Class 3: C++ part I

C-inherited syntax

• C++ has become very complicated (or "rich"), but it's based on C.
• C and C++ have a "nested" syntax structure.
• At the top level, C has two main possibilities:
  • Define a function, or
  • Declare something (say what it is):
    • a new "user-defined" type
    • a function (without defining it)
    • a global variable (but avoid that)
  • Within a function definition, other declarations can occur. (That's the nesting.)
• C++ has a few other options, such as the using namespace statement.

Function definitions

Basic function definitions consist of

function_type function_name ( argument_list ) { function_body }

where function_type is the type of the functions return value (like int or double), and argument_list is a comma-separated list of variable declarations.

For example:

double FahrenheitToCelsius(double T_degF)
{
  double T_degC;
  T_degC = (T_degF - 32.0)/1.8;
  return T_degC;
}

Pass-by-value vs. pass-by-reference

Parameters (aka arguments) in C are generally [1] "passed by value": the value is copied into a new variable to be used as the function argument.

• Changing the value in the function doesn't change any values in the calling program.

C++ also allows "pass by reference":

• The caller must provide a variable as the argument. It doesn't have to have the same name as the argument.
• Changes to the argument in the function change the value of the variable in the calling program.
• Internally, the called function is really being given the address of the variable in the calling function.

Exercise 1: pass by value vs. pass by reference

Compare these two function definitions:

double FahrenheitToCelsius1(double T)   // pass by value
{
  T = (T - 32.0)/1.8;  // reusing T -- bad practice!
  return T;
}

double FahrenheitToCelsius2(double & T)  // pass by reference
{
  T = (T - 32.0)/1.8;
  return T;
}

Try them out and see what they do. (Write a main() for this.) Can you call FahrenheitToCelsius1(32.0)? Can you call FahrenheitToCelsius2(32.0)?

Moral of Exercise 1

Be careful to remember which function arguments are pass-by-value and which are pass-by-reference.

Scope of variable definitions

A variable defined inside the body of one function is completely different from a variable of the same name defined in the body of another function:

double FahrenheitToCelsius3(double T_degF)
{
  double T_degC;
  T_degC = (T_degF-32.0)*5.0/9.0;
  return T_degC;
}

double FahrenheitToCelsius4(double T_degF)
{
  double T_degC;
  T_degC = (T_degF+40.0)/1.8-40.0;
  return T_degC;
}

Break

Flow control

• Conditional execution (branching): if and if-else
• Looping: while, do-while, for
• Loop shortcuts: continue and break

basic if

if (expr) {
   statements;
}

Does statements if expr is non-zero, where expr can be any expression.

Example:

if ( x > 100 && u > 0.0 ) {
   u= -u;
   x= 100;
}

basic if/else

if (expr) {
    statements;
}
else {
    statements;
}

Does first block if expr is non-zero, otherwise does second.

if / else if / ... / else

if (expr) {
    statements;
}
else if (expr2) {
    statements;
}
else if ...

and so on and so forth, optionally ending in:

else {
    statements;
}

looping: while and do

 while (expr) {
    statements;
}

do {
    statements;
} while (expr);

loop shortcuts: break, and continue

• break will jump out of a loop, continuing after the end of the loop
• continue will jump to the test and loop again if non-zero.

looping: for

for (expr1; expr2; expr3) {
  statements;
}

expr1;
while (expr2) {
  statements;
  expr3;
}

The one way a for is different from a while is that a continue inside the for loop will execute expr3 before going back to the beginning.

Example: print an asterisk in column n

void printStarAt(int n)
{
  using namespace std;
  int i;
  for (i=0; i<n; i=i+1)
    cout << ' ';
  cout << "*\n";
}

Exercise 2: 1-d bouncing "ball" (asterisk)

Goal:

• bounce a ball between two walls
• let x be the position of the ball
• let v be its velocity (+1 moving to the right, -1 to the left)
• start with x=0, v=+1
• reverse direction if it hits a wall at x=79 or x=0
• "animate" on terminal using the printStar() function

Work together to define the algorithm, and I'll code it according to your directions.

Assignment: "Adventure"

Make a simple interactive "text adventure game" that repeatedly asks user for "commands" and then gives responses.

Simple example: http://www-personal.ksu.edu/~gahs/john/adventure0.html

• Note: this was written by my 8-year-old son, with a little help from me.
• Uses JavaScript, but only works with Firefox, not Internet Explorer.

Classic example: http://www.ifiction.org/games/play.phpz?cat=&game=1&mode=html

• This is Will Crowther's 1973 version.

You don't have to set up a really big adventure, just implement a few commands in a loop and some reasonable responses. Have fun.