check if n is a power of 2

[Cliff]

Hi,
I have a function that accepts an input n from a user.I want to force
the user to supply a number that is always a power of 2,i.e one of
2,4,8,16,....
This is what i have as a requirement for the number;

n = power(2,x)
therefore
x = log10(n)/log10(2)
All numbers{n} that give whole numbers i.e 1,2,3 etc when put into
the above equation satisfy my requirement.Now how do I ask Matlab to
check if a number is a whole number?

I thought isinteger(x) would work but later realise it want meant for
such use.

I want to make sure if say x=1.234 i reject the given n unless if n
gives x={whole number}

Thanks

[Jos]

Post by Cliff
Hi,
I have a function that accepts an input n from a user.I want to force
the user to supply a number that is always a power of 2,i.e one of
2,4,8,16,....
This is what i have as a requirement for the number;
n = power(2,x)
therefore
x = log10(n)/log10(2)
All numbers{n} that give whole numbers i.e 1,2,3 etc when put into
the above equation satisfy my requirement.Now how do I ask Matlab to
check if a number is a whole number?
I thought isinteger(x) would work but later realise it want meant for
such use.
I want to make sure if say x=1.234 i reject the given n unless if n
gives x={whole number}
Thanks

why not "isinteger(log2(n))"?

hth
Jos

[Theophanes Raptis]

Post by Jos

Post by Cliff
Hi,
I have a function that accepts an input n from a user.I want to force
the user to supply a number that is always a power of 2,i.e one of
2,4,8,16,....
This is what i have as a requirement for the number;
n = power(2,x)
therefore
x = log10(n)/log10(2)
All numbers{n} that give whole numbers i.e 1,2,3 etc when put into
the above equation satisfy my requirement.Now how do I ask Matlab to
check if a number is a whole number?
I thought isinteger(x) would work but later realise it want meant for
such use.
I want to make sure if say x=1.234 i reject the given n unless if n
gives x={whole number}
Thanks

why not "isinteger(log2(n))"?
hth
Jos

[dpb]

Post by Jos

Post by Cliff
the user to supply a number that is always a power of 2,i.e one of
2,4,8,16,....

...

Post by Jos
why not "isinteger(log2(n))"?

'Cuz ISINTEGER only checks the _class_ (which will still be double), not
the _value_ of the argument.

Several ways, one is

isPow2=(fix(log2(n))==n);



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[Steve Amphlett]

Post by Cliff
Hi,
I have a function that accepts an input n from a user.I want to force
the user to supply a number that is always a power of 2,i.e one of
2,4,8,16,....
This is what i have as a requirement for the number;
n = power(2,x)
therefore
x = log10(n)/log10(2)
All numbers{n} that give whole numbers i.e 1,2,3 etc when put into
the above equation satisfy my requirement.Now how do I ask Matlab to
check if a number is a whole number?
I thought isinteger(x) would work but later realise it want meant for
such use.
I want to make sure if say x=1.234 i reject the given n unless if n
gives x={whole number}

Why not take a hint from <nextpow2>? ML has a built-in
<log2> function, which probably (based on some recent TMW posts
here) checks for exact powers of two to give exact integer return
values where possible. So...

n=1024;
floor(log2(n))==log2(n)

ans =

1

Ta da!

[Steve Eddins]

----------------------------------------

Post by Cliff
Hi,
I have a function that accepts an input n from a user.I want to force
the user to supply a number that is always a power of 2,i.e one of
2,4,8,16,....
This is what i have as a requirement for the number;
n = power(2,x)
therefore
x = log10(n)/log10(2)
All numbers{n} that give whole numbers i.e 1,2,3 etc when put into
the above equation satisfy my requirement.Now how do I ask Matlab to
check if a number is a whole number?

I suggest that you check n directly using the two-output form of log2:

[f,e] = log2(n);

If f is 0.5, then n is a power of 2.

http://www.mathworks.com/access/helpdesk/help/techdoc/ref/log2.html

[Theophanes Raptis]

Post by Cliff
Hi,
I have a function that accepts an input n from a user.I want to force
the user to supply a number that is always a power of 2,i.e one of
2,4,8,16,....
This is what i have as a requirement for the number;
n = power(2,x)
therefore
x = log10(n)/log10(2)
All numbers{n} that give whole numbers i.e 1,2,3 etc when put into
the above equation satisfy my requirement.Now how do I ask Matlab to
check if a number is a whole number?
I thought isinteger(x) would work but later realise it want meant for
such use.
I want to make sure if say x=1.234 i reject the given n unless if n
gives x={whole number}
Thanks