CSE202: Lecture 12The Ohio State University1 Function Calling.

CSE202: Lecture 12 The Ohio State University 1

Function Calling


Parameter Passing

• There are two ways (in C++) to pass parameters to functions.– Pass by Value (what we’ve seen so far)– Pass by Reference (we’ll see this later)

• In the functions we have seen so far, the input parameters are passed by value. In other words, the values of the parameters are copied.


Arguments and Parameters. . .

double dist(double x1, double y1, double x2, double y2);

. . .

// call function dist()

dist_pq = dist(px, py, qx, qy);

. . .

• px, py, qx and qy are arguments to the function dist().

• x1, y1, x2 and y2 are the function parameters.


Pass by Value


Function dist()

// function distdouble dist(double x1, double y1, double x2, double y2)// four input parameters: x1, y1, x2, y2// All parameters are PASSED BY VALUE{ double dx, dy, d;

dx = x1 - x2; dy = y1 - y2; d = sqrt(dx*dx + dy*dy);

// return the distance from (x1,y1) to (x2,y2) return(d);}


Pass By Value (pointDist3.cpp). . .double dist(double x1, double y1, double x2, double y2);. . . // call function dist() dist_pq = dist(px, py, qx, qy);. . .

• Pass by value: – Value px is copied to x1;– Value py is copied to y1;– Value qx is copied to x2;– Value qy is copied to y2.

• px, py, qx and qy are arguments to the function.• x1, y1, x2, y2 are the function parameters.


Function dist2()

// function dist2double dist2(double x1, double y1, double x2, double y2)// four input parameters: x1, y1, x2, y2// All parameters are PASSED BY VALUE{ double dx, dy, d;

dx = x1 - x2; dy = y1 - y2; d = sqrt(dx*dx + dy*dy);

x1 = 77; // What affect will this have on the program?

// return the distance from (x1,y1) to (x2,y2) return(d);}


Pass By Value (pointDist3.cpp). . .double dist2(double x1, double y1, double x2, double y2){ . . .

x1 = 77; // What affect will this have on the program? . . . return(d);}

. . . // call function dist2() dist_pq = dist2(px, py, qx, qy);

// What is the value of px?

dist_pr = dist2(px, py, rx, ry); dist_qr = dist2(qx, qy, rx, ry);


Pass by Value

• When a parameter is passed by value, any change to the parameter inside the function has absolutely no effect outside the function!

• If all the parameters are passed by value, the only effect the function has on the calling program is through the returned value.


Pass by Reference


Pass by Reference

• A C++ program can pass to a function the memory locations (references) of the variables used to make the function call, instead of copying their values.

• Any change to the value of parameters will change the value of the variables of the calling function.


Passing by Reference: Example

• The following function manipulates the parameters area and perimeter directly.

// prototype. // The & symbol represents the address of the variable.// It indicates pass by reference, not by value. void RectCalc(double l, double w, double & area, double & perim);

// definitionvoid RectCalc(double l, double w, double & area, double & perim){

area = l * w;perim = 2 * (l + w);

}


Breakdown of RectCalc() (1)

• The function takes 4 parameters. The first two, length and width, are passed by value as before.

• The last two, area and perimeter, are passed by reference (in other words, by their memory locations).

• The syntax for pass-by-reference parameters:data-type & reference-name


Breakdown of RectCalc() (2)

• Notice that because the function caller’s values are directly manipulated, this function does not have to return anything; the function is declared void

• Because of this, when calling the function, one need not assign a variable to its output (because it has none!)


Using RectCalc() (1)

//function prototypevoid RectCalc(double l, double w, double & area, double & perim); int main(){

double rectArea, rectPerim, rectlength = 2.0, rectwidth = 3.0;

//call the function. Do not use & modifier here!!//don’t assign it to anything! It has no return value!RectCalc(rectlength, rectwidth, rectArea, rectPerim);

//rectArea and rectPerim now hold modified values!cout << “The area is: “ << rectArea

<< “ and the perimeter is: “ << rectPerim << endl;

return 0;}

//Place Function Definition here


Using RectCalc() (2)

• Notice that because rectArea and rectPerim were passed by reference, their values were manipulated.

