Input/Output cheat sheet
Input in C
These examples require the following line at the top:
#include <stdio.h>

Read an integer, skipping leading whitespace.
int x;
scanf( "%d", &x );

Read integers, one at a time, skipping whitespace, until the end of input.
int x;
while( scanf( "%d", &x ) )
{
    /* Do something with x. */
}

Read a character, skipping leading whitespace.
char c;
scanf( "%c", &c );

Read the next character, whatever it is. Returns -1 when there is nothing left to read.
int c = getchar();

Read characters one by one until the end of input.
int c;
while( ( c = getchar() ) >= 0 )
{
    char ch = ( char )c;
    /* Do something with ch. */
}

Read a word, skipping leading whitespace. Make sure w[] has enough space.
char w[1024];
scanf( "%s", w );

Read one line at a time until the end of input. This code gives a warning at compile time. Make sure s[] has enough space.
char s[1024];
while( gets( s ) )
{
    /* s[] contains the line without the '\n' at the end. */
}

Output in C
int p = 17;
long long x = 69;
printf( "Case #%d: %Ld\n", p, x );
Result: "Case #17: 69".

double x = 0.1234, y = 0.1235;
printf( "%.3f %.3f\n", x, y );
Result: "0.123 0.124".

int x = 13;
printf( "%d|%4d|%04d|%-4d|%0*d|\n", x, x, x, x, 8, x );
Result: "13| 13|0013|13 |00000013|".
Input in C++
These examples require the following lines at the top:
#include <iostream>
#include <string>
using namespace std;

Read an integer, skipping leading whitespace.
int x;
cin >> x;

Read integers, one at a time, skipping whitespace, until the end of input.
int x;
while( cin >> x )
{
    // Do something with x.
}

Read a character, skipping leading whitespace.
char c;
cin >> c;

Read a word, skipping leading whitespace.
string w;
cin >> w;

Read one line at a time until the end of input.
string s;
while( getline( cin, s ) )
{
    // s contains the line without the '\n' at the end.
}

Output in C++
int p = 17;
long long x = 69;
cout << "Case #" << p << ": " << x << endl;
Result: "Case #17: 69".

For more complicated output formatting, printf() is probably the best choice (see above).
Input in Java 1.5
These examples require the following lines at the top:
import java.io.*;
import java.util.*;
Scanner in = new Scanner( System.in );

Read an integer, skipping leading whitespace.
int x = in.nextInt();

Read integers, one at a time, skipping whitespace, until the end of input.
while( in.hasNextInt() )
{
    int x = in.nextInt();
    // Do something with x.
}

Read the next character, whatever it is. Returns -1 if there is nothing more to read.
int c = System.in.read();

Read a word, skipping leading whitespace.
String w = in.next();

Read one line at a time until the end of input.
while( in.hasNextLine() )
{
    String s = in.nextLine();
    // s contains the line, including the '\n' at the end.
}

Output in Java 1.5
int p = 17;
long x = 69;
System.out.println( "Case #" + p + ": " + x );
Result: "Case #17: 69".

double x = 0.1234, y = 0.1235;
System.out.printf( "%.3f %.3f\n", x, y );
Result: "0.123 0.124".

int x = 13;
System.out.printf( "%d|%4d|%04d|%-4d|%0*d|\n", x, x, x, x, 8, x );
Result: "13| 13|0013|13 |00000013|".