Source code in the purebasic programming language

EnableExplicit

#Separator$ = ","

Define fInput$ = "input.csv"; insert path to input file
Define fOutput$ = "output.csv"; insert path to output file  
Define header$, row$, field$
Define nbColumns, sum, i

If OpenConsole()
  If Not ReadFile(0, fInput$)
    PrintN("Error opening input file")
    Goto Finish
  EndIf
  
  If Not CreateFile(1, fOutput$)
    PrintN("Error creating output file")
    CloseFile(0)
    Goto Finish 
  EndIf
  
  ; Read header row
  header$ = ReadString(0)
  ; Determine number of columns
  nbColumns = CountString(header$, ",") + 1
  ; Change header row
  header$ + #Separator$ + "SUM"
  ; Write to output file
  WriteStringN(1, header$)
  
  ; Read remaining rows, process and write to output file
  While Not Eof(0)
    row$ = ReadString(0)
    sum = 0
    For i = 1 To nbColumns
      field$ = StringField(row$, i, #Separator$)
      sum + Val(field$)
    Next
    row$ + #Separator$ + sum
    WriteStringN(1, row$)
  Wend
  
  CloseFile(0)
  CloseFile(1)
  
  Finish:
  PrintN("")
  PrintN("Press any key to close the console")
  Repeat: Delay(10) : Until Inkey() <> ""
  CloseConsole()
EndIf