I/O in contests

In competitions, the input for your code has to read from standard input (most of the time) and you print the output to standout output.

In C++, two most common choices to do is

• iosteams - cin / cout (NEW)
• stdio - scanf / printf (C style)

Though it seems inconsequential, handling I/O correctly can make your solution milliseconds or even a few seconds. This becomes relevant when the input is huge (for example, reading $10^6$ integers).

Before moving on to I/O optimization, let’s quickly see how to use each I/O function.

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Stdio

Include statement: #include <cstdio>

scanf and printf need to know the type of the variable you are reading/printing, denoted by using a format specifier (%).

Here are the most common ones, for a complete list you can go here

scanf("%d", &x); // read int
scanf("%lld", &x); // read long long int
scanf("%s", &s); // read string
scanf("%f", &d); // read float

Similarly,

printf("%d", x); // print int
printf("%lld", x); // print long long int
printf("%s", s); // print string
printf("%f", d); // print float

Two space separated integers can also be read in a single scanf statement

scanf("%d %d", &a, &b);

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IOStream

Include statement: #include <iostream>

Unlike cstdio functions, it’s not required to define the type of the variable. I/O becomes much simpler as x can be any data type here.

cin>>x; // read x
cout<<x; // print x

Synchronization

In a single program, scanf and cin can be interleaved to read from input because synchronization is on by default.

Without going too much into the language architecture, this synchronization comes at a cost. Turning this off will cause unexpected bugs when scanf and cin and used together for the same stream but will speed up cin.

Add this statement at the start of the program:

ios_base::sync_with_stdio(false);

Note: With synchronization turned off, only use cin to read input.

Another option is to not use iostreams at all. scanf is fast enough and should almost never cause issues.

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We’ll kick off the next chapter with algorithm analysis.