Creating a 3d plane using Frank Luna's technique: what are sinf and cosf?

I am creating a 3d plane that lies on the x and z axis, and has hills that extend on the y axis. The bulk of the code looks like this:

float PeaksAndValleys::getHeight(float x, float z)const
{
    return 0.3f*( z*sinf(0.1f*x) + x*cosf(0.1f*z) );
}

void PeaksAndValleys::init(ID3D10Device* device, DWORD m, DWORD n, float dx)
{
    md3dDevice = device;

    mNumRows  = m;
    mNumCols  = n;

    mNumVertices = m*n;
    mNumFaces    = (m-1)*(n-1)*2;


    // Create the geometry and fill the vertex buffer. 

    std::vector<Vertex> vertices(mNumVertices);
    float halfWidth = (n-1)*dx*0.5f;
    float halfDepth = (m-1)*dx*0.5f;
    for(DWORD i = 0; i < m; ++i)
    {
        float z = halfDepth - i*dx;
        for(DWORD j = 0; j < n; ++j)
        {
            float x = -halfWidth + j*dx;

            // Graph of this function looks like a mountain range.
            float y = getHeight(x,z);

            vertices[i*n+j].pos = D3DXVECTOR3(x, y, z);

            // Color the vertex based on its height.
            if( y < -10.0f )
                vertices[i*n+j].color = BEACH_SAND;
            else if( y < 5.0f )
                vertices[i*n+j].color = LIGHT_YELLOW_GREEN;
            else if( y < 12.0f )
                vertices[i*n+j].color = DARK_YELLOW_GREEN;
            else if( y < 20.0f )
                vertices[i*n+j].color = DARKBROWN;
            else
         开发者_开发技巧       vertices[i*n+j].color = WHITE;
        }
    }

    D3D10_BUFFER_DESC vbd;
    vbd.Usage = D3D10_USAGE_IMMUTABLE;
    vbd.ByteWidth = sizeof(Vertex) * mNumVertices;
    vbd.BindFlags = D3D10_BIND_VERTEX_BUFFER;
    vbd.CPUAccessFlags = 0;
    vbd.MiscFlags = 0;
    D3D10_SUBRESOURCE_DATA vinitData;
    vinitData.pSysMem = &vertices[0];
    HR(md3dDevice->CreateBuffer(&vbd, &vinitData, &mVB));


    // Create the index buffer.  The index buffer is fixed, so we only 
    // need to create and set once.

    std::vector<DWORD> indices(mNumFaces*3); // 3 indices per face

    // Iterate over each quad and compute indices.
    int k = 0;
    for(DWORD i = 0; i < m-1; ++i)
    {
        for(DWORD j = 0; j < n-1; ++j)
        {
            indices[k]   = i*n+j;
            indices[k+1] = i*n+j+1;
            indices[k+2] = (i+1)*n+j;

            indices[k+3] = (i+1)*n+j;
            indices[k+4] = i*n+j+1;
            indices[k+5] = (i+1)*n+j+1;

            k += 6; // next quad
        }
    }

    D3D10_BUFFER_DESC ibd;
    ibd.Usage = D3D10_USAGE_IMMUTABLE;
    ibd.ByteWidth = sizeof(DWORD) * mNumFaces*3;
    ibd.BindFlags = D3D10_BIND_INDEX_BUFFER;
    ibd.CPUAccessFlags = 0;
    ibd.MiscFlags = 0;
    D3D10_SUBRESOURCE_DATA iinitData;
    iinitData.pSysMem = &indices[0];
    HR(md3dDevice->CreateBuffer(&ibd, &iinitData, &mIB));
}

My question pertains to the cosf and sinf. I am familiar with trigonometry and I understand sin, cosine, and tangent, but I am not familiar with cosf and sinf and what they do. From looking at this example, they have a lot to do with finding a y value.

cosf and sinf are simply the float versions of cos and sin. The normal cos and sin functions return double values instead of floats. Note that all these functions work in radians, not degrees.

Combined in the way above, they give a landscape that looks somewhat like a mountain range, as in this plot.