The University of Iowa
The College of Liberal Arts and Sciences
Department of Computer Science

Programming Languages and Tools:

CS:3210:0001

Lecture/Lab #8

Programming with C++

Scope of a name, user-defined types (struct)

Warm-up

What's an idiomatic way to increment an integer in C++?

Is this valid C++? If so, what will be the output of this snippet when x is equal to 17?

if ( x >= 0 )
   if ( x < 10 ) 
       std::cout << "x is in the [0, 10) interval\n";
else
   std::cout << "x is negative\n";

Is this valid C++? If so, what does it do?

for ( ; ; )
    std::cout << "Hi there...\n";

Warm-up (cont.)

What does the following function do?

void foo( int n )
{
    switch ( n )
    {
        case 9: std::cout << "9 ";
        case 8: std::cout << "8 ";
        case 7: std::cout << "7 ";
        case 6: std::cout << "6 ";
        case 5: std::cout << "5 ";
        case 4: std::cout << "4 ";
        case 3: std::cout << "3 ";
        case 2: std::cout << "2 ";
        case 1: std::cout << "1 ";
        case 0: std::cout << "0 ";
    }

    std::cout << "the end\n";
}

Scope of a name

A scope of a name is a part of the program in which a name has a particular meaning.

The same name can refer to different entities in different scopes.

Most scopes in C++ are delimited by curly braces.

Names are visible from the point where they are declared until the end of the scope.

Scopes can be nested; The contained (or nested) scope is referred to as an inner scope, the containing scope is the outer scope.

void foo() { n = 42; }

int main() {
    int n = 0;
    foo();
    std::cout << n << "\n";
}

def foo():
    n = 42

def main():
    n = 0
    foo()
    print(n)

main()

Scopes (cont.)

https://repl.it/@agurtovoy/lab8-scope

#include <iostream>

int sum = 0;

void compute( int x, int y ) {
    sum = x + y;
}

int main() {
    int sum = -1;
    compute( 2, 2 );

    float pi = 3.14;
    std::cout << pi << "\n";

    if ( sum < 0 ) {
        double pi = 3.14159;
        std::cout << pi << "\n";        
    }
    
    std::cout << sum << "\n";
}

Global aka namespace scope

sum

compute

main

main function scope

sum

if block scope

Exercise 1

Use your understanding of variable scopes and programming best practices to correct the following program

https://repl.it/@agurtovoy/lab8-exercise1

Open the exercise template

Write your code, press Run to test

When you're done, grab your Repl's link and send it as a direct message to me (agurtovoy)

Click on the corresponding option in the "Lab8 exercises" poll in #general

User-defined types

Built-in types are deliberately low-level; they directly and efficiently reflect the capabilities of conventional computer hardware.

C++ is one of the few mainstream languages that let us design and implement our own higher-level types that are efficient, easy to use, and are often indistinguishable from built-in language facilities.

Types built out of other types using C++’s abstraction mechanisms are called user-defined types.

char, int, double, unsigned long long

string("hello, ") + name

struct

Defining a basic data structure type

#include <iostream>

void draw_line( int x1, int y1, int x2, int y2 )
{
    std::cout 
        << "draw_line: (" 
        << x1 << ", " << y1 << ")-(" 
        << x2 << ", " << y2 << ")\n";
}

void draw_rect( int x1, int y1, int x2, int y2 );
void draw_triangle( int x1, int y1, int x2, int y2, 
                    int x3, int y3 );

int main() {
    draw_line( 0, 0, 10, 10 );
    draw_line( 10, 0, 10, 0 );
}

https://repl.it/@agurtovoy/lab8-struct

struct (cont.)

struct point
{
    point( int x, int y )
        : x( x ), y( y )
    {}
    
    int x;
    int y;
};

point p( 1, -1 );
std::cout << p.x + p.y << "\n";

class point():
    def __init__( self, x, y ):
        self.x = x
        self.y = y

p = point( 1, -1 )
print( p.x + p.y )

Constructor is a function defined within the struct scope that has the same name as the struct

Data members have to be declared explicitly

Data members are initialized in constructor using member initialization list

A new object can be creating by calling its constructor

Data members are accessed using the standard dot notation

Exercise 2

Write a function that adds two points and a function that compares two points for equality

https://repl.it/@agurtovoy/lab8-exercise2

Open the exercise template

Write your code, press Run to test

When you're done, grab your Repl's link and send it as a direct message to me (agurtovoy)

Click on the corresponding option in the "Lab8 exercises" poll in #general