A loop for scanning all binary sequences of length n with k 1's and n-k 0's

I want to write a loop that scans all binary sequences of length n with k 1's and n-k 0's.

Actually, in each iteration an action is performed on the sequence, then it goes to next sequence, if necessary. (I'm not looking for the functions like nchoosek or perms that return all such a sequences).

How to write such a loop? Thanks

[Azzi Abdelmalek]

Is it a homework?

[Mahdi]

No! I'm testing an approach that I've developed! I need to perform process on such strings to find at least one with the desired property.

[Azzi Abdelmalek]

Then why are you asking to not use some functions?

[Mahdi]

Because for large n's it takes so much time (if memory allows) to return all such strings. But producing one by one is practical since as soon as the criterion is satisfied the loop will broke. Also, in this way no surplus computations are performed, increasing efficiency of my approach. Thanks