Pascal Programming

Local Identifiers, Functions, Modularity and Data Flow

Pascal Programming

Problem: In our procedure we want to exchange black for white. A simple exchange will not work.

So, we need Temporary for storage. We may have used Temporary in other

procedures. So, what do we do?

Pascal Programming

We declare Temporary within our procedure.

The declaration . . . Inside the procedure and before begin (Sound familiar?)

The declaration terminates when the procedure finishes its work.

Pascal Programming

e g:– procedure ChangeColor;

• {Changes one color for another}

– var• Temporary, Black, White : integer;

– begin• Temporary := Black• Black := White• White :=Temporary . . . .

Pascal Programming

Variables that affect the entire program -- Global Variables

Variables that affect the procedure ONLY Local Variables

A variable of the same name may have two different uses. The local variable controls.

Pascal Programming

Local rule . . . Any declaration allowed in a Pascal program is allowed in a Pascal procedure.

Local identifiers make the procedure self-contained and self-sufficient.

A variable declared in a procedure . . .– Is local to that procedure– its meaning confined to the procedure– you need only know it when writing or using that

procedure

Pascal Programming

Block . . .parameter list, declaration set and body of statements under their influence.

All identifiers in the block are said to be local to the block

Scope . . .the area of influence of an identifier--may be global or local.

Pascal Programming

Functions may be. . .– Predefined . . .sqrt, trunc and round– Programmer defined

A function call is an expression . . .not a statement like a procedure.

Functions return value(s). Functions can act like procedures . . . But that will

create unintended side-effects. Use a function for its intended purpose . . . creating and delivering value.

Pascal Programming

e g: function Cube (X: integer): integer; {returns cube of X} begin {cube};

– Cube :=X*X*X; end {cube};

Pascal Programming

The declaration . . . Starts with the reserved word function. Includes the type for the function in the

heading. In the body . . . The function name must be on the left

side of an assignment statement.

Pascal Programming

Data flow . . . When we break a problem into

component parts, we need to trace the data through each of the elements or sub-tasks.

We use a data flow diagram for this. This structured diagram clarifies the role

and relationship of each sub-task.

Pascal Programming

Get Data

Determine balance after down payment

Determine monthly interest

Determine length of loanCreate amortization table

Pascal Programming

Data Flow Analysis . . . will allow you to track the movement of

data. will help you decide whether a sub-task

should be a function or a procedure. will help a programmer test the

soundness of the program.

Pascal Programming

Summary . . . Local variables hold data within a

procedure . . . Not outside of it. Parameters should establish the

interaction of a procedure with the larger program.

A formal value parameter is equal to the value held at the time of the call. Continue. . .

Pascal programming

When an actual variable parameter does not change a value when expected . . . Check the var in the parameter list.

When a sub-task requires computation of a value . . . Use a function.

Eliminate side-effects in functions by not making them into procedures.

Data flow analysis helps determine parameters and their type.