Select previous input jQuery

Question

I have a form with many input=text and I have neraby two image (-,+) I want to increase or decrease the previous input text. I don't want to put into the input text an id because I can have a lot of this line with input text and minus or plus. With my code the minus button works but the plus button not why?

My jQuery code

$('.minus').click(function(){
        var intRegex = /^\d+$/;
        var floatRegex = /^((\d+(\.\d *)?)|((\d*\.)?\d+))$/;

        var num = $(this).prev('input').val();
        if(intRegex.test(num) || floatRegex.test(num)) {
            if (num>0){
                $(this).prev('input').val(num-1);
            }
        }
        else{
            //non è un numero
        }

    });

    $('.plus').click(function(){
        var intRegex = /^\d+$/;
        var floatRegex = /^((\d+(\.\d *)?)|((\d*\.)?\d+))$/;

        var num = $(this).prev.prev('input').val();
        if(intRegex.test(num) || floatRegex.test(num)) {
                $(this).prev.prev('input').val(num+1);
        }
        else{
            //non è un numero
        }

    });

HTML:

<input type="text" name="qta" id="qta" style="width:50px;"/>
                            <img src="img/shop/meno.jpg" alt="" class="minus" style="cursor:pointer"/>
                            <img src="img/shop/piu.png" alt="" class="plus" style="cursor:pointer"/>

Answer 1

var num = $(this).prev().prev('input').val();

and

$(this).prev().prev('input').val(num+1);

You missed a () twice

anyway I would wrap every input and images in their own wrapper and I would find the input with $(this).parent().find('input')

Answer 2

use this to go previous

 var num = $(this).prev().prev('input').val();

if you want to go next use this

 var num = $(this).next().next('input').val();

Answer 3

if you have only one input , you can use the ID of that input

$('#qta').val(num+1)

and

$('#qta').val(num-1)