Built-in functions.

Prototype of the islower function: Prototype shows number and types of the arguments and the return type. Prototypes of the built-in functions are in the included header files. 
bool islower(char)
To use/invoke/call the function (correct syntax).
One argument (or parameter). Type is char.
Return value (evaluates to) a bool. Precedes name of function.
A function can return only one value; it thus has only one return type. 

char c;
cin >> c;
// call the function, pass it the value of variable c.  
//It returns false or true, used here as condition of the if
if (islower(c))      
  cout << "It's a lowercase letter";
else
  cout << "It's not a lowercase letter";

-------------------

do {
  cin >> c;
} while (!islower(c));    //return value is logically negated.

-------------------

// unlikely code, but illustrates function's 
//return value; what it evaluates to
cout << islower(c);    // 0 or 1 output (bool converted to 0 or 1)

int i;
i = islower(c) * 3;		//i will be 0 or 3

// character constant argument, also unlikely code since know the g is lowercase
cout << islower('g');  // output 1
cout << islower('$');  // output 0


**********************

double sqrt (double)   One arg (double), return value is double.

cin >> f;
cout << sqrt(f);  	// f unchanged
cout << sqrt(f) / 2;	// half of sqrt of f
cout << sqrt(f/2);	// sqrt of half f
cout << sqrt(sqrt(f)); 	// fourth root of f
cout << sqrt(2);        // arg is int, but compiler converts to double.  
                        //cmath has prototype which informs compiler 
                         about args and return type
y = sqrt(x);   // x unchanged
x = sqrt(x);   //x is now the sqrt of what it was



double pow (double,double)

  Two args: first is double, second is double.
Returns double: the first arg raised to the second arg power.

cout << pow(x,y);    // x to the y power
cout << pow(2,y);    // 2 raised to the y power
cout << pow(x,0.5);  // sqrt(x) 
cout << pow(sqrt(x),y);    //some math
cout << pow(x,3);      //x cubed
cout << pow(x,1.0/3);  //cube root of x

********************************************

char toupper (char)

One arg,:char. Returns char: the uppercase version of the arg. The arg
itself is not changed.

char c1, c2;
cin >> c1;
cout << toupper(c1);   // c1 unchanged
cout << toupper('a');  // A output
c2 = toupper(c1);
cout << c2;
c1 = toupper(c1);    // c1 now uppercase version of what it was

Next (making functions)

©David Wills