How to calculate bit transitions using bitset < > - c++

Question

I am new to C++. I want to calculate the no of transitions from 0 to 0, 0 to 1, 1 to 0 and 1 to 1 in a 9 bit sequence. I have written the following code;
int main {
srand((unsigned)time(0));
unsigned int x;
for (int i=0:i<=512;i++) // loop-1
{
x=rand()%512;
bitset<9>bitseq(x);
for(int j=0;j<=bitseq.size();j++) // loop-2
{
bool a= bitseq.test(j);
bool b= bitseq.test(j+1)
if ((a==0)&(b==0)==0)
{
transition0_0 = transition0_0 + 1; // transition from 0 to 0
}
else if ((a==0)&(b==1)==0)
{
transition0_1 = transition0_1 + 1;
else if ((a==1)&(b==0)==0)
{
transition1_0 = transition1_0 + 1;
else
{
transition1_1 = transition1_1 + 1;
cout<<transition0_0<<" "<<transition0_1<<endl;
cout<<transition1_0<<" "<<transition1_1<<endl;
}
}
Somebody please guide me on the following
how to save the last bit value in loop-2 to check the transition from last bit of the last bitset output to the 1st bit of the next bitset output?
If this does not work, How I can save it in vector and use iterators to check the transitions?

Answer 1

First of all, the loop index j is running past the end of the bitset. Indices go from 0 to bitseq.size()-1 (inclusive). If you're going to test j and j+1 the largest value j can take is bitseq.size()-2.
Second, the ==0 part that appears in your ifs is strange, you should just use
if( (a==0)&&(b==0) )
Notice the use of two &&. While a single & works for this code, I think it's better to use the operator that correctly conveys your intentions.
And then to answer your question, you can keep a "last bit" variable that is initially set to a sentinel value (indicating you're seeing the first bitseq just now) and compare it to bitseq[0] before the start of loop 2. Here's a modified version of your code that should do what you ask.
int main {
srand((unsigned)time(0));
unsigned int x;
int transition0_0 = 0,
transition0_1 = 0,
transition1_0 = 0,
transition1_1 = 0;
int prev = -1;
for (int i=0:i<=512;i++) // loop-1
{
x=rand()%512;
bitset<9> bitseq(x);
if( prev != -1 ) // don't check this on the first iteration
{
bool cur = bitseq.test(0);
if( !prev && !cur )
++transition0_0;
else if( !prev && cur )
++transition0_1;
else if( prev && !cur )
++transition1_0;
else
++transition1_1;
}
for(int j=0;j+1<bitseq.size();j++) // loop-2
{
bool a= bitseq.test(j);
bool b= bitseq.test(j+1)
if ((a==0)&&(b==0))
{
transition0_0 = transition0_0 + 1; // transition from 0 to 0
}
else if ((a==0)&&(b==1))
{
transition0_1 = transition0_1 + 1;
}
else if ((a==1)&&(b==0))
{
transition1_0 = transition1_0 + 1;
}
else
{
++transition1_1 = transition1_1 + 1;
}
} // for-2
prev = bitseq.test(bitseq.size()-1); // update prev for the next iteration
cout<<transition0_0<<" "<<transition0_1<<endl;
cout<<transition1_0<<" "<<transition1_1<<endl;
} // for-1
} // main

Answer 2

Would something like this be better for you? Use an array of 4 ints where [0] = 0->0, [1] = 0->1, [2] = 1->0, [3] = 1->1.
int main {
int nTransition[] = { 0,0,0,0 };
bool a,b;
unsigned int x;
int j;
srand ((unsigned)time(0));
for (int i = 0: i < 512; i++) {
x = rand () % 512;
bitset<9> bitseq(x);
if (i == 0) {
a = bitseq.test (0);
j = 1;
} else
j = 0;
for (; j < bitseq.size (); j++) {
b = bitseq.test(j);
int nPos = (a) ? ((b) ? 3 : 2) : ((b) ? 1 : 0);
nTransition[nPos]++;
a = b;
}
}
}