Xcode6 从字符串Xcode中删除字符

我使用以下代码向可变字符串添加一个字符，然后从自定义键盘将其放入文本视图

- (IBAction) press:(id)sender {
    [[UIDevice currentDevice] playInputClick];
    if (sender == AEE) {
        self.BACK.hidden = NO;
        A = @"A";
        NSLog(@"sender:%@",sender);
    }
    if (sender == BEE) {
        self.BACK.hidden = NO;
        A = @"B";
    }

    if (sender == CEE) {
        self.BACK.hidden = NO;
        A = @"C";
    }

    if (sender == DEE) {
        self.BACK.hidden = NO;
        A = @"D";
    }

    if (sender == EEE) {
        self.BACK.hidden = NO;
        A = @"E";
    }

    if (sender == EFF) {
        self.BACK.hidden = NO;
        A = @"F";
    }

    if (sender == ONE) {
        self.BACK.hidden = NO;
        A = @"1";
    }

    if (sender == TWO) {
        self.BACK.hidden = NO;
        A = @"2";
    }

    if (sender == THREE) {
        self.BACK.hidden = NO;
        A = @"3";
    }

    if (sender == FOUR) {
        self.BACK.hidden = NO;
        A = @"4";
    }

    if (sender == FIVE) {
        self.BACK.hidden = NO;
        A = @"5";
    }

    if (sender == SIX) {
        self.BACK.hidden = NO;
        A = @"6";
    }

    if (sender == SEVEN) {
        self.BACK.hidden = NO;
        A = @"7";
    }

    if (sender == EIGHT) {
        self.BACK.hidden = NO;
        A = @"8";
    }

    if (sender == NINE) {
        self.BACK.hidden = NO;
        A = @"9";
    }

    if (sender == ZERO) {
        self.BACK.hidden = NO;
        A = @"0";
    }




    if (display.length <= 7) {
        [self.display appendString:A];
         DONE.hidden = YES;
    }
    if (display.length == 8) {
        DONE.hidden = NO;
    }


    else {
        [self.display appendString:@""];
    }


    //**THIS IS WHERE I DELETE CHARATERS**
    if (sender == BACK) {


                [display deleteCharactersInRange:NSMakeRange([display length]-2, 2)];
                       NSLog(@"display2:%@", display);
        if (self.display.length <1) {
            BACK.hidden = YES;
        }
        if (display.length < 8) {
            DONE.hidden = YES;
        }
    }


    you.text = display;
}

它只删除一个字符一次。如果我把

[display deleteCharactersInRange:NSMakeRange([display length]-1, 2)];

它做同样的事情。但是当我把

[display deleteCharactersInRange:NSMakeRange([display length]-2, 1)];

---

我的应用程序崩溃，我得到一个越界错误。我尝试过使用if display=我发现，如果我创建了一个单独的操作来删除，并使用了下面的代码，那么一切似乎都正常

iAction删除：idsender{ [[UIDevice currentDevice]播放输入点击]； 如果发送方==返回{ 如果[显示长度]>0{ [显示设置字符串：[显示子字符串到索引：[显示长度]-1]]； you.text=显示

}
else {
    you.text = display;
    //no characters to delete... attempting to do so will result in a crash
}
if (self.display.length <1) {
    BACK.hidden = YES;
}
if (display.length < 8) {
    DONE.hidden = YES;
}

}

}
else {
    you.text = display;
    //no characters to delete... attempting to do so will result in a crash
}
if (self.display.length <1) {
    BACK.hidden = YES;
}
if (display.length < 8) {
    DONE.hidden = YES;
}