Function ( I )

Ying Wu Electrical & Computer Engineering

Northwestern Universityyingwu@ece.northwestern.edu

ECE230 Lectures Series

The GOAL for Week-3/4• Understand arrays• Fully understand addressing• Understand pointers (angel vs. devil)• Understand references• Master C/C++ control structures• Fully understand the use of functions• Fully understand call-by-value and call-by-

reference• Start modular design• DEBUG_2: Basic skills: dump results• Do the MP#2: A Command Line Interpreter

MP#2: Command Interpreter

• MP#2 description

• MP#2 demo

• So, to do that, what shall we learn?

• Make small “tools” or building blocks?• Modular design?• Put modules together?• Find bug?

What to learn today?

• What motivates to use “function calls”?

• What is a function call?

• What is local variable?

• What is the return of a function call?

• What is exactly happening when invoking a function call?

Motivation …• When you were doing your MP#1, you might have typed

many times the following…if ( (buffer[st]>=‘a’ && buffer[st]<=‘z’) ||

(buffer[st]>=‘A’ && buffer[st]<=‘Z’) ){.. ..

}

Orchar buffer[500], piece[100];for(int i=st; i<=ed; i++)

piece[i] = buffer[i+st];piece[ed-st+1] = ‘\0’;

• This way, your code may look lengthy and messy • You may wonder: why should I type the same thing so

many times?• Is there a way to write it once, and use it whenever I want?

Motivation …• Divide and conquer

– Construct a program from smaller pieces or components

– Each piece more manageable than the original program

• Boss/worker and blackbox

– A boss asks a worker to perform a task and return (i.e., report back) the results when the task is done.

– The boss does not care how the task is done, but only concerning about “inputs” and “outputs”. So, everything the worker has done is a blackbox to the boss

– E.g., get the sqrt of a number!

• Can my program has such a mechanism?

Good News!

• Functions are invoked by a function call

– Analogy: “Boss & worker”The boss --- the calling function or caller

The worker --- the called function

The boss does not need to care about how the worker gets the job done

The boss just wants it done

– A function call specifies the function name and provides information (as arguments) that the called function needs

• Function definitions

– Only written once

– These statements are hidden from other functions.

function( )input output

Think before coding!

• If I want to check if or not a char is a letter:– Input?

– Output?

• If I want to copy a string to another char array:– Input?

– Output?

• If I want to find a “segment” in a command line:– Input?

– Output?

How to define a function?• Create customized functions to

– Take in data

– Perform operations

– Return the result

• Format for function definition:return-value-type function-name( parameter-list )

{ declarations and statements;}

• Example:

int square( int y)

{

return y * y;

}

1. Function prototype

2. Loop

3. Function definition

Program Output

1 // Fig. 3.3: fig03_03.cpp

2 // Creating and using a programmer-defined function

3 #include <iostream>

4

5 using std::cout;

6 using std::endl;

7

8 int square( int ); // function prototype

9

10 int main()

11 {

12 for ( int x = 1; x <= 10; x++ )

13 cout << square( x ) << " ";

14

15 cout << endl;

16 return 0;

17 }

18

19 // Function definition

20 int square( int y )

21 {

22 return y * y;

23 }

1 4 9 16 25 36 49 64 81 100

Notice how parameters and return value are declared.

1. Function prototype (3 parameters)

2. Input values

2.1 Call function

1 // Fig. 3.4: fig03_04.cpp

2 // Finding the maximum of three integers

3 #include <iostream>

4

5 using std::cout;

6 using std::cin;

7 using std::endl;

8

9 int maximum( int, int, int ); // function prototype

10

11 int main()

12 {

13 int a, b, c;

14

15 cout << "Enter three integers: ";

16 cin >> a >> b >> c;

17

18 // a, b and c below are arguments to

19 // the maximum function call

20 cout << "Maximum is: " << maximum( a, b, c ) << endl;

3.Function definition

Program Output

21

22 return 0;

23 }

24

25 // Function maximum definition

26 // x, y and z below are parameters to

27 // the maximum function definition

28 int maximum( int x, int y, int z )

29 {

30 int max = x;

31

32 if ( y > max )

33 max = y;

34

35 if ( z > max )

36 max = z;

37

38 return max;

39 }

Enter three integers: 22 85 17Maximum is: 85

Enter three integers: 92 35 14Maximum is: 92

Enter three integers: 45 19 98Maximum is: 98

What can be in the inputs?

