Functions

• Why we use functions

• C library functions

• Creating our own functions

Why functions

• average human mind can hold 6 things at once. (directions example)

• Construct a large program from smaller pieces or modules

• in C++ the pieces are called functions

• final program made up of library pieces plus pieces you write.

C Library Functions• mathematical calculations sqrt, exp, log, sin, cos,

tan, pow, etc. • string manipulations strcat, strcpy, strcmp, strstr • character manipulations isalpha, isdigit, islower,

isupper, atol • input/output cout, cin, printf, fopen, fclose,

fwrite, fread • memory allocation alloc, free • searching and sorting bsearch (binary search),

qsort (quick sort) • www.cplusplus.com

Invoking a library function

Functions are called by writing the name of the function, followed by a left parentheses, followed by the argument list,followed by the right parentheses.

For example: cout << sqrt (900.0);

This code displays 30

Invoking a library function#include <iostream> #include <cmath>using namespace std;

int main(){ for (double i = 1; i <= 10; i++) { cout << "The square root of " << i << " is "; cout << sqrt (i) << endl; }

return 0;}

Invoking a library function#include <iostream> #include <cmath>using namespace std;

int main(){ double squareRoot; for (double i = 1; i <= 10; i++) { cout << "The square root of " << i << " is "; squareRoot = sqrt (i) cout << squareRoot << endl; } return 0;}

Functions as black boxes

sqrt

y square root of y 

    

double double

Function arguments• constants

double rootD1;

rootd1 = sqrt (900.0) ;

• variables double rootD1, d1 = 900.0;

rootd1 = sqrt (d1) ;

• expressionsdouble rootD1, d1 = 900.0;

rootd1 = sqrt (d1 + 20) ;

More than one argument

#include <cmath>

  for (double i = 1; i <= 10; i++) { cout << i << " to the power of 10 is "; cout << pow (i, 10) << endl; }

pow

x (double)y (double)

x to the power y(double)

Common programming errorusing a library function without reading the whole description

For example: the pow library function description says “ pow does not recognize integral floating point values greater than 264, such as 1.0E100” A library function will explain it’s deficiences and limitations.

Make sure you know what they are before using it!

Exercise - use abort instead #include <fstream> // provides ifstream, ofstream#include <iostream> // provides cout

using namespace std;

int main(){ ifstream infile; infile.open("yards.in");

if (!infile) { cout << "Unable to open input file" << endl; cout << "Abnormal termination program" << endl; return 0; }

return 0;}

Exercise - using rand

write a code segment to generate and display 5 random numbers between 0 and 1000.

assume srand has already been called with the current time to properly initialize the random generator

function definition

return-value-type function-name (argument list)

{declarations and statements

}

Example: int Square (int y) { int result; result = y * y; return result;}

function definitionreturn-value-type function-name (argument list)

{declarations and statements

}

• function-name - any valid identifier. Our standard - verb, each word capitalized (GetInput)

• return-value-type - any valid data type, plus void void  abort ();

• argument list - comma separated list of arguments. Each must have a data type. Okay if function has no arguments. bool IsEmpty();

Example - Square functionint main () { int xSquared, int x = 10;

xSquared = Square (x); cout << “ the square of “ << x << “ is “ << xSquared; return 0;}int Square (int y) { int result; result = y * y; return result;}

What happens to xSquared, x, y, result? Walk through

variables inside a function are not visible outside the function

int main () { int xSquared; for (int x = 1; x <= 10; x++) { xSquared = Square (x); cout << xSquared << “ “ ; } cout << result; // would cause a compiler error return 0;}int Square (int y) { int result; result = y * y; return result;}

y is not visible outside the function

int main () { int xSquared; for (int x = 1; x <= 10; x++) { xSquared = Square (x); cout << xSquared << “ “ ; } cout << y; // would cause a compiler error return 0;}int Square (int y) { int result; result = y * y; return result;}

Returning control to caller - no return value

void DisplayErrorMessage(string errorMessage){ cout << errorMessage << endl;

}or void DisplayErrorMessage(string errorMessage){ cout << errorMessage << endl;

return ;}

Returning control to caller with return value

bool IsEmpty(){ if (0 == bufferCount ) return true; else return false;}

function prototype

Tells compiler:• type of data returned from function• number of arguments function expects to receive• type of arguments the function expects to receive• order in which those arguments are expected.

