Input/Output Tutorial

In our first meeting, we introduced you to the format of the CCC — given a problem, you are tasked with writing a program that solves it for some input. We then looked at some past problems and worked through them by hand.

Now, let's see how to convert the process you used to solve a problem with pen and paper into a program you can submit on the CCC.

CCC Input/Output Format

One rather intriguing feature of the CCC is that your solutions are autograded in real time. That is, you can submit a solution to the CCC grader and a couple seconds later you can see the score it received (i.e., whether your program worked.) Further, if your solution turns out to be incorrect, you can edit it and submit again, to a maximum of fifty attempts.

To facilitate autograding, however, the CCC expects your program to accept input and produce output in a specific way. In particular, your program should read from standard input (stdin) and output to standard output (stdout.) Colloquially, your program needs to take input from the console and print output to the console as well.

The way to do this will differ depending on the language you are using and what form the input takes (is it a number? a string? multiple numbers? etc.) Sample code in Python, Java, and C++ follows.

Introductory Example — CCC '20 J1

The most common kind of input in the CCC is numerical — you are given some numbers as input and are to determine the corresponding output. For instance, consider CCC '20 J1 — Dog Treats. The premise of this problem is that you have a dog whose happiness depends on the number of small, medium, and large treats he receives in a day; you are tasked with determining whether the dog is happy or sad.

Your program should, therefore, read three numbers from the console, representing the amount of treats of each type. More precisely, the input specification reads:

There are three lines of input. Each line contains a non-negative integer less than $10$. The first line contains the number of small treats, $S$, the second line contains the number of medium treats, $M$, and the third line contains the number of large treats, $L$, that Barley receives in a day.

Worked solutions in Python, Java, and C++ are provided below.

Python

In Python, use the input function to get one line of input from the console; use print for output. Note, however, that since input returns a string, we will need to convert it to an integer ourselves using the int constructor like such:

small_treats = int(input())
medium_treats = int(input())
large_treats = int(input())

From there, we can finish solving the problem using the formula given and a conditional (if) construct.

small_treats = int(input())
medium_treats = int(input())
large_treats = int(input())

happiness_score = 1 * small_treats + 2 * medium_treats + 3 * large_treats
if happiness_score >= 10:
    print("happy")
else:
    print("sad")

Java

In Java, use java.util.Scanner to read input from the console; use System.out.println for output. In this case, your input-reading logic might look like:

// Assuming java.util.Scanner is imported...
Scanner scanner = new Scanner(System.in);
int smallTreats = scanner.nextInt();
int mediumTreats = scanner.nextInt();
int largeTreats = scanner.nextInt();

From there, we can finish solving the problem using the formula given and a conditional construct.

Scanner scanner = new Scanner(System.in);
int smallTreats = scanner.nextInt();
int mediumTreats = scanner.nextInt();
int largeTreats = scanner.nextInt();

int happinessScore = 1 * smallTreats + 2 * mediumTreats + 3 * largeTreats;
System.out.println(happinessScore >= 10 ? "happy" : "sad");

C++

In C++, use std::cin to read input from the console; use std::cout for output. In this case, your input-reading logic might look like:

// Assuming std::cin and std::cout are in scope...
int small_treats{}, medium_treats{}, large_treats{};
std::cin >> small_treats >> medium_treats >> large_treats;

From there, we can finish solving the problem using the formula given and a conditional construct.

int small_treats{}, medium_treats{}, large_treats{};
std::cin >> small_treats >> medium_treats >> large_treats;

int happiness_score = 1 * small_treats + 2 * medium_treats + 3 * large_treats;
std::cout << (happiness_score >= 10 ? "happy" : "sad") << '\n';

Suggested Practice

Practice Problems

Now that you know how to take numerical input, try solving some of the following problems (arranged in roughly increasing difficulty):

• CCC '21 J1 — Boiling Water
• CCC '19 J1 — Winning Score
• CCC '15 J1 — Special Day
• CCC '17 J2 — Shifty Sum
• CCC '20 J2 — Epidemiology
• CCC '17 J3 — Exactly Electrical

Reading a Variable Number of Inputs — CCC '22 J2

Sometimes the amount of input for a problem will not be fixed. To understand what is meant by this, consider CCC '22 J2 — Fergusonball Ratings. The premise of this problem is that you have some information about all the players in a sports team; you are tasked with finding the star players. However, the number of players may vary from case to case; the input specification reads:

The first line of input consists of a positive integer $N$ representing the total number of players on the team. This is followed by a pair of consecutive lines for each player. The first line in a pair is the number of points that the player scored. The second line in a pair is the number of fouls that the player committed. Both the number of points and the number of fouls, are non-negative integers.

To account for this, your program should therefore read input in a loop, stopping only after you have gathered information about all players on the team. Sample code in Python, Java, and C++ is provided below.

Python

First, read the number of players on the team $N$; this will determine how many times we need to loop.

num_players = int(input())

Now, for each player, read the number of points they scored and the number of fouls committed using a loop:

num_players = int(input())

for _ in range(num_players):
    points_scored = int(input())
    foul_count = int(input())

Your turn! Try to complete the solution above.

Java

First, read the number of players on the team $N$; this will determine how many times we need to loop.

Scanner scanner = new Scanner(System.in);
int playerCount = scanner.nextInt();

Now, for each player, read the number of points they scored and the number of fouls committed using a loop:

Scaner scanner = new Scanner(System.in);
int playerCount = scanner.nextInt();

for (int i = 0; i < playerCount; i++) {
    int pointsScored = scanner.nextInt();
    int foulCount = scanner.nextInt();
}

Your turn! Try to complete the solution above.

C++

First, read the number of players on the team $N$; this will determine how many times we need to loop.

int player_count{};
std::cin >> player_count;

Now, for each player, read the number of points they scored and the number of fouls committed using a loop:

int player_count{};
std::cin >> player_count;

for (int i = 0; i < player_count; i++) {
    int points_scored{}, foul_count{};
    std::cin >> points_scored >> foul_count;
}

Your turn! Try to complete the solution above.

Suggested Practice

Practice Problems

Now that you know how to take input in a loop, try solving some of the following problems (arranged in roughly increasing difficulty):

• CCC '21 J2 — Silent Auction
• CCC '20 J3 — Art
• CCC '19 J2 — Time to Decompress
• CCC '19 J3 — Cold Compress
• CCC '13 S2 — Bridge Transport

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Great job, you've reached this end of this tutorial! Here's a challenge problem to try if you're done all the suggested practice above: CCC '21 S1 — Crazy Fencing.