCASE(name)           // "COMPUTER"
OUTPUT LCASE(name)           // "computer"

(The first character may be counted as position 0 or position 1, depending on the language.)

In exam pseudocode the first character is at position 1, and SUBSTRING(name, start, length) takes length characters starting at start.

Worked example. Output the surname from a full name such as "Ada Lovelace".

FullName ← "Ada Lovelace"
Position ← 0
FOR I ← 1 TO LENGTH(FullName)
    IF SUBSTRING(FullName, I, 1) = " "
      THEN
        Position ← I
    ENDIF
NEXT I
OUTPUT SUBSTRING(FullName, Position + 1, LENGTH(FullName) - Position)

1. LENGTH(FullName) is 12 (the space counts as a character).
2. The loop checks one character at a time; the space is at position 4, so Position is 4.
3. The surname starts at position 5 and has $12 - 4 = 8$ characters: SUBSTRING(FullName, 5, 8) is "Lovelace".

A nested statement 嵌套语句 is one control structure placed inside another. You can nest selection and iteration.

FOR I ← 1 TO 3
    IF MOD(I, 2) = 0
      THEN
        OUTPUT I, " is even"
      ELSE
        OUTPUT I, " is odd"
    ENDIF
NEXT I

You will not have to write more than three levels of nesting.

To avoid repeating code, you can break a program into named blocks.

• A procedure 过程 is a named block of code that does a task. You run it with CALL.
• A function 函数 is like a procedure, but it returns a value back to where it was called.

A parameter 参数 is a named input in a procedure or function; a call supplies its value. Exam questions use up to three parameters.

PROCEDURE Greet(PersonName : STRING)
    OUTPUT "Hello ", PersonName
ENDPROCEDURE

CALL Greet("Sam")

FUNCTION Square(N : INTEGER) RETURNS INTEGER
    RETURN N * N
ENDFUNCTION

Answer ← Square(5)   // answer is 25

Worked example. Write a function that returns the larger of two numbers, and use it.

FUNCTION Larger(A : INTEGER, B : INTEGER) RETURNS INTEGER
    DECLARE Result : INTEGER
    IF A > B
      THEN
        Result ← A
      ELSE
        Result ← B
    ENDIF
    RETURN Result
ENDFUNCTION

Biggest ← Larger(12, 30)
OUTPUT Larger(Biggest, 25)

Larger(12, 30) returns 30, which is stored in Biggest; Larger(30, 25) returns 30, which is output. A, B and Result are local: they exist only while the function runs. A function's result must be used, in an assignment or an output; a procedure is run on its own line with CALL and gives nothing back.

Local and global variables

• A local variable 局部变量 is declared inside a procedure or function. It can only be used there.
• A global variable 全局变量 is declared in the main program. It can be used anywhere.

Local variables are safer, because they cannot be changed by mistake from another part of the program.

A library routine 库程序 is a ready-made piece of code you can use. You must know these:

MOD(Number, 2) = 0 tests whether a number is even; DIV(Seconds, 60) and MOD(Seconds, 60) split 135 seconds into 2 minutes and 15 seconds.

A maintainable 可维护的 program is easy for other people to read and change later. To make one:

• use meaningful identifiers 有意义的标识符 — clear names for variables, constants, arrays, procedures and functions (totalScore, not x);
• add comments 注释 to explain what parts of the code do;
• use procedures and functions to split the work into small blocks.

An array 数组 is a single variable that holds many values of the same type, found by an index 索引 (a position number).

One-dimensional arrays

A one-dimensional (1D) array 一维数组 is like a single list.

DECLARE Scores : ARRAY[1:5] OF INTEGER
Scores[1] ← 90
Scores[2] ← 75
OUTPUT Scores[1]

You can use a loop to fill or read an array:

FOR I ← 1 TO 5
    INPUT Scores[I]
NEXT I

Worked example. Find the highest score in Scores and its position. Scores holds 90, 75, 98, 61, 98.

DECLARE Scores : ARRAY[1:5] OF INTEGER
Scores[1] ← 90
Scores[2] ← 75
Scores[3] ← 98
Scores[4] ← 61
Scores[5] ← 98

Highest ← Scores[1]
Place ← 1
FOR I ← 2 TO 5
    IF Scores[I] > Highest
      THEN
        Highest ← Scores[I]
        Place ← I
    ENDIF
NEXT I
OUTPUT "Highest ", Highest, " at position ", Place

The output is Highest 98 at position 3. Starting Highest at the first element, not at 0, makes it work for negative values too; with >= instead of >, the last 98 would be found (position 5).

Two-dimensional arrays

A two-dimensional (2D) array 二维数组 is like a table with rows and columns. It uses two indexes.

DECLARE Grid : ARRAY[1:3, 1:3] OF INTEGER
Grid[1, 1] ← 5
Grid[2, 3] ← 8

You read a 2D array with a loop inside a loop (nested iteration).

Worked example. Marks[1:3, 1:2] holds two test marks for each of three students (rows). Output each student's total.

DECLARE Marks : ARRAY[1:3, 1:2] OF INTEGER
Marks[1, 1] ← 12
Marks[1, 2] ← 15
Marks[2, 1] ← 9
Marks[2, 2] ← 11
Marks[3, 1] ← 14
Marks[3, 2] ← 18

FOR Row ← 1 TO 3
    Total ← 0
    FOR Col ← 1 TO 2
        Total ← Total + Marks[Row, Col]
    NEXT Col
    OUTPUT "Student ", Row, ": ", Total
NEXT Row

The outer loop picks a student; the inner loop adds that student's two marks. The output is 27, 20 and 32. Total ← 0 must be inside the outer loop: placed before it, the totals would run on to 27, 47 and 79.

A program can store data in a file 文件 so it is kept after the program stops. You must open the file, use it, then close it.

OPENFILE "data.txt" FOR WRITE
WRITEFILE "data.txt", "Hello"
CLOSEFILE "data.txt"

OPENFILE "data.txt" FOR READ
READFILE "data.txt", Line
CLOSEFILE "data.txt"

You can read and write single items of data or a whole line of text. Always close a file when you have finished with it.

Worked example. Save the five values of Scores to a file, one per line, then read the first one back.

DECLARE Scores : ARRAY[1:5] OF INTEGER
Scores[1] ← 90
Scores[2] ← 75
Scores[3] ← 98
Scores[4] ← 61
Scores[5] ← 98

OPENFILE "scores.txt" FOR WRITE
FOR I ← 1 TO 5
    WRITEFILE "scores.txt", Scores[I]
NEXT I
CLOSEFILE "scores.txt"

OPENFILE "scores.txt" FOR READ
READFILE "scores.txt", FirstScore
CLOSEFILE "scores.txt"
OUTPUT FirstScore

Each WRITEFILE adds one line, so the file holds five lines; reading starts at the top, so FirstScore is the first score. Opening a file FOR WRITE starts it empty: anything already in it is lost.

• A variable can change while the program runs; a constant is fixed. Learn the five data types: integer, real, char, string, Boolean.
• Every program is built from three structures: sequence, selection (IF / CASE), and iteration (loops).
• A WHILE loop tests before the body, so it may run zero times; a REPEAT … UNTIL loop tests after, so it always runs at least once.
• MOD gives the remainder and DIV the whole-number part of a division: 17 MOD 5 is 2, 17 DIV 5 is 3.
• A function returns a value; a procedure does not. A local variable works only inside its block; a global one works anywhere.