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Absolute C++
These test questions are true-false, fill in the blank, multiple choice, and free form
questions that may require code. The multiple choice questions may have more than one
correct answer. You are required to mark and comment on correct answers.. Mark all of
the correct answers for full credit. The true false questions require an explanation in
addition to the true/false response, and, if false, also require a correction.
True False:
1. The if, while and for statements control only one statement.
Answer: True
Explanation: The one statement may be a block (statements that are enclosed with
curly braces { }) or a simple statement.
2. Given the declaration
int x = 0;
The following expression causes a divide by zero error:
(x !=0) || (2/x < 1);
Answer: False.
Explanation: The || operator uses short-circuit evaluation. The first member of this
expression is true; the truth value of the complete expression can be determined
from this; consequently, the second expression is not evaluated. There is no divide-
by-zero error.
3. Suppose we have these declarations,
int x = -1, y = 0, z = 1;
This Boolean expression is correct and it does what the programmer intends.
x < y < z
Answer: False
Explanation: Unfortunately, the expression compiles without error and runs. The <
operator associates (groups) left to right, so the expression evaluates as
(x < y) < z
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The left hand expression evaluates to true, which, if compared to a numeric type,
converts to 1. When compared to 1, this is false. What the programmer intends,
expressed as mathematacs might is -1 < 0< 1, a result that is clearly true.
4. You want to determine whether time has run out. The following code correctly
implements this.
!time > limit
Answer: False.
Explanation: The expression always evaluates to false. This cannot be what the
programmer intended. The compiler doesn’t catch the problem because the code is
legal, correct C++. Corrected code is !(time > limit)
Code execution proceeds as follows: The operator ! takes a bool argument. It
returns the opposite bool value. The value of time is converted to a bool. The
value of time is certainly nonzero, hence !time is !true, i.e., false. The >
compares this result with a numeric value, limit, (an int or perhaps some kind of
floating point). The value on the left (false) is converted to a 0 value of that type.
The value of limit is unlikely to be a negative number and we are concerned about
time running out, so it is unlikely that time is zero. Consequently, the inequality
becomes 0>limit, where limit is nonzero. This is false.
5. The value of count is 0; limit is 10. Evaluate:
(count == 0)&&(limit < 20)
Answer: true
6. The value of count is 0; limit is 10. Evaluate:
count == 0 && limit < 20
Answer: true
Explanation: The operators == and < have higher precedences than &&, hence the
expressions, count == 0 and limit < 10 are evaluated (to true) then the
&& is executed.
7. The value of count is 0; limit is 10. Evaluate:
(count != 0)||(limit < 20)
Answer: true.
Explanation: The first expression evaluates to false, the value of the || expression
is determined by the second expression. The second expression is true so the ||
expression evaluates to true.
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8. In a while loop, the Boolean_Expression is executed before each execution of the
loop body.
Answer: true
9. In a do-while loop, a continue statement terminates the loop.
Answer: False
Explanation: The continue statement causes the Boolean_Expression to be
executed. If true, the body executes, otherwise the loop terminates.
10. A break statement is used in loops only.
Answer: False.
Explanation: In addition to its use in loops, a break statement is used in the
switch statement to transfer control to the next statement after the switch block.
11. When a loop is nested in side another loop, a break or continue statement
terminates or restarts the outermost loop of the nested loop structure.
Answer: False
Explanation: A break or continue terminates or restarts only the innermost loop
containing the break or continue.
Free Form Questions:
1. Assume variables first and second are declared to be double and are
initialized. Write a sequence of lines of code that cause the values stored in first
and second to be exchanged if the value of first is not less than second.
Answer:
// double first, second;
// these have been initialized
if (!(first < second))
{
double temp = first;
first = second;
second = temp;
}
//assert: first <= second
2. Write multiway if-else statements for which the output is “Alarm: Boiler
Pressure: TOO HIGH” if the value of the variable boiler_pressure is
greater than 1000 (psi), and the output is “Boiler Pressure: TOO LOW” if
the value of boiler_pressure is below 100(psi), otherwise the output is
“Boiler Pressure: within normal limits.”