• So this simulates the effect of returning 2 values back to the caller!


pointDist3.cpp. . .// main functionint main(){ double px, py, qx, qy, rx, ry; double dx, dy, dist_pq, dist_pr, dist_qr;

// read input cout << "Enter point 1 (2 floats): "; cin >> px >> py; cout << "Enter point 2 (2 floats): "; cin >> qx >> qy; cout << "Enter point 3 (2 floats): "; cin >> rx >> ry;


Function inputCoord()

// function inputCoord()void inputCoord(int i, double & x, double & y)// three input parameters: i, x, y// Parameter i is passed by value// Parameters x and y are PASSED BY REFERENCE { // read input cout << "Enter point " << i << " (2 floats): "; cin >> x >> y;}


pointDist5.cpp. . .// main functionint main(){ double px, py, qx, qy, rx, ry; double dist_pq, dist_pr, dist_qr;

// read input inputCoord(1, px, py); inputCoord(2, qx, qy); inputCoord(3, rx, ry);. . .


pointDist5.cpp. . . // calculate distances dist_pq = dist(px, py, qx, qy); dist_pr = dist(px, py, rx, ry); dist_qr = dist(qx, qy, rx, ry);

// output distances output_distance(px, py, qx, qy, dist_pq); output_distance(px, py, rx, ry, dist_pr); output_distance(qx, qy, rx, ry, dist_qr);

return 0;}

// function definitions. . .


Notes on Pass by Reference (1)

• The argument to a reference parameter must be a variable, not a constant or an expression. (Why?)

• The following function calls are illegal:

void RectCalc(double l, double w, double & a, double & p);

double a = 3.0, b = 4.0, c = 5.0, d = 6.0, e = 7.0;

const double x = 5.0, y = 6.0;

RectCalc(3.0, 4.0, 5.0, 6.0);

RectCalc(a, b, 5.0, 6.0);

RectCalc(a, b, c*d, e);

RectCalc(a, b, x, y);


Notes on Pass by Reference (2)

• Note the function call gives no indication whether a parameter is passed by reference. You need to look at the function definition to make sure.

• If you do not pay attention to this, variables could be changed that you were not expecting to change! If a variable’s value is not supposed to change, then it should (usually) be passed by value.


A Closer Look at Pass-by-Reference


swap() Function Definition

// swap() function definitionvoid swap(double & num1, double & num2)// swap the values of num1 and num2{double temp;

temp = num1; // Why do we need temp?num1 = num2;num2 = temp;

}


swap() Function Usage

int main(){double oneNum, anotherNum;oneNum = 3.3;anotherNum = 5.6;

swap(oneNum, anotherNum); //swap!

//oneNum now holds 5.6//anotherNum now holds 3.3

}


A Closer Look at swap():From main()

• In main(), two variables are declared and defined:

oneNum = 3.3

anotherNum = 5.6

• Memory cells in the computer are allocated to hold these variables


A Closer Look at swap(): After calling swap()

• The main() function calls swap()

• Because swap defined num1 and num2 as reference parameters, the variables oneNum and anotherNum are now referred to as num1 and num2 respectively inside the function

• In other words, they are “aliased” or “linked”


A Closer Look at swap(): Inside swap()

• In swap(), a new variable, temp, is introduced to hold the value of num1

• Without temp, the first variable assignment would overwrite the other!

• We see how in the next slide.


A Closer Look at swap(): Making the Exchange

• Assign num2 to num1.

• Notice that num1 value is now lost. Fortunately, we saved its value inside temp


A Closer Look at swap(): Finishing up

• swap() has completed its routine. Its local variables vanish, and the calling variables have their values swapped.

• The main() function now continues execution beyond the function call.

