declaration of the variable

Question

Hello!

If I write

  for(int i = L-1; i >= 1; --i)
  {
        auto hterm =  sites.op("Adag",i)*sites.op("A",i+1);
        hterm +=  sites.op("A",i)*sites.op("Adag",i+1);
         auto g = Gate(sites,i,i+1,Gate::tReal,tstep/2.,hterm);
    gates.push_back(g);
  }

it is OK, but if I write

for(int i = L-1; i >= 1; --i)
{
    if (i % 2 == 0)
    {
        auto hterm = sites.op("Adag",i)*sites.op("A",i+1);
        hterm += sites.op("A",i)*sites.op("Adag",i+1);
    }

    else
    {
        auto hterm = 2.0*sites.op("Adag",i)*sites.op("A",i+1);
        hterm += 2.0*sites.op("A",i)*sites.op("Adag",i+1);
    }

    auto g = Gate(sites,i,i+1,Gate::tReal,tstep/2.,hterm);
    gates.push_back(g);
}

It says
error: no declaration of the variable «hterm»
auto g = Gate(sites,i,i+1,Gate::tReal,tstep/2.,hterm);

But the following snippet gives "ITensorT::operator+=: different index structure
"

  for(int i = L-1; i >= 1; --i)
{
    if (i % 2 == 0)
    {
        auto hterm =  sites.op("Adag",i)*sites.op("A",i+1);
        hterm +=  sites.op("A",1)*sites.op("Adag",i+1);
        auto g = Gate(sites,i,i+1,Gate::tReal,tstep/2.,hterm);
        gates.push_back(g);
    }

    else
    {
        auto hterm = 2.0*sites.op("Adag",i)*sites.op("A",i+1);
        hterm += 2.0*sites.op("A",i)*sites.op("Adag",i+1);
        auto g = Gate(sites,i,i+1,Gate::tReal,tstep/2.,hterm);
        gates.push_back(g);
    }

How one can curcumvent this issue? I need different hamiltonians for even and odd i

Answer 1

Hello,

In your first code, you just have to initialize hterm before the if statement:

for(int i = L-1; i >= 1; --i)
  {
  IQTensor hterm;
  if (i % 2 == 0)
    {
    hterm = sites.op("Adag",i)*sites.op("A",i+1);
    hterm += sites.op("A",i)*sites.op("Adag",i+1);
    }
  else
    {
    hterm = 2.0*sites.op("Adag",i)*sites.op("A",i+1);
    hterm += 2.0*sites.op("A",i)*sites.op("Adag",i+1);
    }
  auto g = Gate(sites,i,i+1,Gate::tReal,tstep/2.,hterm);
  gates.push_back(g);
  }

(assuming you are using QNs, otherwise you need to initialize with ITensor hterm).

In your second code, it looks like there is just a small mistake in the code. Instead of:

 auto hterm =  sites.op("Adag",i)*sites.op("A",i+1);
 hterm +=  sites.op("A",1)*sites.op("Adag",i+1);

it should be:

 auto hterm =  sites.op("Adag",i)*sites.op("A",i+1);
 hterm +=  sites.op("A",i)*sites.op("Adag",i+1);

Cheers,
Matt

Comments

Thank you very much, Matt!  
I have aready though about "IQTensor hterm;" but I wrote it down without semicolon.
I am used to Python, where there is no semicolon at the end of a sentence.  I am new to C++.

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Yes, C++ can be a bit tricky to get used to at first! I definitely still forget to put semicolons :).