Cambridge Pseudocode Handout · ⁨剑桥伪代码讲义⁩

Pseudocode 伪代码 is the language Cambridge writes algorithms 算法 in. It is not a real programming language — no computer runs it — but the exam marks it as if it were, so the form matters.

Every keyword 关键字 is written in upper case: IF, REPEAT, PROCEDURE, OUTPUT.

• Write IF, never if or If.
• A keyword can never be used as a name of your own: you may not call a variable FOR.

Common mistakes

• Lower-case keywords. The mark scheme expects upper case.
• Inventing keywords. Only the words in this handout are in the syllabus.

An identifier 标识符 is the name you give a variable 变量, a constant 常量, a procedure 过程 or a function 函数.

Four rules, all from the syllabus:

• It must start with a capital letter: Count, not count.
• It may hold only letters and digits — no spaces, no punctuation.
• It is not case-sensitive 不区分大小写, so Count and COUNT are the same name. Pick one spelling and keep it.
• Make it mean something. NumberOfPlayers earns the reader's time; N does not.

Common mistakes

• count ← 0 — a lower-case start. 0478 requires the capital.
• Number of players — spaces are not allowed inside a name.

A comment 注释 starts with // and runs to the end of the line. It is for the human reader; the algorithm ignores it.

Indentation 缩进 shows which statements are inside a structure. The syllabus asks for four spaces per level, with one exception you must know:

• THEN and ELSE are indented two spaces from their IF.
• The statements under them are indented four.
• ENDIF lines up with its own IF.

Common mistakes

• Putting THEN on the IF line. That is the A-Level 9618 layout — see topic 11.
• No indentation at all. A marker has to see the structure to award the structure marks.

0478 pseudocode has exactly five data types 数据类型. Learn the list — naming a type that is not on it loses the mark.

• A STRING is written in double quotes, a CHAR in single quotes.
• TRUE and FALSE are keywords, so they are upper case and carry no quotes.

Common mistakes

• Writing FLOAT, DOUBLE or BOOL. Those are real languages, not this syllabus.
• 'Hello' for a string — single quotes hold one character only.

DECLARE names a variable 变量 and fixes its type before you use it.

Several names of one type go on a single line, separated by commas:

• Declare a variable before its first use, near the top of the algorithm.
• A variable keeps its type for the whole algorithm — an INTEGER never holds "Mei".

Common mistakes

• Leaving out the colon: DECLARE Counter INTEGER.
• Using a variable that was never declared. The exam expects the declaration.

A constant 常量 is a value that is fixed for the whole algorithm — a tax rate, π, the size of a class. Naming it once means a change is made in one place.

• In 0478 the arrow is used, exactly as for an assignment.
• The value must be a literal 字面量 — a number, a string, TRUE — never a calculation and never another variable.
• A constant can never be given a new value later. That is the whole point of it.

Common mistakes

• CONSTANT TaxRate = 0.20. The = form is the A-Level 9618 spelling — see topic 11.
• Assigning to a constant further down the algorithm.

The assignment 赋值 operator is ←, a left-pointing arrow. It means take the value on the right and put it in the box on the left.

Counter ← Counter + 1 looks impossible as mathematics and is ordinary as an instruction: work out the right-hand side with the value the box holds now, then store the result back.

• If you cannot type ←, write <-. Both are accepted, here and in the exam.
• The left side must be a single identifier or one array element. Never a calculation.

Common mistakes

• Counter = 0. In pseudocode = is a comparison, not an assignment — and confusing the two is the single commonest syntax error in this paper.
• Counter + 1 ← Counter. The arrow points at the thing being changed.

INPUT reads a value from the user into a variable. OUTPUT displays one or more values.

• INPUT takes exactly one variable, and that variable must already be declared.
• OUTPUT takes any number of items separated by commas; they are printed one after another with nothing between them.
• A literal 字面量 string in an OUTPUT goes in double quotes; a variable does not.

