I learned that you use "for -ing" and not a 'to' infinitive when you want to say the purpose of a tool, but I am not so sure about it.

People told me I should say:

> 
> This computer is for you to attend lectures. (1a)
> 
> 
> 

or

> 
> This computer is for attending lectures. (1b)
> 
> 
> 

but not

> 
> This computer is to attend lectures. (1c)
> 
> 
> 

However, can't a 'to' infinitive alone be used as an adjective to indicate the purpose? For example, can't I say:

> 
> You have a computer to attend class. (2a)
> 
> 
> 

instead of

> 
> You have a computer **for** you to attend class. (2b)
> 
> 
> 

If I can, what do you think makes such a difference between the 1s and 2s?