I have been placed in TCS ,but I want to do CDAC course also. What should I prefer?

Join TCS. 

You did not mention whether you already have a respectable C-CAT score and are sure of joining a top rated CDAC institute. Here's what I'd suggest -

1. Join TCS
2. Prepare for C-CAT on weekends or whenever you get time. 
3. Get a good rank in C-CAT and see which centre you can get. If you get a top one - you *may* consider joining one. 

We do not know if you wish to make a career in IT/Software domain or wish to get into Electronics domain. If it's IT, then join TCS and build your programming skills-set. That's a much better deal over joining CDAC to get familiar with programming. 

TCS is a good company to start your career - and you can get access to TCS's huge on-campus learning facilities. Use it to build your skills-set while getting paid. If you aren't allotted a project or do not like then environment, you can always go back to preparing for C-CAT and getting a good rank. 

I'd not recommend skipping this opportunity. There are no solid reasons (apparently) to opt for CDAC, ditching a good job in hand. 

Also, don't expect very exceptional job offers after completing CDAC course. most candidates are likely to be absorbed by bit IT companies or startups. 

Invest all your money and time into building your skill-set. There are several courses available online which you can take at your own pace. I'd strongly recommend opting for them.

Do let us know your choice and be an active member here. You'll learn a lot from fellow CEans. 

All the best! 

Thank you for your advice. I want to work in IT so I will join TCS and try to improve my skillset.