Prompting the user

An INPUT on its own shows nothing on screen, so a well-written algorithm prints a prompt 提示 first:

Common mistakes

• INPUT "Enter a number", X — the prompt is a separate OUTPUT statement.
• OUTPUT "Total: " + Total. 0478 joins output items with a comma; + is arithmetic and & belongs to A-Level 9618 (topic 11).
• Forgetting the quotes: OUTPUT Hello looks for a variable called Hello.

Five arithmetic 算术 operators, in the order a calculator uses them.

Brackets come first, then ^, then * and /, then + and - — the usual precedence 优先级. Use brackets whenever the reader would have to stop and think.

Common mistakes

• Expecting / to give a whole number. 7 / 2 is 3.5, not 3; for a whole number use DIV.
• Writing A x B or A ÷ B. Use * and /.

Integer division 整数除法 answers two different questions, and 0478 writes each of them as a function 函数, with the two values in brackets.

• DIV(a, b) — how many whole times b goes into a, the quotient 商.
• MOD(a, b) — what is left over, the remainder 余数.

Both take integers, and both return an integer.

What they are for

MOD is the standard test for divides exactly, which makes it the way to ask whether a number is even 偶数:

DIV converts a total into whole units — seconds into minutes, pence into pounds:

Common mistakes

• 10 MOD 3. That is the A-Level 9618 form; 0478 uses the function form MOD(10, 3).
• Mixing the two up. DIV gives the whole part; MOD gives what does not fit.

A relational operator 关系运算符 compares two values and produces TRUE or FALSE.

The three logic operators 逻辑运算符 join conditions:

• AND — both sides must be TRUE.
• OR — at least one side must be TRUE.
• NOT — turns TRUE into FALSE and back.

Common mistakes

• <> is the not equal sign here — not !=, which belongs to other languages.
• IF Age >= 15 AND <= 18. Each side of AND needs its own complete comparison: Age >= 15 AND Age <= 18.
• Calling these comparison operators in an answer. The syllabus term is relational.

0478 publishes four string 字符串 routines and no others. Three of them are here.

LENGTH(<identifier>) returns how many characters 字符 a string holds — spaces included.

UCASE(<identifier>) returns the string in upper case 大写, LCASE(<identifier>) in lower case 小写. Neither changes the original 原值 — each hands back a new value.

That last line still prints Happy: to keep the change you must assign it back.

Comparing without worrying about case

UCASE on both sides is the standard way to accept an answer however it was typed:

Common mistakes

• Expecting UCASE(Name) to change Name. Assign the result back.
• LENGTH counts characters, not words. LENGTH("Happy Days") is 10.

SUBSTRING(<identifier>, <start>, <length>) pulls a piece out of a string — starting at position start, taking length characters.

⚠️ Counting starts at 1, not 0. The first character of "Happy Days" is at position 1.

Taking one character, and the rest

LENGTH inside SUBSTRING is what makes the last two lines work for a word of any size — hard-coding 8 would break the moment the word changed.

Reading a string one character at a time

Common mistakes

• Starting at 0. Position 1 is the first character.
• Reading the third argument as an end position. It is a count — SUBSTRING(W, 2, 3) takes three characters beginning at the second.
• Asking for more characters than the string has.

Selection 选择 runs one group of statements or another, depending on a condition 条件.

Add ELSE for the other case. Exactly one of the two branches 分支 runs — never both, never neither.

The layout is the one part students lose marks on, so learn it as a shape:

• IF and its condition on one line, nothing after the condition.
• THEN and ELSE on their own lines, indented two spaces.
• The statements indented four.
• ENDIF back at the column of its IF.

Nesting 嵌套

An IF inside an IF handles more than two outcomes. Each one needs its own ENDIF.

Common mistakes

• Forgetting ENDIF. Every IF closes, and a missing one is a syntax error 语法错误 the marker will see at a glance.
• IF Mark >= 50 THEN OUTPUT "Pass" on one line. No such form exists in either syllabus; write the three lines.
• IF Mark = 50 THEN written as IF Mark ← 50. The arrow assigns; = compares.

When one variable is tested against several single values, CASE OF says it far more clearly than a stack of nested IFs.

• Each branch is a value, a colon, then what to do.
• OTHERWISE catches everything the listed values missed. It is optional, and writing one is good practice — it is what happens when the user types something unexpected.
• The first matching branch runs, and the rest are skipped.

