Why local variables are preferred

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Quick Questions

Shade in the A or B circle for each question.

1. Why are local variables usually preferred over global variables?

A They stop other parts of the program changing the value by mistake

B They make the program run faster on every computer

2. Two procedures both declare a local variable called Total. What happens?

A They share one Total, so changing it in one procedure changes it in the other

B Each procedure has its own separate Total, so they do not affect each other

3. Which is the better way to give a subroutine a value it needs?

A Set a global variable before calling the subroutine

B Pass the value in as a parameter

4. A procedure changes the global variable Score by mistake. What is the problem?

A Other parts of the program that use Score may now get wrong values

B Nothing, because only that procedure can see Score

5. Which subroutine is easier to test on its own?

A One that uses only its parameters and local variables

B One that reads and changes several global variables

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1DECLARE Total : INTEGER (inside PROCEDURE ShowTotal)		AA local variable: it exists only inside this subroutine, so no other part of the program can change it by mistake
2FUNCTION AddTax(Price : REAL) RETURNS REAL		BOUTPUT Double(5) // should output 10
3Two subroutines can each use a local variable with the same name without affecting each other		CThe procedure changes the global variable Score by mistake, which can break other parts of the program. Use a local variable instead.
4A subroutine that uses only local variables and parameters can be tested on its own		DThe value is passed in as a parameter, so the function does not rely on a global variable and can be tested alone
5Score ← 0 (global) PROCEDURE Reset() Score ← 100 ENDPROCEDURE		EThe function depends on global variables Width and Height, so it is hard to test alone. Pass them in as parameters instead.
6FUNCTION Area() RETURNS INTEGER RETURN Width * Height ENDFUNCTION		FPROCEDURE A() DECLARE i : INTEGER ... PROCEDURE B() DECLARE i : INTEGER

Definitions

local variable

Fill in the Blanks

Complete each line of code by filling in the missing parts.

PROCEDURE AddBonus(Score : INTEGER) _______ NewScore : INTEGER NewScore ← Score + 10

FUNCTION AddTax(Price : REAL) RETURNS REAL DECLARE Total : ____ Total ← Price * 1.2

PROCEDURE Reset() DECLARE Count : INTEGER _____ ← 0

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

// Test Square alone: expect 16 ______ Square(4)

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A PROCEDURE Twice()

B OUTPUT N * 2

C ENDPROCEDURE

Question 2

A PROCEDURE Bonus(P : INTEGER)

B OUTPUT Pts + 10

C ENDPROCEDURE

Question 3

A PROCEDURE Calc(Num : INTEGER)

B DECLARE Temp : INTEGER

C Total ← Num * 2

D OUTPUT Temp

E ENDPROCEDURE

Question 4

A PROCEDURE Add(N : INTEGER)

B DECLARE Sum : INTEGER

C Sum ← Total + 1

D OUTPUT Sum

E ENDPROCEDURE

Question 5

A FUNCTION Inc(N) RETURNS REAL

B DECLARE R : REAL

C R ← N + 1

D RETURN Total

E ENDFUNCTION

Answer Key

Why local variables are preferred

Section A: Quick Questions

A — They stop other parts of the program changing the value by mistake

B — Each procedure has its own separate Total, so they do not affect each other

B — Pass the value in as a parameter

A — Other parts of the program that use Score may now get wrong values

A — One that uses only its parameters and local variables

Section B: Match the Pairs

DECLARE Total : INTEGER (inside PROCEDURE ShowTotal) → A local variable: it exists only inside this subroutine, so no other part of the program can change it by mistake

FUNCTION AddTax(Price : REAL) RETURNS REAL → The value is passed in as a parameter, so the function does not rely on a global variable and can be tested alone

Two subroutines can each use a local variable with the same name without affecting each other → PROCEDURE A() DECLARE i : INTEGER ... PROCEDURE B() DECLARE i : INTEGER

A subroutine that uses only local variables and parameters can be tested on its own → OUTPUT Double(5) // should output 10

Score ← 0 (global) PROCEDURE Reset() Score ← 100 ENDPROCEDURE → The procedure changes the global variable Score by mistake, which can break other parts of the program. Use a local variable instead.

FUNCTION Area() RETURNS INTEGER RETURN Width * Height ENDFUNCTION → The function depends on global variables Width and Height, so it is hard to test alone. Pass them in as parameters instead.

Section C: Definitions

local variable — A variable declared inside a subroutine. It keeps the subroutine self-contained, so changing it cannot accidentally alter other parts of the program, and the subroutine is easier to test alone.

Section D: Fill in the Blanks

PROCEDURE AddBonus(Score : INTEGER)
   DECLARE NewScore : INTEGER
   NewScore ← Score + 10

FUNCTION AddTax(Price : REAL) RETURNS REAL
   DECLARE Total : REAL
   Total ← Price * 1.2

PROCEDURE Reset()
   DECLARE Count : INTEGER
   Count ← 0

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

// Test Square alone: expect 16
OUTPUT Square(4)

Section E: Spot the Error

A — The header has no parameter, so N is never passed in. The procedure would have to rely on a global variable called N. It should be PROCEDURE Twice(N : INTEGER) so the value is passed in as a parameter.

A — The parameter is called P, but the body uses Pts. Pts is not local to the procedure, so it would only work if a global called Pts existed. The parameter name must match the name used inside: Pts.

C — Total is not declared in this procedure, so this line would change a global variable by mistake. The procedure should store the result in its own local variable: Temp ← Num * 2.

C — Total is not a parameter or a local variable, so the procedure depends on a global variable outside it. That makes it hard to test alone. It should use the parameter: Sum ← N + 1.

D — The function works out its answer in the local variable R, but returns Total, which is some other variable outside the function. It should be RETURN R.