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

pi

if block scope

pi

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

Programming with C++, Spring 2020, Lecture #8

By Aleksey Gurtovoy

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Programming with C++, Spring 2020, Lecture #8

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

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