Question: Just to help other developers, because there is no similar question on SO. 

```
div class=(is_active? ? 'active' : 'inactive')
div class=('active' if is_active?)

```

Answer: 
See the examples below: 

```
div class=(is_active? ? 'active' : 'inactive')
div class=('active' if is_active?)

```

The same approach can be used to assign dynamic values to other attributes.
I use array of classes and nil element if there is no need to include class in list, then compact array to remove nil elements and finally join all together. 

```
div class=(["cday", "col-md-1", day.day == 1 ? "col-md-offset-#{day.cwday-1}" : nil].compact.join(' '))

```