'To revise for an exam' idiom In English, its natural to say the following sentence:

'I will revise for my exam.'

In the above case, it could be argued that the word 'for' is being used as an idiom since you are revising for your own benefit and not the exam's benefit. 

In Mandarin, I have heard the following phrase spoken:

我要替考试复习.

This sounds partly unnatural; I can't put my finger on it - maybe I'm wrong. My question is, is there a succinct  or better way to express doing X for Y as in the above case, maybe comparable to English? Is the use of 替 here wrong?

Edit:

I suppose it wouldn't be so unnatural to say something like 我要复习因为我有考试 however, this is rather long winded.
You can  say 我(正在)为考试而复习. Colloquially, we usually separate it into two pieces: 快考试了，我得复习了。；我得复习了，要考试了。

There are some other alternatives like 我要备考了，我正在备考，etc. , which means I'm preparing for my exam.