Problems upsampling data using accelerate - ios

Question

After I downsample a vector with a constant decimating factor, I want to upsample the vector back to the original sample rate (after performing some analyses). However, I am struggling with the upsampling.
For the downsampling I apply vDSP_desamp from the Accelerate framework, and for the upsampling I tried to apply vDSP_vlint:
// Create some test data for input vector
float inputData[10] = {0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9};
int inputLength = 10;
int decimationFactor = 2; // Downsample by factor 2
int downSampledLength = inputLength/decimationFactor;
// Allocate downsampled output vector
float* downSampledData = malloc(downSampledLength*sizeof(float));
// Create filter (average samples)
float* filter = malloc(decimationFactor*sizeof(float));
for (int i = 0; i < decimationFactor; ++i){
filter[i] = 1.0/decimationFactor;
}
// Downsample and average
vDSP_desamp(inputData,
(vDSP_Stride) decimationFactor,
filter,
downSampledData,
(vDSP_Length) downSampledLength, // Downsample to 5 samples
(vDSP_Length) decimationFactor );
free(filter);
The output of downSampledData using this code is:
0.05, 0.25, 0.45, 0.65, 0.85
To upsample the (processed) data vector back to the original sample rate I use the following code:
// For this example downSampledData is just copied to processedData ...
float* processedData = malloc(downSampledLength*sizeof(float));
processedData = downSampledData;
// Create vector used by vDSP_vlint to indicate interpolation constants.
float* b = malloc(downSampledLength*sizeof(float));
for (int i = 0; i < downSampledLength; i++) {
b[i] = i + 0.5;
}
// Allocate data vector for upsampled data
float* upSampledData = malloc(inputLength*sizeof(float));
// Upsample and interpolate
vDSP_vlint (processedData,
b,
1,
upSampledData,
1,
(vDSP_Length) inputLength, // Resample back to 10 samples
(vDSP_Length) downSampledLength);
However, the output of upSampledData is
0.15, 0.35, 0.55, 0.75, 0.43, 0.05, 0.05, 0.05, 0.08, 0.12
which is not correct, apparently. How should I apply vDSP_vlint? Or should I use other functions for upsampling the data?