Hunter Liu's Website

2.1. Practise Problems

≪ 2. Week 1 Thursday: If Statements and Conditionals | Table of Contents | 3. Week 2 Tuesday: Practise with If Statements ≫

It’s common for exam questions about predicting output to have obfuscated code, i.e. code where variable names are intentionally misleading or nonsensical, or where strange indentation, bracketing, and spacing can make the code appear misleading. Be very careful when reading the code, and keep close track of each variable. It may be helpful to step through the code line-by-line with pencil and paper. For these problems, if you get stuck, you can put the code into an IDE to see their output.

1. A Sign

Write a C++ program that asks a user for an integer, then tells the user if it is positive, negative, or neither.

Sample runs:
OUTPUT:  Enter a number.
INPUT:   -7
OUTPUT:  -7 is a negative number.

OUTPUT:  Enter a number.
INPUT:   8
OUTPUT:  8 is a positive number.

OUTPUT:  Enter a number.
INPUT:   0
OUTPUT:  0 is neither positive nor negative.
Solution

In English, you may describe a solution as follows:

  1. Print “Enter a number.”
  2. Store the provided input in a variable x
  3. If x is negative, print “x is a negative number.”
  4. If x is positive, print “x is a positive number.”
  5. If neither (3) nor (4) are true, print “x is neither positive or negative.”

The above is called pseudocode. We can provide a one-to-one translation of the pseudocode as follows:

 1#include <iostream>     // needed for cout, cin, and endl 
 2
 3using namespaace std; 
 4
 5int main() {
 6    // Step 1 
 7    cout << "Enter a number." << endl; 
 8
 9    // Step 2 
10    int x; 
11    cin >> x; 
12
13    // Step 3 
14    if(x < 0) {
15        cout << x << " is a negative number." << endl; 
16    }
17
18    // Step 4 
19    else if(x > 0) {
20        cout << x << " is a positive number." << endl; 
21    } 
22
23    // Step 5 
24    else {
25        cout << x << " is neither positive nor negative." << endl; 
26    }
27
28    return 0; 
29}

2. Big Brother

What is the output of the following code for these inputs?

 1#include <iostream>
 2
 3using namespace std;
 4
 5int main() {
 6    int big_brother;
 7    cin >> big_brother;
 8
 9    if(big_brother > 50) {
10        cout << "I like dogs." << endl;
11    } else if(big_brother == 5 || big_brother <= 0) {
12        cout << "I like cats." << endl;
13    } else {
14        cout << "I despise animals." << endl;
15    }
16
17    return 0;
18}

3. Rainy Weather

What is the output of the following code for the following inputs?

 1#include <iostream>
 2
 3using namespace std;
 4
 5int main() {
 6    int a;
 7    cin >> a;
 8
 9    if(a > 10)
10        cout << "I can see storm clouds." << endl;
11        a = a - 10;
12
13    if(a < 0)
14        cout << "It looks like it will rain." << endl;
15
16    return 0;
17}

4. Soggy Sandwiches

What is the output of the following code?

 1#include <iostream>
 2
 3using namespace std;
 4
 5int main() {
 6    int a = 15;
 7    float b = 4.6;
 8    double c = -9.7;
 9    int d = 100;
10    float e = 2.718;
11    double f = 4.178e18;    // scientific notation - 4.178 * 10^18
12
13    if(a == 10 || b > 3 && c < 0 || d * e < f && f < 0 && e > 0) {
14        cout << "I love C++ so much, I want to marry C++." << endl;
15    } else {
16        cout << "C++ is as pleasant as a soggy sandwich." << endl;
17    }
18
19    return 0;
20}
Solution

This amounts to checking what the condition on line 13 is, i.e. if it’s true or false. We can work out the individual conditions without any problem: a == 10 is false, b > 3 is true, c < 0 is true, d * e < f is false, f < 0 is false, and e > 0 is true. So, the expression becomes:

false || true && true || false && false && true

Remember that && is always computed before || (unless parentheses are present). true && true is true, while false && false && true is false. Thus, this becomes false || true || false Since at least one of these is true, the full expression is true. Thus, the code on line 14 will be run, and the output will be I love C++ so much, I want to marry C++.