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How To Think Like a Computer Scientist C++ Edition The Pretext Interactive Version

Section 8.3 Accessing instance variables

You can read the values of an instance variable using the same syntax we used to write them:
int x = blank.x;
The expression blank.x means “go to the object named blank and get the value of x.” In this case we assign that value to a local variable named x. Notice that there is no conflict between the local variable named x and the instance variable named x. The purpose of dot notation is to identify which variable you are referring to unambiguously.
You can use dot notation as part of any C++ expression, so the following are legal.
cout << blank.x << ", " << blank.y << endl;
double distance = sqrt(blank.x * blank.x + blank.y * blank.y);
Listing 8.3.1. In this active code, we access the instance variables of Point object black using dot notation and output their values. Next, we output the distance from the origin.

Checkpoint 8.3.1.

In string x = thing.cube;, what is the object and what is the instance variable we are reading the value of?
  • string is the instance variable, cube is the object
  • string is a data type.
  • x is the instance variable, thing is the object
  • x is the local variable.
  • thing is the instance variable, cube is the object
  • Consider the placement of thing – it is before the .
  • cube is the instance variable, thing is the object
  • Yes, we access the instance variable cube of the object thing using the dot operator.
  • cube is the instance variable, string is the object
  • string is a data type.

Checkpoint 8.3.2.

What will print?
struct Blue {
  double x, y;
};

int main() {
  Blue blank;
  blank.x = 7.0;
  blank.y = 2.0;
  cout << blank.y << blank.x;
  double distance = blank.x * blank.x + blank.y * blank.y;
  cout << distance << endl;
}
  • 2.0 7.0 53
  • Spaces need to be printed out like any other output.
  • 2753
  • There are no spaces in the correct output.
  • 7253
  • The order in which the variables are printed out do not need to match the order in which they are declared.
  • 7.02.053
  • The order in which the variables are printed out do not need to match the order in which they are declared.

Checkpoint 8.3.3.

You want to go to the object named circle and get the integer value of y, then assign it to the local variable x. How would you do that?
  • int y = circle.x();
  • No parentheses are needed.
  • int circle = x.y;
  • You should be assigning to the local variable x.
  • int y = circle.x;
  • You should be assigning to the local variable x.
  • int x = circle.y;
  • This is the correct way to assign the value of y to x.
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