C++ :: Mixer With Wave Files

I'm doing a mixer with wave files. I can play and even apply the reverse effect to them. the logarithm is like this:

for( unsigned int i=0; i<sampleCount/2; i++ )
{
// swapping with the other half
double temp = samples[i];
samples[i] = samples[sampleCount-i-1];
samples[sampleCount-i-1] = temp;
}

Now I want to apply these effects to the files: delay, slow, and fast but I don't know how to do it. The teacher told me that to do it slow I need to double the Samples so I tried to do this samples[i] = samples[sampleCount*2]; it does not work......Also I apply the contrary to do make the sound fast samples[i] = samples[sampleCount/2]; the samething I don't get the sound desired.

For the daley part I'm thinking on play the audio one without touch it and then repit it 8 times or more to create the delay.... I did it like this:

double temp = samples[i];
samples[i] = samples[sampleCount-i-1]+ (samples[sampleCount -i-1]*8);

It didn't work..why the use of -i-1 after sampleCount!!