Suppose your file looks like:
$ cat in.txt
Day 1
1 10
2 20
3 15
4 20
Day 2
1 25
2 30
3 40
4 35
Day 3
1 40
2 45
3 30
4 15
A little Unix shell magic can be used to help Julia parse this as follows:
julia> using CSV, DataFrames
julia> CSV.read(pipeline(`awk 'BEGIN { day = "0"; print "day\ttime\tvalue" } /^Day/ { day = $2} /^[^D]/ {print day,"\t" $0}' in.txt`), DataFrame; delim='\t')
12×3 DataFrame
Row │ day time value
│ Int64 Int64 Int64
─────┼─────────────────────
1 │ 1 1 10
2 │ 1 2 20
3 │ 1 3 15
4 │ 1 4 20
5 │ 2 1 25
6 │ 2 2 30
7 │ 2 3 40
8 │ 2 4 35
9 │ 3 1 40
10 │ 3 2 45
11 │ 3 3 30
12 │ 3 4 15
The key useful info is the way awk tool is used to separate the important data lines from the intra-file headers, and to process the data in the header to enable adding it later to each line separately. This is the awk command on its own:
awk 'BEGIN { day = "0"; print "day\ttime\tvalue" } /^Day/ { day = $2} /^[^D]/ {print day,"\t" $0}' in.txt
Hopefully the awk logic is clear enough.
P.S. screenshots are annoying in a post. Try to insert copy-pasteable stuff