if/else statements without comparison operators and curly braces
About the missing conditions, e.g.:

if (pressed[i])


If is supposed to be a boolean, this is the prefered way, as long as the boolean has value zero (0) when false and nonzero when true. It is the same as writing:

if (pressed[i] != 0)


It is not usual to do this for integers, only for booleans.

About the brackets: If you have only one statement the brackets can be omitted. But it is quite a bad programming practice, since sooner or later you add a statement, without the brackets added and you have a bug introduced.

If you really want to shorten it than instead of

if (justpressed[i])
  digitalWrite(led, HIGH);
else
  digitalWrite(led, LOW);


use

digitalWrite(justpressed[i] ? HIGH : LOW);


This is called the Ternary operator.

update

Using the remark of wondra but with the ternary operation your code can be

for (int i = 0; i &lt; 15; i++)
{
    lcd.setCursor(i, 1);
    lcd.print   (     pressed    [i] ? "X"  : " ");
    digitalWrite(led, justpressed[i] ? HIGH : LOW);
}