vars readLatestPrices(string AssetName)
{
    string FilePath = strf("C:\\Users\\chu_h\\Desktop\\output\\%s.csv", AssetName);
    string content = file_content(FilePath);

    .....rest of the code
    
    vars ClosePrice= series(0, 1500);
    int i, idx;
    
    for(i = 0; i < size; i++) {
        idx = (head - 1 - i + 1500) % 1500;
        ClosePrice[i] = Prices[idx];
    }

    int i;
    for(i = 0; i < 10; i++) {
        printf("#Prices[%d] = %.6f\n", i, ClosePrice[i]); //Prints correct latest 10 prices
    }
    
    return CloseSeries;
}

vars Prices = readLatestPrices("GC_J");  //This gives the correct price series
vars PricesDiff = series(Prices[0] - Prices[1]);   //This always give same value, for example if ClosePrice[0]-ClosePrice[1] = 10, the series will just be 10,10,10,...
    
// Print the 10 most recent PricesDiff 
printf("\n#Asset: %s - Latest 10 PricesDiff (newest first):\n", "GC_J");
int i;
for(i = 0; i < 10; i++) {
    printf("#PricesDiff[%d] = %.6f\n", i, PricesDiff[i]);
}

#Asset: GC_J - Latest 10 PricesDiff (newest first):
PricesDiff[0] = 10
PricesDiff[1] = 10
PricesDiff[2] = 10
PricesDiff[3] = 10
PricesDiff[4] = 10
PricesDiff[5] = 10
PricesDiff[6] = 10
PricesDiff[7] = 10
PricesDiff[8] = 10
PricesDiff[9] = 10

vars Prices = readLatestPrices("GC_J");  //This gives the correct price series
vars PricesDiff = series(Prices[0] - Prices[1]);   //will just be 10,10,10,...

vars Prices = readLatestPrices("GC_J");  //This gives the correct price series
vars PricesDiff = series(0);  
int i;
for(i = 0; i < LookBack; i++) PricesDiff[i] = Prices[i] - Prices[i+1]; //will be 10, 3, -5, 4,...