JQuery: How to use a for loop variable in a jquery selector?

Question

It's my first post here.

I am trying to make a photo gallery and I got the code below to work with my website. I want the same nth-child to be affected in .gallery when the same nth-child is hovered in the .preview class.

$(document).ready(function() {
    $(".preview > :nth-child(1)").mouseover(function() {
        $(".gallery > :nth-child(1)").css("opacity","1");
        $(".gallery :not( > :nth-child(1))").css("opacity","0");
    });
    $(".preview > :nth-child(2)").mouseover(function() {
        $(".gallery > :nth-child(2)").css("opacity","1");
        $(".gallery :not( > :nth-child(2))").css("opacity","0");
    });
    $(".preview > :nth-child(3)").mouseover(function() {
        $(".gallery > :nth-child(3)").css("opacity","1");
        $(".gallery :not( > :nth-child(3))").css("opacity","0");
    });
    $(".preview > :nth-child(4)").mouseover(function() {
        $(".gallery > :nth-child(4)").css("opacity","1");
        $(".gallery :not( > :nth-child(4))").css("opacity","0");
    });
});

I then thought about an easier way to do that using a for-loop. Later I am planning to add more children to both .gallery and .preview, so a for-loop will make the code much simpler. I think that in the code below the problem is with the for-loop variable i. Can you please look at the code below and see what I am doing wrong?

$(document).ready(function() {
    for (i = 1; i < preview.length; i++) {
        var selector1 = ".preview > :nth-child(" + i + ")";
        var selector2 = ".gallery > :nth-child(" + i + ")";
        var selector3 = ".gallery :not( > :nth-child((" + i + "))";
        $(selector1).mouseover(function() {
            $(selector2).css("opacity","1");
            $(selector3).css("opacity","0");
        });
    }
});

EDIT: Here is the HTML it applies to:

<div class="gallery">
        <div></div>
        <div></div>
        <div></div>
        <div></div>
        <div class="preview">
            <div></div>
            <div></div>
            <div></div>
            <div></div>
        </div>
    </div>

EDIT: And here is CSS

.gallery > :nth-child(1) {
    background-image: url('https://sp.yimg.com/ib/th?id=HN.607997559404560656&pid=15.1&P=0');
    height: 500px;
    background-size: cover;
    opacity: 1;
}
.gallery > :nth-child(2) {
    background-image: url('http://www.kamionek.waw.pl/images/stories/2012/stadion_narodowy_ii_2012_kamionek_0001.jpg');
    height: 500px;
    background-size: cover;
    position: relative;
    top: -500px;
    opacity: 0;
    margin-bottom: -500px;
}
.gallery > :nth-child(3) {
    background-image: url('http://www.twojezaglebie.pl/wp-content/uploads/2012/02/stadion-narodowy-luty-2012_6521.jpg');
    height: 500px;
    background-size: cover;
    position: relative;
    top: -500px;
    opacity: 0;
    margin-bottom: -500px;
}
.gallery > :nth-child(4) {
    background-image: url('http://upload.wikimedia.org/wikipedia/commons/a/ac/Stadion_Narodowy_w_Warszawie_20120422.jpg');
    height: 500px;
    background-size: cover;
    position: relative;
    top: -500px;
    opacity: 0;
    margin-bottom: -500px;
}
.preview {
    height: 100px;
    width: 100px;
    width: 100%;
}
.preview div {
    height: 100px;
    width: 100px;
    background-size: cover;
    display: inline;
    float: left;
}
.preview > :nth-child(1) {
    background-image: url('https://sp.yimg.com/ib/th?id=HN.607997559404560656&pid=15.1&P=0');
}
.preview > :nth-child(2) {
    background-image: url('http://www.kamionek.waw.pl/images/stories/2012/stadion_narodowy_ii_2012_kamionek_0001.jpg');
}
.preview > :nth-child(3) {
    background-image: url('http://www.twojezaglebie.pl/wp-content/uploads/2012/02/stadion-narodowy-luty-2012_6521.jpg');
}
.preview > :nth-child(4) {
    background-image: url('http://upload.wikimedia.org/wikipedia/commons/a/ac/Stadion_Narodowy_w_Warszawie_20120422.jpg');
}

Answer 1

You can do it using your existing system like this: http://jsfiddle.net/TrueBlueAussie/wn3c84ke/5/

$(document).ready(function () {
    $(".preview div").mouseover(function () {
        var $previews = $('.preview div');
        var $children = $(".gallery").children().not('.preview');
        var $selected = $children.eq($previews.index(this));
        $children.not($selected).css("opacity", "0");
        $selected.css("opacity", "1");
    });
});

Which uses the index position of the hover for the selection index, but I would actually recommend a data-driven approach like this: http://jsfiddle.net/TrueBlueAussie/wn3c84ke/7/

HTML:

<div class="gallery">
    <div id="one"></div>
    <div id="two"></div>
    <div id="three"></div>
    <div id="four"></div>
    <div class="preview">
        <div data-target="#one"></div>
        <div data-target="#two"></div>
        <div data-target="#three"></div>
        <div data-target="#four"></div>
    </div>
</div>

jQuery

$(document).ready(function () {
    $(".preview > div").mouseover(function () {
        var $children = $(".gallery >  *:not(.preview)");
        var $selected = $($(this).attr("data-target"));
        $children.not($selected).css("opacity", "0");
        $selected.css("opacity", "1");
    });
});

Answer 2

Try this:

$(".preview").on("mouseover", function () {
    $(".gallery").children().index(this).css("opacity", "0");
    $(this).children().css("opacity", "1");
});

Answer 3

A way to do it with a loop would be:

$(document).ready(function() {
    $(".preview div").each(function(index,object){
        $(object).mouseover(function() {
            $(".gallery div").css("opacity","0");
            $(".gallery div")[index].css("opacity","1");
        });
    });
});