Apply The Result Of A Query That Includes Variable

The following query is failing when trying to apply the
MAX(field_x_order)
to the variable @max
Note the x is represented by the string variable @stri

declare @i int
declare @stri varchar(10)
declare @max int
set @i = 18
set @max = 0
while @i < 49
begin
set @i = @i + 1
set @stri = cast(@i as varchar(10))
select @max = MAX(field_ + @stri + _ORDER) FROM table_name WHERE field_ + @stri + IS NOT NULL -- error: Incorrect syntax near the keyword 'IS'.
exec ('UPDATE display_1a SET field_' + @stri + '_order = field_' + @stri + '_order ' + @max + 'WHERE field_' + @stri + ' IS NULL')
end

I have also tried:
select MAX(field_ + @stri + _ORDER) INTO @max = FROM display_1a WHERE field_ + @stri + IS NOT NULL -- error: Incorrect syntax near '@max'.

and:
select @max = ('SELECT MAX(field_' + @stri + '_ORDER) FROM display_1a WHERE field_' + @stri + ' IS NOT NULL') -- error: Conversion failed when converting the varchar value 'SELECT MAX(field_19_ORDER) FROM display_1a WHERE field_19 IS NOT NULL' to data type int.

Thanks,
Lou

Lou