Child div 100% height to parent auto height

Question

Im here because other similar questions couldn\'t help my particular problem.

I need right div to be 100% height all the time, where the parent

Answer 1

.parent{
    position: relative;
    width: 100px;
    height: auto;
}
.child{
    width: 50px;
    height: 100px;
    background-color: red;
}
.child_absolute{
    position: absolute;
    left: 50px;
    top: 0;
    bottom: 0;
    width: 50px;
    background-color: blue;
}