• This was all done without actually “returning” any values


Another example using swap()int main(){

double a, b;

cout << “Enter two numbers: “;cin >> a >> b;

if (a > b) { swap(a,b); }

cout << a << “ <= “ << b << endl;

return 0; }


Standard Template Library: swap()

• swap() is such a useful function, that it is part of the standard template library.

...#include <algorithm>using namespace std;

int main(){ int a, b; double c, d;

. . . swap(a,b); swap(c,d);. . .


Standard Template Library: swap()

#include <algorithm>#include <iostream>using namespace std;

int main(){ double a, b; cout << “Enter two numbers: “;

cin >> a >> b;

if (a > b)

{ swap(a,b); }

cout << a << “ <= “ << b << endl;

return 0; }


Expressions as Arguments• Expressions can be arguments to function parameters

which are passed BY VALUE:

double circleArea(double radius); // Pass by valuedouble r(3.5);double area = circleArea(3*r+2.0); // valid

• Constants and expressions CANNOT be arguments to function parameters which are passed BY REFERENCE:

double circleArea(double & radius); // Pass by referencedouble r(3.5);double area1 = circleArea(3*r+2.0); // SYNTAX ERROR!double area2 = circleArea(3.5); // SYNTAX ERROR!

Pass by Value vs. Pass by Reference

• Use pass by reference when you want the function to change the value of an argument;

• Otherwise, use pass by value.



What’s the error?. . . void rect_calc(double & l, double & w, double & area, double & perim);

int main(){ double rectArea(0.0), rectPerim(0.0), rectLength(2.0), rectWidth(3.0);

rect_calc(rectLength, rectWidth, rectArea, rectPerim);

cout << "Area: " << rectArea << endl; cout << "Perimeter: " << rectPerim << endl;

return 0;}

void rect_calc(double l, double w, double & area, double & perim){ area = l * w; perim = 2 * (l + w);}


> g++ rectCalcError1.cpp

/tmp/ccJFlGrU.o: In function `main':

rectCalcError1.cpp:(.text+0x4e): undefined reference to `rect_calc(double&, double&, double&, double&)'

collect2: ld returned 1 exit status

>

void rect_calc(double & l, double & w, double & area, double & perim);


rect_calc(rectLength, rectWidth, rectArea, rectPerim);...void rect_calc(double l, double w, double & area, double & perim){ area = l * w; perim = 2 * (l + w);}


Function Prototype

• The function prototype should look EXACTLY LIKE the function header (except that the prototype ends with a ‘;’.)

• Protoype:

void rect_calc(double l, double w, double & area, double & perim);

• Header:

void rect_calc(double l, double w, double & area, double & perim);


What’s the error?. . . void rect_calc(double l, double w, double & area, double & perim);

int main(){ int rectArea(0.0), rectPerim(0.0), rectLength(2.0), rectWidth(3.0);



return 0;}

void rect_calc(double l, double w, double & area, double & perim){ area = l * w; perim = 2 * (l + w);}



rectCalcError2.cpp: In function ‘int main()’:

rectCalcError2.cpp:16: error: invalid initialization of reference of type ‘double&’ from expression of type ‘int’

rectCalcError2.cpp:8: error: in passing argument 3 of ‘void rect_calc(double, double, double&, double&)’

>

8. void rect_calc(double l, double w, 9. double & area, double & perim);10. 11. int main()12. {13. int rectArea(0.0), rectPerim(0.0), 14. rectLength(2.0), rectWidth(3.0);15. 16. rect_calc(rectLength, rectWidth, rectArea, rectPerim);...28. void rect_calc(double l, double w, 29. double & area, double & perim)30. {31. area = l * w;32. perim = 2 * (l + w);33. }


What’s the error?. . . void rect_calc(int l, int w, int & area, int & perim);




return 0;}

void rect_calc(int l, int w, int & area, int & perim){ area = l * w; perim = 2 * (l + w);}



rectCalcError2.cpp: In function ‘int main()’:

rectCalcError2.cpp:16: error: invalid initialization of reference of type ‘double&’ from expression of type ‘int’

rectCalcError2.cpp:8: error: in passing argument 3 of ‘void rect_calc(double, double, double&, double&)’

