# Function calculates wrong value

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Nik joung on 14 Nov 2017
Edited: Michal Kvasnicka on 14 Nov 2017
Hello,
the following function computes the number of binary words of length m and certain weight r (number of ones), which do not contain l consecutive zeros:
if (r==0)
if (m<=l-1)
numberOfStrings = 1;
return
else
numberOfStrings = 0;
return
end
else
tmp = 0;
for j=0:min(m,l-1)
for s=0:r-1
bin1=0;
bin2=0;
if ((m-j)-1-s*l < r-1)
bin1 = 0;
else
bin1 = nchoosek((m-j)-1-s*l,r-1);
end
if ((m-j)-1-(1+s)*l < r-1)
bin2 = 0;
else
bin2 = nchoosek((m-j)-1-(1+s)*l,r-1);
end
tmp = tmp + (-1)^s * nchoosek(r-1,s) * (bin1-bin2);
end
end
numBin = tmp;
end
In the first If-case we consider r=0 (also no ones in the sequences), in the else case we consider weights r >= 1.
I have problems with the case, where no consecutive zeros are allowed in the words (variable l=1). For a certain length m there exists only one possible binary sequence. It is the word with the weight equal to the length (r=m).
For example: m=2 --> possible sequence is 11 (has weight r=2) or for m=8 --> possible sequence is 11111111 (has weight r=8). For all other weights r there don't exists possible sequences (the function should output 0).
Until the word length m = 48 the function seems to work well. With m=48, there exists one word (containing no consecutive zeros) with the weight r=48. For all other weights r it outputs 0.
When I use larger lengths m than 49, the function is not working well anymore. The function outputs the number 1 (for r=49). For all other weights r it should be zero. But if I use r=25, it outputs -2 instead of 0. It gets worser for larger lengths.
Can anyone help me with this problem?

#### 1 Comment

Have you used the debugger to look at what is going on line-by-line? It's by far the best way for a function that you know better than anyone else (assuming you wrote it and if you didn't it is even better as a way to understand someone else's function!)

Michal Kvasnicka on 14 Nov 2017
Edited: Michal Kvasnicka on 14 Nov 2017