NIDAQmx analog output C++ constant voltage

Hi,

I'm currently implementing a DAQ in my C++ software which will generate the user voltage. It works fine with Matlab and I get nice changes when I put different voltages.

In spite of that, when I implement in my main the following code:

TaskHandle taskHandle = 0;
  float64 data[1000];
  int var=1;
  while(var<10){
    cin >> var;
    for (int i = 0; i < samplesReceived; ++i){
      data[i] = var;
      // - Initiation PART - //
      DAQmxCreateTask("",&taskHandle);
      DAQmxCreateAOVoltageChan(taskHandle,"Dev1/ao1","",-10.0,10.0,DAQmx_Val_Volts,NULL);
      DAQmxCfgImplicitTiming(taskHandle, DAQmx_Val_HWTimedSinglePoint, 1000);
      DAQmx // - WRITE PART - // 
      //DAQmxWriteAnalogF64(taskHandle,1000,0,10.0,DAQmx_Val_GroupByChannel,data,NULL,NULL); 
      DAQmxWriteAnalogScalarF64(taskHandle, 0, 1, var, NULL); 
// START 
        DAQmxStartTask(taskHandle);
      //DAQmxStopTask(taskHandle); } }

I don't see any changes on a oscilloscope graph U=f(t).

However, when I put something this way:

  TaskHandle taskHandle = 0;
  float64 data[1000];
  int var=1;
  while(var<10){
    cin >> var;
    for (int i = 0; i < samplesReceived; ++i){
      data[i] = var;
      // - Initiation PART - //
      DAQmxCreateTask("",&taskHandle);
      DAQmxCreateAOVoltageChan(taskHandle,"Dev1/ao1","",-10.0,10.0,DAQmx_Val_Volts,NULL);
      DAQmxCfgSampClkTiming(taskHandle, "", 10000.0, DAQmx_Val_Rising, DAQmx_Val_FiniteSamps, 1000);
      DAQmx
      // - WRITE PART - //      
      DAQmxWriteAnalogF64(taskHandle,1000,0,10.0,DAQmx_Val_GroupByChannel,data,NULL,NULL);
      //DAQmxWriteAnalogScalarF64(taskHandle, 0, 1, var, NULL);

//START
      DAQmxStartTask(taskHandle);

      //DAQmxStopTask(taskHandle);

   }
  }

 It works but with graphs not smooth at all and if I put var=1 it evolves with big noise to 1V and if I write it again it switches to 0V (it depends in fact, sometimes it stays for 2 or 3 times at 1V ...).

I don't understand why on Matlab using just something like (after createSession etc.) outputSinglescan(s, 1) gives me really nice results while in C++ I get noisy graph.

Probably I'm not using the right commands, so if someone could help me having the Matlab equivalent of outputSinglescan for C/C++ NIDAQmx,

Thanks,

Complementary informations; I work with a NI USB-6221 with an oscilloscope in BNC ao1.