Problem setting video frame rate using AVAssetWriter/AVAssetReader

Here is an idea to select frames. Suppose the fps of source video is F and target fps is TF. rate = TF/F

Initiate a variable n equal to -rate and add rate each time,
when the integer part of n changed, select the frame.

e.g. rate = 0.3
          n: -0.3 0 0.3 0.6 0.9 1.2 1.5 1.8 2.1
                  ^              ^           ^
frame index:      0  1   2   3   4   5   6   7
select 0 4 7

float rate = 0.39999f; // TF/F 
float n =  -rate; // to make sure first frame will be selected
for (int i = 0; i < 100; ++i, n += rate) { // i stands for frame index, take a video with 100 frames as an example
    int m = floor(n);
    int tmp = n+rate;
    // if rate > 1.0 repeat i
    // if rate < 1.0 some of the frames will be dropped
    for (int j = 0; m+j < tmp; ++j) {
        // Use this frame
        printf("%d ", i);
    }
}