>

8. void rect_calc(int l, int w, 9. int & area, int & perim);10. 11. int main()12. {13. double rectArea(0.0), rectPerim(0.0), 14. rectLength(2.0), rectWidth(3.0);15. 16. rect_calc(rectLength, rectWidth, rectArea, rectPerim);...28. void rect_calc(int l, int w, 29. int & area, int & perim)30. {31. area = l * w;32. perim = 2 * (l + w);33. }


Pass by Reference

• When passing by reference:– Arguments of type int must be passed to

parameters of type int;– Arguments of type double must be passed to

parameters of type double;– Arguments of type string must be passed to

parameters of type string;– etc.


Other Notes on Functions

• If a calculation or process needs to execute repeatedly, then make that a function.

• Keep your functions from doing too much. If a function is long and complex, it is hard to debug.

• Split long and complex functions into more functions.


Return


The return Statement• All functions that have a return value must have

a return statement.

• A function may have more than one return statement.

• When a return is executed, it will immediately exit your function and return back to the calling procedure.

• At each function call, one and only one return statement is executed.


#include <iostream>using namespace std;

int maxInt(int a, int b);

int main(){

cout << maxInt(88, 102) << endl;

return 0;}

// function maxInt has two return statementsint maxInt(int a, int b){

if (a >= b) { return a; }else { return b; }

// Note: The program never reaches here!}


const Parameters


const Parameters

• Parameters can be declared constant.

• Example:

void rect_calc(const double l, const double w, double & area, double & perim);


rectCalc2.cpp. . .void rect_calc(const double l, const double w, double & area, double & perim);



cout << "Area: " << rectArea << endl; cout << "Perimeter: " << rectPerim << endl;. . .}

// function definitionvoid rect_calc(const double l, const double w, double & area, double & perim){ area = l * w; perim = 2 * (l + w);}


const Parameters

• Values of const parameters cannot be modified.

• Pass by reference parameters can be declared const!!

• Why would we this? Pass a “large” object by reference (to save memory) but ensure it is not modified, i.e., pass a C++ string.


Passing a const string#include <string>...void read_input(const string & prompt, double & l, double & w){ cout << prompt;

cin >> l; cin >> w;}


rectCalc3.cpp...#include <string>using namespace std; // Must come before using type string

void read_input(const string & prompt, double & l, double & w);void rect_calc(const double l, const double w, double & area, double & perim);

int main(){ double rectArea(0.0), rectPerim(0.0), rectLength(2.0), rectWidth(3.0); string prompt = "\nThis program computes the area and perimeter of a rectangle\nfrom

its length and width.\nThe formula for area is (length*width).\nThe formula for perimeter is 2*(length+width).\n\nEnter length and width: ";

read_input(prompt, rectLength, rectWidth); rect_calc(rectLength, rectWidth, rectArea, rectPerim);

cout << "Area: " << rectArea << endl; cout << "Perimeter: " << rectPerim << endl; return 0;}...


> rectCalc3.exe

This program computes the area and perimeter of a rectanglefrom its length and width.The formula for area is (length*width).The formula for perimeter is 2*(length+width).

Enter length and width: 3 5Area: 15Perimeter: 16

... double rectArea(0.0), rectPerim(0.0), rectLength(2.0), rectWidth(3.0); string prompt = "\nThis program computes the area and perimeter of a

rectangle\nfrom its length and width.\nThe formula for area is (length*width).\nThe formula for perimeter is 2*(length+width).\n\nEnter length and width: ";

read_input(prompt, rectLength, rectWidth);...


const Parameters

• Can pass strings by value, but then a new copy is made of the entire string.

• Both:void read_input(const string & prompt, double & l, double & w);

and

void read_input(const string prompt, double & l, double & w);

will have the same behavior but the second will use more memory.


