Concatenation Interview Questions: A Comprehensive Guide to Mastering String Manipulation

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I expect they wanted something like this:

It doesn’t return the end of the string that was joined like strcat does, but that doesn’t seem to be needed.

I’m not sure if this kind of question is good for an interview. It shows that you can write short code and know what strcat does, but that’s about it.

Edit: as aib writes, the statement

…is perhaps a more conventional way of writing the second loop (instead of using for).

Since the task was to join two strings together and not to make a copy of strcat, I would choose the easy option of making a new string that is a mix of the two.

The right answer to that question is that it would show that you have a skill that you shouldn’t have. They are wanting you to demonstrate the ability to write hacker code. They want you to come up with your own way to do things that every C compiler already does. This is a waste of time. They are wanting you to write streamlined code which, by definition, is not readable. The 15 line implementation is probably better if it is more readable. Most projects do not fail because the developers wasted 150 clock cycles. Some do fail because someone wrote unmaintainable code. If you did have to write that, it would need a 15 line comment. So, my answer would be, “Show me the performance metrics that show why you shouldn’t use the standard libraries and why you need the best solution.” Time is much better spent on design and gathering those performance metrics.

Never forget – you are also interviewing them.

After reading some other solutions to this, I think the following is probably the best code answer:

But I would follow that up with a safe version of that:

Also, here’s the full answer (which compiler will make the above answer work best) along with the application, which was the real question:

EDIT: Im very upset that people are so against strcpy and strchr. Waah! So, I thought Id play by the spirit of the rules:

I still cant understand how anyone would take 2 whole lines ;-P.

I tested this bit in VS2008, and it worked fine.

Any function can be made to fit in a single line by simply removing all the .

However, I think youre looking for this answer:

The explanation is rather simple:

src++ returns a pointer to the current character being copied before incrementing to point to the next one. * dereferences this pointer, and a similar expression on the LHS copies it to dst. The character that was copied is what the whole = expression returns, so a simple while loop copies it until it finds a .

However:

strcat() is easier to read and possibly much faster. Any other solution is feasible only when strcat() is not available. (Or when youre in an interview, apparently. ) And replace strcat() above with strncat() unless youre really really sure the destination string is big enough.

Edit: I missed the part about strlen() being disallowed. Heres the two-statement function:

Note that the standard strcat() function returns the original value of dst.

One line:

(Note that this might possibly invoke undefined behavior.)

Addendum

The ISO C99 standard states that: If copying takes place between objects that overlap, the behavior is undefined.

That being said, the code above will still probably work correctly. It works with MS VC 2010.

I have a feeling such questions are meant to be elimination questions rather than selection. Because the questions are so hard to understand, it is easier to get rid of candidates than to choose candidates by asking them more real-world questions. It was just a rant from me because I’m also looking for work and getting questions like that. Thanks to SO, I was able to answer a lot of them.

Destination string must end with NULL terminated.

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