Hunter Liu's Website

1.1. Practise Problems

≪ 1. Week 1 Tuesday: Input/Output, Variables, and Errors | Table of Contents | 2. Week 1 Thursday: If Statements and Conditionals ≫

We went over two of these problems in class. I had prepared six of them, and I’ve put them all here if you need some additional practise. I’ve included the solutions that we have worked out together in class.

Each of these problems describes a task that a program needs to perform. If you are asked to write such a program, you will be provided with one or more examples of expected inputs and outputs from the program. Otherwise, you’ll be asked to either predict the program’s output or identify any (build, runtime, or logic) errors present. Fix these errors if you can!

Problem 1. Favourite Animals

Write a program that asks the user for their favourite animal. Then, print a sentence that says “I love [animal] too!”

Sample run:
OUTPUT   What is your favourite animal?
INPUT    Jellyfish
OUTPUT   I love Jellyfish too!
Solution

Here is the solution we came up with in class:

 1#include <iostream>     // needed for cout, cin, and endl 
 2#include <string>       // needed for string variables 
 3
 4using namespace std; 
 5
 6int main() {
 7    // prints the prompt for the user 
 8    cout << "What is your favourite animal?" 
 9
10    // stores user input in the string variable animal 
11    string animal; 
12    cin >> animal; 
13
14    // prints the result to the screen. 
15    cout << "I love " << animal << " too!" << endl; 
16
17    return 0;           // don't forget this! 
18}

I encourage you to try inputting Bald eagle as the favourite animal. What is the output? What does this tell you about using cin? Currently, we are not equipped to deal with this issue, but I encourage you to look up how to resolve this.

Problem 2. Squaring a Number

Write a program that takes a decimal number as input, then prints out the square of that number to the screen.

Sample run:
INPUT    -3.7
OUTPUT   13.69
Remark
Hopefully you’re reading this after solving the problem. The key detail is that you should use a float or a double to store the user’s input and the squared number, as the problem specifies using a decimal. Try a few inputs with varying levels of precision (such as 1, 1.02, 1.0002, etc.) and see how the output changes depending on which data type you used.

Problem 3. The Pythagorean Theorem

Write a program that asks the user for the length of two legs of a right triangle, then outputs the length of the hypotenuse of that triangle.

Sample run:
OUTPUT   Enter the first leg of the triangle.
INPUT    3
OUTPUT   Enter the second leg of the triangle.
INPUT    4
OUTPUT   The hypotenuse has length 5.
Hint
If you’re stuck on how to take a squareroot, you may want to look at the cmath library. Do you need to put any #include directives?

Problem 4. Finding Errors

Explain the error(s) in the following code, if there are any. What kind of errors are they?

 1#include <iostream>
 2
 3using namespace std
 4
 5int main() {
 6    // Asks user for their first name
 7    cout << "What is your first name?" << endl
 8    string first_name
 9    cin >> first_name
10
11    // asks user for last name
12    cout << "What is your last name?" << endl
13    string last_name
14    cin >> last_name
15
16    // print out the result to the screen
17    cout << "Your full name is " << first_name
18         << last_name << "." << endl
19    return 0
20}
Solution

This is the corrected code:

 1#include <iostream>
 2#include <string>
 3
 4using namespace std;
 5
 6int main() {
 7    // Asks user for their first name
 8    cout << "What is your first name?" << endl;
 9    string first_name;
10    cin >> first_name;
11
12    // asks user for last name
13    cout << "What is your last name?" << endl;
14    string last_name;
15    cin >> last_name;
16
17    // print out the result to the screen
18    cout << "Your full name is " << first_name
19         << last_name << "." << endl;
20    return 0;
21}

The first major error was that there were no semicolons; we have added some on lines 4, 8-10, 13-15, 19, and 20. Note that semicolons do not need to be present on every line of code, but rather at the end of every C++ statement. This includes declaring variables, changing the value of a variable, calling functions, returning a value, printing, or taking input. This does not include #include directives, empty lines, or int main() { ... }.

The second major error was that we needed to #include <string>: we were using string variables, and the compiler won’t know what that is until we include the right library.

Problem 5. Circular Logic

The intention of this program is to compute the area of a circle, given its radius. What are the error(s) in this program? What kind of errors are they?

 1#include <iostream>
 2
 3using namespace std;
 4
 5int main() {
 6    // get the radius of the circle from the user
 7    cout << "Please enter the radius of the circle." << endl;
 8    int radius;
 9    cin >> radius;
10
11    // compute the area with A = pi * r^2.
12    int area = 3.1415926535 * radius * radius;
13
14    // print the result to the screen.
15    cout << "The area of the circle is " << area << "." << endl;
16    return 0;
17}
Solution

If you put this code in your IDE and try to run it, it will compile and run smoothly without crashing. This should immediately indicate that there are no build errors or runtime errors. However, if you input a radius of 1, you should get an area of 3, which isn’t quite right. An input of 1.8 will also give an area of 3, which makes no sense.

The issue is that they used an integer to store both the radius and the area, causing anything after the decimal to be truncated (i.e., cut off and rounded down). The fix would be to replace every int with float or double.

Warning: on an exam, you will not have the leisure of running the code on a computer! These types of questions often assess two different skills: can you tell what the code will do, and can you identify the logical errors?

Problem 6. Bad Timing

This program intends to ask the user for some number of hours and minutes, then prints out the equivalent number of minutes. What are the error(s) in this code, if any? What kind of errors are they? How could you fix them?

Sample of expected output:
OUTPUT   Enter the number of hours.
INPUT    3
OUTPUT   Enter the number of minutes.
INPUT    30
OUTPUT   That is equal to 210 minute(s).

 1#include <iostream>
 2
 3using namespace std;
 4
 5int main() {
 6    // ask for the number of hours
 7    cout << "Enter the number of hours." << endl;
 8    int hours;
 9    cin >> hours;
10
11    // ask for the number of minutes
12    cout << "Enter the number of minutes." << endl;
13    int minutes;
14    cin >> minutes;
15
16    // compute the total number of minutes
17    int minutes = 60 * hours + minutes;
18
19    // print the result.
20    cout << "That is equal to " << minutes << " minute(s)." << endl;
21    return 0;
22}
Solution
The error is that there are two separate declarations of the variable minutes — one on line 13, which holds the user input, and one on line 17, which holds the computed result. This is a build error, as the compiler will complain about this. To fix it, one can simply remove the int on line 17, which will overwrite the user input with the total number of minutes. Alternatively, one could rename it to a different variable (say, total_minutes) and update line 20 accordingly.