Passing IO Streams


writeFile4.cpp. . .int main(){ ofstream fout; // declare an output file stream string file_name("outfile.txt");

fout.open(file_name.c_str(), ios::out); // open file file_name for output

if (!fout.is_open()) // check if file is opened for output { cerr << "Unable to open file " << file_name << endl; exit(10); }

fout << “x = “ << 3 << endl;fout << “y = “ << 7.5 << endl;

fout.close();return 0;

}


Function write_data()void write_data(ostream & out, int x, double y){ out << "x = " << x << endl; out << "y = " << y << endl;}

• Note: Pass string streams by reference.


writeFunc.cppvoid write_data(ostream & out, int x, double y);

int main(){ ofstream fout; // declare an output file stream string file_name("outfile.txt");

fout.open(file_name.c_str(), ios::out); // open file file_name for output

if (!fout.is_open()) // check if file is opened for output . . .

write_data(cout, 3, 7.5);write_data(fout, 3, 7.5);

fout.close();return 0;

}


Function read_data()int read_data(istream & in){ int x;

in >> x; x = abs(x);

return(x);}

• Note: Pass string streams by reference.


readFunc.cpp. . .int read_data(istream & in);

int main(){ ifstream fin; // declare an input file stream string file_name("infile.txt"); int x,y;

fin.open(file_name.c_str(), ios::in); if (!fin.is_open()) . . . // check if file is opened for input

x = read_data(fin); cout << "Enter integer: "; y = read_data(cin);

cout << "x = " << x << endl; cout << "y = " << y << endl;

fin.close(); // close file stream fin. . .


> readFunc.exeEnter integer: -17x = 23y = 17>

…

x = read_data(fin);

cout << "Enter integer: "; y = read_data(cin);

cout << "x = " << x << endl; cout << "y = " << y << endl;…

File: infile.txt

-23

int read_data(istream & in){ int x;

in >> x; x = abs(x);

return(x);}

Function Exercises



passExample1.cpp

...// function prototypevoid f(int a, int & b);

int main(){ int x(5), y(10);

f(x,y);

cout << "x = " << x << endl; cout << "y = " << y << endl;

return 0;}

// functionvoid f(int a, int & b){ a++; b = 2*b;

cout << "a = " << a << endl; cout << "b = " << b << endl;}


passExample2.cpp

...// function prototypesvoid f(int & a);void g(int & b);

int main(){ int x(4);

f(x);

cout << "x = " << x << endl;

return 0;}

// functionvoid f(int & a){ a++; g(a); cout << "a = " << a << endl;}

// functionvoid g(int & b){ b = 2*b; cout << "b = " << b << endl;}


What is the problem?

...// function prototypesvoid f(int & a);void g(int & b);


f(x);

cout << "x = " << x << endl;

return 0;}

// functionvoid f(int & a){ a++; g(a); cout << "a = " << a << endl;}

// functionvoid g(int & b){ b = 2*b; f(b); cout << "b = " << b << endl;}


passExample3.cpp

...// function prototypevoid f(int & a);

int main(){ int a(3), x(8);

f(x);

cout << "a = " << a << endl; cout << "x = " << x << endl;

return 0;}

// functionvoid f(int & a){ int x;

a = 2*a; x = 7;

cout << "a = " << a << endl; cout << "x = " << x << endl;}


passExample4.cpp

...// function prototypevoid f(int a, int b);


f(x, x);

cout << "x = " << x << endl;

return 0;}

// functionvoid f(int a, int b){ a++; b = 2*b;



passExample5.cpp

...// function prototypevoid f(int & a, int & b);


f(x, x);

cout << "x = " << x << endl;

return 0;}

// functionvoid f(int & a, int & b){ a++; b = 2*b;


CSE202: Lecture 12The Ohio State University1 Function Calling.

Documents

double x2

double y1

double y2

function parameters

value of parameters

value slide

double dx

x1 value py