Dear all,
I'd like to perform the same band math expression (to calculate NDVI) on three different Sentinel-2 images, located in different folders.
I'd also like to save them with different names that maintain a part of the names of the original files.
I wrote this code where first I list the paths of the images ( in Files=[... ) and then I run a for cycle to perform the NDVI of each of them and to save the results with different names, which recall the name of the original images:

pro idl_bandmath
compile_opt idl2, hidden
on_error, 2

Files=[ 'D:\\Sentinel-2\\PROVA - S2A_R079_2A\\S2A_MSIL2A_20180513T095031_N0207_R079_T33SUC_20180513T115532.SAFE\\MTD_MSIL2A.xml', 'D:\\Sentinel-2\\PROVA - S2A_R079_2A\\S2A_MSIL2A_20180523T095031_N0208_R079_T33SUC_20180524T183727.SAFE\\MTD_MSIL2A.xml', 'D:\\Sentinel-2\\PROVA - S2A_R079_2A\\S2A_MSIL2A_20180602T095031_N0208_R079_T33SUC_20180602T142826.SAFE\\MTD_MSIL2A.xml' ]


for f=0,n_elements(Files)-1 do begin

task_1 = ENVITask('PixelwiseBandMathRaster')

stringa = '{' + $
' "url": "xxxxx",' + $
' "factory": "URLRaster",' + $
' "dataset_index": 0' + $
'}'
stringa2=stringa.replace("xxxxx",Files[0])

string_file_time=files[f].Substring(strlen(Files[0])-70,strlen(Files[0])-22)

task_1.input_raster = IDLHydrate(TYPE='ENVIRASTER', JSON_Parse(stringa2 ))

task_1.expression = '(float(b4) - float(b3)) / (float(b4) + float(b3))'
temp_name = "D:\Sentinel-2\PROVA - S2A_R079_2A\OUTPUT\S2A_MSIL2A_20180602T095031_N0208_R079_T33SUC_20180602T142826_ndvi_prova3"
temp_name= temp_name.Substring(0,51) + string_file_time + "_ndvi_prova6"
task_1.output_raster_uri =temp_name
task_1.Execute
endfor

end


The code actually creates three files with correct names (thus taking part of the names of the corresponding original images) but all of them are the NDVI of the first image listed in Files, thus they result all the same. I don't understand why it does not calculate the NDVI of each of the images but only of the first one (three times).
Any help will be appreciated, thanks a lot!