A range, and a character

Common mistakes

• Using CASE OF for a condition. It tests one variable against values; Mark >= 50 needs an IF.
• Forgetting ENDCASE.
• Leaving out OTHERWISE and then wondering why an unexpected value does nothing at all.

Iteration 迭代 repeats statements. Use a FOR loop when you know before you start how many times — a count-controlled 计数控制 loop.

The counter 计数器 starts at the first value, and the loop runs once for every value up to and including the second. 1 TO 5 runs five times.

STEP

STEP changes the size of each jump. A negative step counts down.

Running totals

The commonest use: a variable outside the loop that the loop adds to.

Total ← 0 must sit before the loop. Inside, it would reset on every pass.

Common mistakes

• NEXT naming the wrong counter. Nested loops must close in the reverse order they opened: the inner NEXT first.
• Assuming 1 TO 5 runs four times. Both ends are included.
• Changing the counter inside the loop. Let FOR own it.

A WHILE loop tests before each pass, so it can run zero times. Use it when the number of repeats depends on something that happens while the loop runs.

Something inside the loop must eventually make the condition false, or the loop never ends — an infinite loop 无限循环.

Common mistakes

• Forgetting to change the variable the condition tests, giving an infinite loop.
• Leaving out DO or ENDWHILE.
• Using WHILE where the count is known. A FOR says what you mean in one line.

A REPEAT loop tests after each pass, so it always runs at least once.

That "at least once" is the reason to choose it: validating 验证 input, where you must ask before you can judge the answer.

⚠️ UNTIL states the condition that stops the loop; WHILE states the one that keeps it going. They are opposites, and swapping them is the classic error.

Common mistakes

• Writing UNTIL Number < 100 when you mean UNTIL Number >= 100.
• Using REPEAT where the body must be able to run zero times.
• Forgetting that UNTIL ends the structure — there is no ENDREPEAT.

An array 数组 holds many values of one type under one name. Each value sits at an index 下标.

• The bounds 边界 are written lower:upper, and both are included — [1:3] has three elements 元素.
• Cambridge arrays normally start at 1, not 0.
• Every element has the same type, fixed by OF.

Filling and reading with a FOR loop

An array and a counted loop belong together: the counter is the index.

Finding the largest

Start Largest at the first element, never at 0 — with negative data, 0 would win and the answer would be wrong.

Common mistakes

• Reading Scores[6] from an array declared [1:5]. That is out of bounds 越界.
• Writing Scores(3). Arrays use square brackets; round brackets call a function.
• Declaring [1:5] and then looping 0 TO 4.

A 2-D array 二维数组 is a table: rows and columns, two indexes.

The first index is the row 行, the second the column 列.

Nested loops walk a table

One loop per dimension 维度. The inner loop finishes a whole row before the outer moves on.

⚠️ The inner NEXT closes the inner counter. NEXT Row before NEXT Col crosses the loops over and is nonsense — a marker spots it instantly.

Common mistakes

• Swapping row and column. Grid[2,3] is row 2, column 3.
• Grid[Row][Col]. Cambridge writes one pair of brackets with a comma inside.
• Crossing the NEXT lines over, as above.

A procedure 过程 is a named block of statements. Writing one once and calling it from several places is decomposition 分解 — the idea the whole of paper 2 rests on.

A procedure is called by name, with the keyword CALL:

• The definition 定义 does nothing on its own. Only CALL runs it.
• Control returns to the line after the CALL when the procedure ends.
• A procedure does not give a value back. If you need one, you want a function.

Common mistakes

• Writing DefaultLine on its own to run it. 0478 requires CALL.
• Leaving out ENDPROCEDURE.
• Expecting the procedure's own variables to exist outside it.

A function 函数 is a procedure that hands a value back. It is written where that value is needed — inside an OUTPUT, on the right of an assignment, inside a condition.

• RETURNS <data type> on the header says what type comes back. It is not optional.
• RETURN ends the function immediately — nothing after it runs.
• A function is never called with CALL; it is used wherever its value belongs.

