Converting binary to decimal and vice versa, please help!!

I'm having trouble converting strings that represent binary numbers to decimal and vice versa. The main issue is that I'm unsure of how to deal with the decimal point, say converting -11.11 to decimal. This is a homework assignment, so I'm not allowed to use the built in functions.

Here's my code for one of the two:

function decimal= mybin2real(binarystring)
decimal = 0;
for i = 1 : length(binarystring)
decimal = decimal + str2num(binarystring(i)) * 2^(length(binarystring) - i);
end
decimal
end

I was thinking about finding where the decimal point was using strfind, but i'm not sure how to implement it in the matlab code.

[Daniel Shub]

You realize that your example code is littered with built in functions. As bin2dec is not a builtin function, why not just use it. Or maybe retype the function yourself. Have you looked at how bin2dec solves the problem?

[SB]

My bad, I meant I can't use the bin2dec function.

[Daniel Shub]

I will repeat myself: Have you looked at how bin2dec solves the problem. In general the code included in MATLAB is of pretty high quality.

[SB]

I tried to look at documentation, but didn't find much. Could you either supply me with where to look so I could understand the methodology, or could you look at my code below and help me through there?

[Daniel Shub]

At the command line type

type bin2dec

or

edit bin2dec

this will let you see the source code.

[SB]

Thanks alot, that's really helpful, I didn't know we could even do that.

Answer 1

[Matt Fig]

Here I have simply fixed your code. You were really close!

function decimal= mybin2real(binarystring)
% Converts an input string of 1s and 0s to a number.
Ind = strfind(binarystring, '.');
L = length(binarystring);

if isempty(Ind)
    Ind = L+1;
end

Num1 = binarystring(1:Ind-1);
LN1 = length(Num1);
Num2 = binarystring(Ind+1:end);
LN2 = length(Num1);
dec1=0;

for ii = 1 : LN1
    dec1 = dec1 + str2double(Num1(LN1-ii+1)) * 2^(ii-1);
end

dec2=0;

for ii = 1 : length(Num2) 
    dec2 = dec2 + str2double(Num2(ii)) * 2^-(ii);
end

decimal=dec1+dec2;

[SB]

The code returns NaN when I run it, additionally, I think I should have the output as well as the input as character strings.

[Matt Fig]

What input did you give?

>> mybin2real('10101.101')

ans =

                      21.625

>> mybin2real('1010.101')

ans =

                      10.625

>> mybin2real('1010.101111')

ans =

                   10.734375

>> mybin2real('11010111.101111001')

ans =

               215.736328125

[SB]

I don't think the code works for negative signs, since mybin2real('-11.11') returns NaN.

[Matt Fig]

Well that's an easy fix, yes? As the first step, simply check if there is a negative sign. If so, remove it and negate the number at the end.... No Big Deal....

[SB]

so like, if strcmp(binarystring(1),'-')==1 decimalvalue=-dec1-dec2

or something like that? I'm not quite sure, to be honest.

[Matt Fig]

No. As the very first thing....

if binarystring(1)=='-'
    binarystring = binarystring(2:end);
    F = -1;
else
    F = 1;
end

Then at the end:

decimal  = decimal*F;

Answer 2

[Andrei Bobrov]

decimal = (binarystring-'0')*pow2(size(binarystring,2)-1:-1:0).';

ADD

binarystring = '-11.11';

b = binarystring - '0';
[~,s] = ismember([-3,-2],b);
p = b(b >= 0);
decimal = sign(b(1))*p*pow2(diff(s) + (-2:-1:-1-numel(p)))';

[SB]

That doesn't work since something like mybin2real('-11.11') outputs -77 instead of -3.75 (it doesnt look for the decimal point)

[Andrei Bobrov]

see ADD part

Answer 3

[prince kumar]

n=input('input binary no.='); 
n=int2str(n); s=0; 
for i=1:numel(n) 
    s=s+(str2num(n(i))*(2.^(numel(n)-i))); 
end 
s