How to navigate in an Array of strings left and right - arrays

Question

I have an array of strings
var characters = ["Jim", "Anna", "Nick", "Chris", "Peter"];
my initial position is Jim[0]. if i type right i want to go to the next one
In this case to "Anna". I can do this by adding one to the current index.
The thing is getting more tricky when i type left from the initial position?
So how can i go from Jim[0] -> Peter[4]?
Something that works is
Jim[0] - 1 + characters.length which equals to 0 - 1 + 5 = 4
and this is Peter. But this seems incorrect because i cannot do that in the case that the current value is Anna[1] and i want to go to Jim[0]. As 1 - 1 + 5 = 5

Answer 1

Something like this would work.
var characters = ["Jim", "Anna", "Nick", "Chris", "Peter"];
var index = 0; // Currently at Jim.
var direction = -1; // 1 for right, -1 for left.
// What's left of our current position?
characters[(index + characters.length + direction) % characters.length]; // Peter!

Answer 2

You could do a conditional. Keep your index in a variable instead of hard-coding it. This will allow you to do something similar to the pseudocode following (Sorry, wasn't sure which language this was for):
new_index = index--
if (new_index < characters.length)
index = characters.length - 1
You can then use that determined index to access the array. You could something similar to wrap around going forwards as well.

Answer 3

There seems to be no single formula to calculate the left and right index without using conditions.
If you don't intend to use if-else or just want a single line solution, you can use the ternary operator as follows:
new_index = (direction=="left")?(index==0)?characters.length-1:index-1:(index==characters.length-1)?0:index+1
Please note that the above code assumes there are only 2 possible directions.

Answer 4

This is what you're looking for
(i % characters.length) + characters.length) % characters.length
var characters = ["Jim", "Anna", "Nick", "Chris", "Peter"];
var index;
for( var i = -10; i < 10; i++ ){
index = ((i % characters.length) + characters.length) % characters.length;
console.log(i + " > " + index + " = " + characters[index]);
}