A function inside a condition

Common mistakes

• CALL SumSquare(10, 20) — that throws the answer away.
• A function with no RETURN on some path. Every route out must return a value.
• Leaving RETURNS INTEGER off the header.

A parameter 参数 is a value a procedure or function is given when it is called. Each one carries its own type.

Several parameters are separated by commas, and the order at the call must match the order in the definition:

⚠️ In 0478 every parameter is passed by value 传值: the procedure works on a copy, so changing it inside changes nothing outside. To get a value back, use a FUNCTION and RETURN it. (A-Level 9618 adds BYREF for the other behaviour — topic 11.)

Common mistakes

• PROCEDURE Line(Size) with no type.
• Calling with the arguments in the wrong order — Greet(2, "Mei") is a type error.
• Expecting a procedure to change the variable you passed in. In 0478 it cannot.

A file 文件 keeps data after the program ends. 0478 uses four commands, and every one names the file it works on.

Three file modes 文件模式:

• One WRITEFILE writes one line.
• CLOSEFILE is not optional. An unclosed file can lose the data still waiting to be written, and that is a stock exam answer.

Common mistakes

• FOR WRITE when you meant FOR APPEND, which silently destroys the old contents.
• Forgetting CLOSEFILE.
• Opening a file that is already open.

READFILE takes one line into a variable. The variable must be declared first.

Normally you do not know how many lines there are, so you read until the end of the file — which is what the EOF function reports.

EOF("names.txt") is TRUE once every line has been read, so WHILE NOT EOF(...) means while there is still something to read.

Counting while reading

Common mistakes

• A WHILE EOF(...) loop — that runs only when there is nothing left. You want NOT.
• Reading past the end because the loop has no EOF guard.
• Opening FOR READ a file that was never written.

Cambridge publishes two pseudocode specifications, and they disagree. Everything in topics 1-10 is IGCSE 0478. A-Level 9618 changes the details below — and a student who studies both loses marks by writing one in the other's paper.

This is the 0478 form, and it runs here:

The 9618 form of the same algorithm is written like this — note THEN on the IF line, = in the CONSTANT, and MOD between its two values:

⚠️ Which one is right depends only on which paper you are sitting. Neither is a mistake; each is a mistake in the other's exam. The playground on this page runs the 0478 rules, so a 9618 construct is refused with a message naming the syllabus it belongs to — which is a quick way to check your own habits.

Common mistakes

• Writing THEN on the IF line in an 0478 answer, because that is what a textbook or a website showed. It is the single commonest cross-syllabus slip.
• 10 MOD 3 in 0478. There it is MOD(10, 3).
• ROUND(3.14159, 2) in a 9618 answer. 9618 has no ROUND at all.

0478 passes every parameter by value 传值 (topic 9). 9618 lets you choose, and says so in the header.

• BYVAL — the procedure gets a copy. Changing it changes nothing outside. This is the default when neither word is written.
• BYREF — the procedure works on the caller's own variable 引用, so a change is visible after the call.

This is 9618 only. It does not run here, and it is not accepted in an 0478 answer:

The 0478 way to get a value back

Use a FUNCTION and assign what it returns. This is the whole of the 0478 answer, and it runs:

Common mistakes

• Writing BYREF in an 0478 answer. The keyword is not in that syllabus.
• Expecting a plain 9618 parameter to behave like BYREF. Without the word it is BYVAL.

9618 has data structures 数据结构 that 0478 does not. You will meet them only at A-Level, and none of them runs under the 0478 rules of this page.

A record 记录 groups fields of different types under one name (9618 section 4):

An enumerated type 枚举类型 lists the only values a variable may take:

9618 also adds a DATE data type, random-access files (OPENFILE … FOR RANDOM), pointers, and classes with CLASS … ENDCLASS for its object-oriented paper.

⚠️ 0478 has five data types and arrays, and nothing else (topic 2). A 0478 answer that declares a record is answering a different syllabus.

Common mistakes

• Using a record in an 0478 answer. Use parallel arrays instead — one array per field, sharing an index.
• Thinking TYPE means the same as DECLARE. TYPE defines a new type; DECLARE creates a variable of an existing one.