no of bits increases after decimal to binary conversion

I performed 

a =

       5
      -5

   b=dec2bin(2^4+a)

b =

10101
01011

I have given 4 bits but why my output gives 5 bit value,also for 5 why may MSB is 1`,for -5 why it is 0,also if i perform 

a=0;

   b=dec2bin(2^4+a)

b =

10000

In this why my MSB is 1

Answer 1

[Azzi Abdelmalek]

 2^4+5 requires at least 5 bits

If you want to represent a with 4 bits

b=dec2bin(a,4)

also, to use dec2bin(a), a must be non negative integer

[Pat]

dec2bin(-5,4) ??? Error using ==> dec2bin at 33 D must be a non-negative integer smaller than 2^52.

[Azzi Abdelmalek]

Yes, That what I said above, to represent signed binary number use 2's complement representation http://en.wikipedia.org/wiki/Two%27s_complement

[Pat]

Azzi can u please provide me an example ,i understood concept ,but dont know how to start with