PLSQL dynamically truncate string to varchar2 column size - oracle

Question

I'm looking for a way to dynamically truncate a string to the max size of a varchar2 column.
See example below.
Table definition
CREATE TABLE some_table
(
log varchar2(50)
);
Code
DECLARE
v_str some_table.log%TYPE;
BEGIN
v_str := SUBSTR('a text longer than 50 chars ...', 1, 50 /*HOW TO GET 50 DYNAMICALLY HERE ?*/ );
END;
Any ideas?

Answer 1

select char_length
from user_tab_columns
where table_name = 'SOME_TABLE'
and column_name = 'LOG';
would return a value of 50 (the length of the log column in the some_table table)

Answer 2

Getting (and caching) the column length is probably the way to go, but just for fun and riffing from an approach in this discussion:
DECLARE
v_str some_table.log%TYPE;
v_ret pls_integer;
v_len pls_integer;
BEGIN
v_ret := anydata.convertchar('-').getchar(v_str);
v_len := length(v_str);
dbms_output.put_line(v_len);
v_str := substr('a text longer than 50 chars, a text longer than 50 chars, a text longer than 50 chars...', 1, v_len);
dbms_output.put_line(v_str);
END;
/
dbms_output:
50
a text longer than 50 chars, a text longer than 50
Because it's using convertchar and getchar rather than the varchar2 versions, the result is the literal char - padded to its maximum length based on the variable's data type.
db<>fiddle
You can read more about anydata at Oracle-base or in the documentation.
You could still cache that, or work it out on exception. If you were going to do this often and performance was acceptable without caching/catching, you could possibly wrap it in a procedure:
CREATE PROCEDURE trunc_string(p_str out varchar2, p_val varchar2)
AS
v_ret pls_integer;
v_len pls_integer;
BEGIN
v_ret := anydata.convertchar('-').getchar(p_str);
v_len := length(p_str);
p_str := substr(p_val, 1, v_len);
END;
/
and then call that as:
DECLARE
v_str1 some_table.log%TYPE;
v_str2 varchar2(30);
BEGIN
trunc_string(v_str1, 'a text longer than 50 chars, a text longer than 50 chars, a text longer than 50 chars...');
dbms_output.put_line(v_str1);
trunc_string(v_str2, 'a text longer than 30 chars, a text longer than 30 chars, a text longer than 30 chars...');
dbms_output.put_line(v_str2);
END;
/
dbms_output:
a text longer than 50 chars, a text longer than 50
a text longer than 30 chars, a
db<>fiddle
But that's probably overkill... though that could be said about using anydata at all.