User-Defined Functions II TK1914: C++ Programming.

User-Defined Functions II

TK1914: C++ Programming

FUNCTIONS

• In this second part of the chapter on User-Defined Functions, you will learn about– Passing parameters– Value and reference parameters– Scope of an identifier

• Local variables• Global variables

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PARAMETER PASSING

• A function may define parameters which a caller needs to provide when calling that function.

• The provision of parameters when calling a function is sometimes referred to as parameter passing.

• We will look at two ways of passing parameters to functions:– Pass by value (value parameters)– Pass by reference (reference parameters)

• Analogy: Paying for something by cash / direct debit

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VALUE PARAMETERS

• If a formal parameter is a value parameter– The value of the corresponding actual parameter is

copied into it

• The formal parameter has its own copy of the value of the corresponding actual parameter.

• During program execution– The value parameter manipulates the data stored in its

own memory space

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CLUSTER ACTIVITY 6.7

• Refer prog06.7.cpp– Examine the source code. What do you think the

program is trying to do?– What do you think the program will output if the user

inputs 53?– Run the program to see if it behaves as expected.– Could you explain your observation?

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REFERENCE PARAMETERS

• If a formal parameter is a reference parameter

– It receives the address of the corresponding actual parameter

• A reference parameter stores the address of the corresponding actual parameter

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REFERENCE PARAMETERS

• When a function with reference parameters is executed

– The address stored in those parameters are used to refer to the memory spaces of the corresponding actual parameters

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REFERENCE PARAMETERS

A reference parameter receives the address of the actual parameter

Using reference parameters, you can:

Pass one or more values from a function

Change the value of the actual parameter

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REFERENCE PARAMETERS

Reference parameters are useful in three situations:

Returning more than one value

Changing the actual parameter

When passing the address would save memory space and time

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CLUSTER ACTIVITY 6.8

• Refer prog06.8.cpp– Examine the source code. How is it different from prog06.7.cpp?

– What do you think the program will output if the user inputs 53?

– Run the program to see if it behaves as expected.– Could you explain your observation?

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PARAMETERS AND MEMORY ALLOCATION

• When a function is called– Memory for its formal parameters and variables

declared in the body of the function (called local variables) is allocated in the function data area

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PARAMETERS AND MEMORY ALLOCATION

• In the case of a value parameter– The value of the actual parameter is copied into the

memory cell of its corresponding formal parameter

• In the case of a reference parameter

– The address of the actual parameter is passed to the formal parameter

– In the other words, the content of the formal parameter is an address

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CLUSTER ACTIVITY 6.9

• Refer prog06.9a.cpp– Examine the source code. What do you think the

program is trying to do?– What do you think the program will output?– Run the program. Is the output generated as

expected? Explain.• Refer prog06.9b.cpp

– Examine the source code. How is it different from the previous program?

– What do you think the program will output?– Run the program. Is the output generated as

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SCOPE OF AN IDENTIFIER

• The scope of an identifier refers to where in the program an identifier is accessible

• Local identifier - identifiers declared within a function (or block)

• Global identifier – identifiers declared outside of every function definition

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CLUSTER ACTIVITY 6.10

• Refer prog06.10.cpp– Examine the source code. What do you think the

program is trying to do?– Compile the source code.

• Study the compilation error messages.• Could you explain the compile-time errors?

– Can you suggest one way of correcting the program?

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CLUSTER ACTIVITY 6.11

• Refer prog06.11.cpp– Examine the source code. How is it different from prog06.10.cpp?

– Compile the program. Are there any compilation errors?

– Run the program to test it. Does the program produce correct results?

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CLUSTER ACTIVITY 6.12

• Refer prog06.12.cpp– Examine the source code.

• How many variables in the program are named area?

• How are they different?– Compile the program. Are there any compilation

errors?– What do you think the program will output if the user

inputs 20 for width and 10 for height?– Run the program. Is the output as expected?

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CLUSTER ACTIVITY 6.13

• Refer prog06.13.cpp– Examine the source code. What do you think the

program does?– Focus on the if statement.

• What is the purpose of the if statement?• Observe that the variable temp is declared in

the body of the if statement.• Insert a cout statement WITHIN the body of

the if statement which outputs the value of temp after swapping.

• Recompile the program and run it.

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CLUSTER ACTIVITY 6.13 (CONT.)

• Insert the same cout statement OUTSIDE the body of the if statement.

• Recompile the program. Could you explain the compilation error?

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CLUSTER ACTIVITY 6.14

• Refer prog06.14.cpp– Examine the source code.

• How is it different from prog06.12.cpp?

• Which variable does ::area in the function calcArea refer to?

– Compile the program. Are there any compilation errors?

– What do you think the program will output if the user inputs 20 for width and 10 for height?

– Run the program. Is the output as expected?

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GLOBAL VARIABLES

• The operator :: is called the scope resolution operator

• By using the scope resolution operator – A global variable declared before the definition of a

function (block) can be accessed by the function (or block) even if the function (or block) has an identifier with the same name as the variable

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GLOBAL VARIABLES

• C++ provides a way to access a global variable declared after the definition of a function

• In this case, the function must not contain any identifier with the same name as the global variable

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CLUSTER ACTIVITY 6.15

• Refer prog06.15.cpp– Examine the source code.

• How is it different from prog06.11.cpp?• What is the purpose of the function outputDebugMessages?

– What do you think the program will output if the user inputs 20 for width and 10 for height?

– Run the program. • Is the output as expected?• Before analyzing the source code in detail, could

you guess which function in the program is most likely to contain the source of the error?

• Analyze the program code for the source of the error. Is it as you suspected?

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GLOBAL VARIABLES

• Using global variables has side effects

• Any function that uses global variables

– Is not independent

– Usually cannot be used in more than one program

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GLOBAL VARIABLES

• If more than one function uses the same global variable and something goes wrong

– It is difficult to find what went wrong and where

• Problems caused by global variables in one area of a program might be misunderstood as problems caused in another area

• A very good practice in programming is to try to avoid the use of unnecessary global variables in programs.

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STATIC AND AUTOMATIC VARIABLES

• Read textbook, pg 380

ROYO

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FUNCTION OVERLOADING

• Read textbook, pg 382

ROYO

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DEFAULT PARAMETERS

• Read textbook, pg 384

ROYO

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PROGRAMMING EXAMPLE: DATA COMPARISON

• Read textbook, pg 392

ROYO

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YOU SHOULD NOW KNOW…

Passing parameters by value and by reference Value and reference parameters

in what situation should each of them be used Scope of an identifier

Local variables Global variables

why you should avoid unnecessary use of global variables in your programs

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