ios - Swift "can not increment endIndex" when deleting Japanese characters -

i'm using swift 2.0/xcode 7.0 ios app. i've built japanese ime convert roman characters japanese equivalent. example:

• typing "a" convert "a" "あ"
• typing "k" nothing (no match in japanese) until next character typed, example typing "k" "a" result in "か"

my problem when try delete japanese character. if there 1 character in text field, delete key/function works expected. however, if there more one, when try delete character, receive can not increment endindex error in following code.

var imeinputlength: int = 0  let currentinputvalue: string = txtfldyourresponse.text!.lowercasestring  if(currentinputvalue.characters.count==0) {     imeinputlength = 0 }  let inputstringtokeep: string = currentinputvalue.substringwithrange(     range<string.index>(start: currentinputvalue.startindex.advancedby(imeinputlength),      end: currentinputvalue.endindex))  let imestringtokeep: string = currentinputvalue.substringwithrange(     range<string.index>(start: currentinputvalue.startindex,      end: currentinputvalue.startindex.advancedby(imeinputlength)))      if let imevaluedc = jimedc[inputstringtokeep] {         txtfldyourresponse.text = imestringtokeep + imevaluedc         imeinputlength = (txtfldyourresponse.text?.characters.count)!-1     }      if let imevalue = jime[inputstringtokeep] {         txtfldyourresponse.text = imestringtokeep + imevalue         imeinputlength = txtfldyourresponse.text!.characters.count     } 

currentinputvalue text text field.

imeinputlength int (initial value = 0) increments total character count in text field after match found.

jimedc , jime key/value pairs handle conversion roman japanese characters.

i'm printing endindex console before code runs. appears increment/decrement expected, code block above fails increment error.

i've been banging head against couple weeks no progress.

edit: clarified handling of imeinputlength , added additional code clarification.

a side conversation l'l'l got me on right track. confused error message can not increment endindex. in reality, creating out-of-bounds error on range.

when backspace key pressed, wasn't modifying imeinputlength reflect new length of input string. result, advancebyamount larger updated endindex value. swift threw can not increment endindex reason startindex greater endindex.

i resolved first checking following:

if(imeinputlength > currentinputvalue.characters.count) {     print("you must have pressed backspace")     imeinputlength = txtfldyourresponse.text!.characters.count } else {     let inputstringtokeep:     ... } 

if backspace key pressed, needed see if imeinputlength value greater current character count of input field. use case imeinputlength ends greater when japanese character deleted using backspace key. when condition caught, reset imeinputlength current character count , exit routine.

i'm happy of use cases test out positive!