Example:

int Square (int y);

function prototypeCompiler must see either the function itself or the function prototype before the function is actually called in the code.

int Square (int y); int main () { int xSquared; for (int x = 1; x <= 10; x++) { xSquared = Square (x); cout << xSquared << “ “ ; } return 0;}

Function itself may be in a different file as it is with library functions

Area of a Triangle

AreaTriangle

side1

side2

side3(float)

(float)

(float)

area of thetriangle (float)

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

float AreaTriangle (float side1, float side2, float side3); // prototype

int main () { float a, b, c; // the three sides of the triangle float area; float area; cout << endl << "This program calculates the area of a triangle"; cout << endl << "with sides of length 3.0, 4.0, and 5.0" << endl;

a = 3.0; b = 4.0; c = 5.0; area = AreaTriangle(a, b, c); cout << endl << "The area of the triangle is " << area << endl; return 0;}

/* * PRE: side1, side2, and side3 are positive numbers that * form the sides of a triangle * POST: returns the area of a triangle with sides side1, * side2, side3 */ float AreaTriangle (float side1, float side2, float side3) { float s; // local variable - the semiperimiter s = (side1 + side2 + side3) / 2.0; return (sqrt ( s * (s - side1) * (s - side2) * (s - side3) ) ); }

variables6.05.04.03.0

6.05.04.03.0

sside3side2side1areacba

          

 

s, side1, side2, side3 are variables in the AreaTriangle function

area, a, b and c are variables in the main program

At runtime a copy of a, b, c is made and used to initialize side1, side2 and side3

miscellaneous

• you could skip writing a prototype all together and just put the function ahead of main. Not intuitive. Makes source code hard to read.

• you could implement AreaTriangle initially as a stub with no other code except return 0;

1) Write a function named Smallest that takes three integer inputs and returns an integer that is the smallest of the three inputs. Write the prototype for the Smallest function. Write a program that gets 3 integers from a user and displays the smallest

2) Write a function that, given a letter of the alphabet, returns true if the letter is a vowel (lower or uppercase) and returns false if the letter is not a vowel.

IsAVowel true if letter is a vowelfalse if letter is not a vowel

letter (char)

3) Write a program to invoke the IsAVowel function. Inputs a letter and prints out whether it is or is not a vowel.

Automatic conversionsWhat if the parameter you want to send is different than that expected by the function?

double Square (double y); int main () { double xSquared; for (int x = 1; x <= 10; x++) { xSquared = Square (x); cout << xSquared << “ “ ; } return 0;}

x converted to double. Works fine.

Automatic conversionsWhat if the parameter you want to send is different than that expected by the function?

int Square (int y); int main () { double d1 = 9.8; cout << Square (d1); // displays 81 return 0; return 0;}

d1 converted to int. Information is lost. Beware!

Promotion rules

• Specify which types can be converted to other types without losing data.

• As long as you follow the promotion rules then conversions are okay.

• Must promote to a data type higher in the hierarchy

Promotion hierarchy

long doubledoublefloatunsigned long intlong intunsigned intintunsigned short intshort intunsigned charchar

Common programming error

• Losing data by allowing the compiler to change a data type and not follow the promotion rules.

Exercises1) Find the error in each of the following program segments and explain how to fix it. a) int sum (int x, int y) {

int result; result = x + y;}

b) int sum (int n) { if (0 == n) return 0; else n = n + n;}

c) in main program: double x = 1E10; cout << "square of 1E10 = " << square (x) << endl; int square (int x) { return x * x; }