Input and Output

Author: Muhammad Hasan (IF 2018)

Understanding input and output in CP is really important, especially when there are a lot of inputs, and also some unusual inputs. In this section, we will discuss how to do proper basic input and output in C++ for CP.

Basic inputs

There are usually two ways to take input, the first one is to use scanf and the second is to use cin. The former is usually faster, but a little bit slower to code, and you have to know what type of input you want to take. Here is an example of inputs using scanf:

int n;
long long m;
string s;

scanf("%d", &n);
scanf("%lld", &m);
scanf("%s", &s);

// you could also do scanf("%d %lld %s", &n, &m, &s);

The second way of taking input is to use cin, which is faster to type, and you don’t need to know what type of input you want to take. But, it is slower than scanf in taking inputs. To solve this, you need the first 3 lines in the template explained before (especially the second line). With those 3 lines, cin is now faster. Here is an example of inputs using cin:

int n; // integer
long long m; // long integer
string s; //

cin >> n >> m >> s;

// you could also do it separately
// cin >> n;
// cin >> m;
// cin >> s;

And then, sometimes you need to get a line of input. You can do that input like this:

string str;
getline(cin, str); // gets all the line in the input

stringstream ss(str);
string token;
while (ss >> token) // useful to take tokens in the line of the input
    cout << token << '\n';

Also, there is some problem which doesn't tell you the specific number of inputs. So you can do your inputs like this:

some_type inp;
while (cin >> inp) {
    solve();
}

To test this efficiently, you need to make the .exe of your program, which you can do like this:

g++ -o name_of_program name_of_program.cpp

And then you can make an input.in file with inputs written on it, and then run the program in terminal like this:

./name_of_program.exe < input.in

Basic Outputs

There are also two common ways to output in C++. The first one is to use printf and the second one is to use cout. This is similar to the two common ways of taking inputs. Here is an example using both of them:

int n = 1;
long long m = 5
string s = "foobar";

printf("%d %lld %s\n", n, m, s); // prints 1 5 foobar

cout << n << " " << m << " " << s << " " << '\n'; // prints 1 5 foobar

For decimal number, you can output it like this:

// prints with 9 decimal digit
double num;
printf("%.9f", num);
cout << fixed << setprecision(9) << num << '\n';

Remember to include the first 3 lines in the template to speed up output using cout.

Common Mistakes

Now, there is a common mistake which beginners usually made. When printing an endline, don’t use endl, and always use '\n', because it is faster and doesn't flush the output. An exception of this would be in interactive problem, and so you can use endl.

Here is an illustration:

cout << answer << endl; // bad
cout << answer << '\n'; // good

Also, when there are many test cases you can output it directly without having to seperately store it first, and then output it in the end.

// Good Example
for (auto test : testcases) {
    cout << result << '\n';
}

// Bad Example
for (auto test : testcases)
    answer[test] = result;

for (auto test : testcases)
    cout << answer[test] << '\n';