• The inputs of the function call is specified in the argument list

• (NOTE) the argument list can also be used to get the output through call-by-reference (you’ll learn that later)

• The argument list can have anything you want it there.

What can be a return?

• Unlike Matlab, C/C++ function only allows ONE return, which can be– A variable, e.g., int, double, bool…– A pointer, e.g., int*, double*, …– A reference, e.g., int&, double&, …– void, i.e., nothing

Function Prototypes• Function prototype

– Function name

– Parameters• Information the function takes in

– Return type

• Type of information the function passes back to caller (default int)• void signifies the function returns nothing

– Only needed if function definition comes after the function call in the program

• Example:int maximum( int, int, int );

– Takes in three ints

– Returns an int

Math Library Functions

• Math library functions– Allow the programmer to perform common mathematical

calculations.used >

• Examplecout << sqrt( 900.0 );

– Calls the sqrt (square root) function. The preceding statement would print 30

– double sqrt(double x);

• Function arguments can be– Constant sqrt( 4 );– Variable sqrt( x );– Expressions sqrt( sqrt( x ) ) ;

sqrt( 3 - 6x );

Header Files

• Header files– Contain function prototypes for library functions– <cstdlib> , <cmath>, etc.

– Load with #include <filename>• Example:

#include <cmath>

• Custom header files– Defined by the programmer

– Save as filename.h– Loaded into program using

#include "filename.h"

Local Variables• Local variables

– Known only in the function in which they are defined

– All variables declared in function definitions are local var.

– The LIFE CYCLE of a local variable is only in the function in which it is used.

• Parameters– Local variables passed when the function is called that

provide the function with outside information

void main()

{

int x = 20;

int y = myfunc(x);

}

int myfunc(t)

{

int x;

x = t+1;

return x;

}

To have a deep understanding…

• Let’s understand the following:– Your program machine code– Loading …– Instruction Pointer (IP)– Stack– Invoking a function call

Compiling and Linking

Machine object code, e.g., a.obj

CompilerLinker

Various libraries

Machine executable code (the stuff for machine to execute, not for you. ) e.g.,

“a.exe”

Your C/C++ source code (the stuff you wrote) e.g., a.cpp

Loading …

• Load .exe to memory (when you execute .exe)

main( )

int func(a,b)func(a,b)

x=func(3,4);00601000

00602000

return c;

Code seg.

Instruction Pointer• How does the computer execute the program?

main( )

int func(a,b)func(a,b)

x=func(3,4);00601000

00602000

return c;

I. P.

Code seg.

Let me demonstrate this process in VC++ debugger!

Question: when a function is called, for the I.P.,

(1) where to go?

(2) how to get back from function call?

A Concept: Stack (LIFO)

20

34

12

20

push(20)

20

34

push(34)

20

34

12

push(12)

20

34

pop(12)

20

pop(34)

pop(20)

Invoking a Function• What happens when a function is invoked?

– where is my code?

– what happens to i.p.?

– what happens to stack?

– how to pass parameters?

– how to get the return?

006010003

4

00601000 00601000c

main( )

int func(a,b)func(a,b)

x=func(3,4);00601000

00602000

return c;

I. P.

Code seg.stack

Question for Today!int fun(int x){

int y;y = x + 2;return y;

}

void main( ){

int y = 5;int x = 1;x = fun(x);

}

Question: after main() is executed, x=?, y=?

Prepare for MP#2

• Take a look at my solution to MP#1, figure out which part can be replaced by function calls.

About MP#2• Attention, we only check 4 types of valid grammars.

1. assignment:   "operand_1   =   operand_2", e.g., a = 1

2. binary operation:   "operand_1    operator    operand_2", e.g., a + b

3. unary operation:    "operand_1    operator", e.g., a++

4. assignment and binary operation:  "result = operand_1     operator    operand_2", e.g., a = b + c

• According to that, we can define– Operations, (e.g., +, -, *, /, ++, --)

– Operation types. (e.g., assignment, unary, binary)

• So, enum OP {ASN, ADD, MIN, MUL, DIV, INC, DEC};

enum OP_TYPE {asn, unary, binary};

A trick to do MP#2• You may want to find the # of segments of the

command line– Since we only allow 2, 3, 5

• Observe the position of operators (if defining a bool pIsOP[5] = {false, false, false, false, false}.

– a + b {F, T, F, F, F}– c ++ {F, T, F, F, F}– a = 1 {F, T, F, F, F}– a = b + 1 {F, T, F, T, F}

• From this observation, we only allow two patterns:– {F, T, F, F, F} and {F, T, F, T, F}