Creating lagged variable

[Daniel Abraham]

* Example generated by -dataex-. To install: ssc install dataex
clear
input str9 date str31 brand_name int demand
"02jun2021" "Baskin Robbins"  3
"03jun2021" "Baskin Robbins"  3
"04jun2021" "Baskin Robbins"  3
"05jun2021" "Baskin Robbins"  1
"06jun2021" "Baskin Robbins"  3
"07jun2021" "Baskin Robbins"  3
"10jun2021" "Baskin Robbins"  1
"11jun2021" "Baskin Robbins"  7
"12jun2021" "Baskin Robbins"  3
"13jun2021" "Baskin Robbins"  8
"14jun2021" "Baskin Robbins"  1
"15jun2021" "Baskin Robbins"  4
"16jun2021" "Baskin Robbins"  1
"17jun2021" "Baskin Robbins"  4
"18jun2021" "Baskin Robbins"  2
"19jun2021" "Baskin Robbins"  5
"20jun2021" "Baskin Robbins"  4
"21jun2021" "Baskin Robbins"  2
"22jun2021" "Baskin Robbins"  2
"23jun2021" "Baskin Robbins"  3
"24jun2021" "Baskin Robbins"  5
"25jun2021" "Baskin Robbins"  3
"26jun2021" "Baskin Robbins"  6
"27jun2021" "Baskin Robbins"  2
"28jun2021" "Baskin Robbins"  3
"29jun2021" "Baskin Robbins"  1
"30jun2021" "Baskin Robbins"  3
"01jul2021" "Baskin Robbins"  3
"02jul2021" "Baskin Robbins"  2
"03jul2021" "Baskin Robbins"  3
"04jul2021" "Baskin Robbins"  2
"05jul2021" "Baskin Robbins"  1
"06jul2021" "Baskin Robbins"  2
"07jul2021" "Baskin Robbins"  5
"08jul2021" "Baskin Robbins"  3
"09jul2021" "Baskin Robbins"  2
"10jul2021" "Baskin Robbins"  6
"11jul2021" "Baskin Robbins"  3
"12jul2021" "Baskin Robbins"  4
"13jul2021" "Baskin Robbins"  1
"16jul2021" "Baskin Robbins"  3
"17jul2021" "Baskin Robbins"  1
"18jul2021" "Baskin Robbins"  1
"19jul2021" "Baskin Robbins"  3
"22jul2021" "Baskin Robbins"  2
"23jul2021" "Baskin Robbins"  2
"24jul2021" "Baskin Robbins"  4
"25jul2021" "Baskin Robbins" 11
"26jul2021" "Baskin Robbins"  6
"27jul2021" "Baskin Robbins"  4
"28jul2021" "Baskin Robbins"  4
"29jul2021" "Baskin Robbins"  4
"30jul2021" "Baskin Robbins"  4
"31jul2021" "Baskin Robbins"  4
"01aug2021" "Baskin Robbins"  5
"03aug2021" "Baskin Robbins"  2
"07aug2021" "Baskin Robbins"  4
"08aug2021" "Baskin Robbins"  5
"09aug2021" "Baskin Robbins"  1
"11aug2021" "Baskin Robbins"  2
"12aug2021" "Baskin Robbins"  2
"13aug2021" "Baskin Robbins"  1
"14aug2021" "Baskin Robbins"  2
"15aug2021" "Baskin Robbins"  4
"16aug2021" "Baskin Robbins"  3
"17aug2021" "Baskin Robbins"  2
"18aug2021" "Baskin Robbins"  3
"19aug2021" "Baskin Robbins"  4
"20aug2021" "Baskin Robbins"  2
"21aug2021" "Baskin Robbins"  3
"22aug2021" "Baskin Robbins"  3
"23aug2021" "Baskin Robbins"  2
"24aug2021" "Baskin Robbins"  1
"25aug2021" "Baskin Robbins"  1
"26aug2021" "Baskin Robbins"  3
"27aug2021" "Baskin Robbins"  5
"28aug2021" "Baskin Robbins"  2
"29aug2021" "Baskin Robbins"  2
"30aug2021" "Baskin Robbins"  2
"31aug2021" "Baskin Robbins"  2
"01sep2021" "Baskin Robbins"  2
"02sep2021" "Baskin Robbins"  4
"03sep2021" "Baskin Robbins"  3
"04sep2021" "Baskin Robbins"  4
"05sep2021" "Baskin Robbins"  7
"06sep2021" "Baskin Robbins"  1
"07sep2021" "Baskin Robbins"  2
"08sep2021" "Baskin Robbins"  1
"09sep2021" "Baskin Robbins"  4
"10sep2021" "Baskin Robbins"  1
"11sep2021" "Baskin Robbins"  3
"12sep2021" "Baskin Robbins"  5
"13sep2021" "Baskin Robbins"  1
"14sep2021" "Baskin Robbins"  1
"15sep2021" "Baskin Robbins"  5
"17sep2021" "Baskin Robbins"  6
"18sep2021" "Baskin Robbins"  6
"19sep2021" "Baskin Robbins" 12
"20sep2021" "Baskin Robbins"  3
"21sep2021" "Baskin Robbins" 11
end

I want to create week lags for the variable demand. For example, 27 Aug 2021 is a Friday; I want to create a variable with last week's Friday demand (Demand on 20 Aug 2021). If that information isn't available, demand on 13 Aug 2021 (Friday) should be captured. If demand on 13 Aug 2021 isn't available, it should return zero. The variable should not capture the demand on other days except Friday. The variable brand_name has multiple brands, not only Baskin Robbins. Hence these lagged values should be created for each of the brands. How should I proceed with the code for creating the new lagged demand variable?

[Emil Alnor]

ren date date_str
gen date = date(date_str, "DMY")
gen dayofweek = dow(date)
gen friday = dayofweek==5
sort brand date
gen lag_demand = .
forval x = 1/14{
    replace lag_demand = demand[_n-`x'] if friday==1 & friday[_n-`x'] == 1 & (date-date[_n-`x']==7) & brand==brand[_n-`x']
    replace lag_demand = demand[_n-`x'] if lag_demand==. & friday==1 & friday[_n-`x'] == 1 & (date-date[_n-`x']==14) & brand==brand[_n-`x']
}