acc = r * s throws away the previous computations of acc for all but the last loop. Right?

void NBodySimpleInteractionEngine::BodyBodyInteraction(const ParticleCpu* const pParticlesIn, 
    ParticleCpu& particleOut, int numParticles) const 
{
    float_3 pos(particleOut.pos);
    float_3 vel(particleOut.vel);
    float_3 acc(0.0f);

    std::for_each(pParticlesIn, pParticlesIn + numParticles, [=, &acc](const ParticleCpu& p)
    {  
        const float_3 r = p.pos - pos;

        float distSqr = SqrLength(r) + m_softeningSquared;
        float invDist = 1.0f / sqrt(distSqr);
        float invDistCube =  invDist * invDist * invDist;
        float s = m_particleMass * invDistCube;

        acc = r * s;
    });

It seems to me that the for_each computes acc for numParticle times.   However on each time except for the last the previous acc value gets overwritten.  In other words, the for_each is pointless and you could just compute instead acc based on pParticlesIn[ numParticles-1 ]

Could somebody explain why my reasoning is wrong, and acc somehow does get the effect of each loop?

This is from page 36 of "C++ AMP"  Gregory & Miller.  I would have expected a sum of accelerations for each other particle, so   acc+= r *s

the code continues :

    vel += acc * m_deltaTime;
    vel *= m_dampingFactor;
    pos += vel * m_deltaTime;

    particleOut.pos = pos;
    particleOut.vel = vel;
}