To make it easier to do some of the other experiments, we'll take a look here at input expressions.
Consider this statement:
cin >> 3;What do you suppose this would mean? Maybe after this executes, every
3 the program encounters will be replaced with the
value the user enters from the keyboard? But this seems quite absurd
and perhaps even dangerous. So, a better question: does C++ even allow
this? Try it. Add the input statement to your program and
recompile.
Question #3.2.1: What is the first compiler error that you get?
Fortunately, C++ doesn't let us do something as undesirable as changing
the value of a constant! Instead, all of the objects in an input expression
must be variables. So comment out the last incorrect input statement by
adding // at the beginning of that line.
Now, let's try this: Add the statement
cin >> i >> j;after the declaration of
i and
j but before the output statement. When you execute your
program, enter these values:
123 456
Question #3.2.2: What values are printed foriandj? Did these values come from their declarations or from your keyboard input?
An input statement replaces the value of a variable used in the input statement. The variable does not remember its old values; that value is gone.
Now try this variation:
Question #3.2.3: Move the input line before the declaration ofiandj. Now what happens?
That's actually a review of the previous lab. Remember that you can't use a variable unless you declare it first. The order of the statements does matter.
Here's a variation that will compile and execute:
Question #3.2.4: Move the input line after the output statement. Whap happens now?
Question #3.2.5: How does this demonstrate that an input statement replaces the values in its variables?
Question #3.2.6: Is it necessary to initializeiandjin their declaration if we read in values for them in the very next statement?
NOTE: Be sure to move the input statement back between the declaration and output statements.