Postgres select concatenated column name - postgresql

Question

For the sake of example, there's five columns in a table month:
month.week1
month.week2
month.week3
month.week4
month.week5
The number of col is determined by a function
EXTRACT(WEEK FROM NOW()) - EXTRACT(WEEK FROM DATE_TRUNC('month', NOW())) + 1
How can I select the column colX? This is what I have so far
SELECT month.week || (
EXTRACT(WEEK FROM NOW())
- EXTRACT(WEEK FROM DATE_TRUNC('month', NOW())) + 1
)::text
FROM month
But that gives me the error
ERROR: column month.week doesn't exist
SQL state: 42703

Answer 1

Use case statement
SELECT
CASE WHEN (week expresion) = 1 THEN month.week1
WHEN (week expresion) = 2 THEN month.week2
WHEN (week expresion) = 3 THEN month.week3
WHEN (week expresion) = 4 THEN month.week4
ELSE month.week5
END as WeekValue
FROM month
OR
SELECT
CASE (week expresion)
WHEN 1 THEN month.week1
WHEN 2 THEN month.week2
WHEN 3 THEN month.week3
WHEN 4 THEN month.week4
ELSE month.week5
END as WeekValue
FROM month

Answer 2

dynamic sql sample:
t=# create table so58(i int,w1 text);
CREATE TABLE
t=# create or replace function so59(_n int) returns table (i int,w text) as $$begin
return query execute format('select i,w%s from so58',_n);
end;
$$ language plpgsql;
CREATE FUNCTION
t=# select * from so59(1);
i | w
---+---
(0 rows)