How to use for loop when I have different length in R? [duplicate]

I have two dataframes df1 and df2 and `df2':

df1 = structure(list(Day = c(19L, 20L, 20L, 20L, 20L, 21L, 21L, 21L, 
21L, 21L, 21L, 21L, 22L, 22L, 22L, 22L, 22L, 22L, 22L, 22L), 
    Month = c(9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 
    9, 9, 9, 9), Year = c(2004, 2004, 2004, 2004, 2004, 2004, 
    2004, 2004, 2004, 2004, 2004, 2004, 2004, 2004, 2004, 2004, 
    2004, 2004, 2004, 2004), Date = structure(c(12680, 12681, 
    12681, 12681, 12681, 12682, 12682, 12682, 12682, 12682, 12682, 
    12682, 12683, 12683, 12683, 12683, 12683, 12683, 12683, 12683
    ), class = "Date")), row.names = c("text1", "text2", "text3", 
"text4", "text5", "text6", "text7", "text8", "text9", "text10", 
"text11", "text12", "text13", "text14", "text15", "text16", "text17", 
"text18", "text19", "text20"), class = "data.frame")

       Day Month Year       Date
text1   19     9 2004 2004-09-19
text2   20     9 2004 2004-09-20
text3   20     9 2004 2004-09-20
text4   20     9 2004 2004-09-20
text5   20     9 2004 2004-09-20
text6   21     9 2004 2004-09-21
text7   21     9 2004 2004-09-21
text8   21     9 2004 2004-09-21
text9   21     9 2004 2004-09-21
text10  21     9 2004 2004-09-21
text11  21     9 2004 2004-09-21
text12  21     9 2004 2004-09-21
text13  22     9 2004 2004-09-22
text14  22     9 2004 2004-09-22
text15  22     9 2004 2004-09-22
text16  22     9 2004 2004-09-22
text17  22     9 2004 2004-09-22
text18  22     9 2004 2004-09-22
text19  22     9 2004 2004-09-22
text20  22     9 2004 2004-09-22

df2 = structure(list(days = structure(c(12680, 12681, 12682, 12683, 
 12684, 12685, 12686, 12687, 12688, 12689, 12690, 12691, 12692, 
 12693, 12694), class = "Date"), week = c(3, 3, 3, 4, 4, 4, 4, 
 4, 4, 4, 4, 4, 1, 1, 1)), row.names = c(NA, -15L), class = 
 "data.frame")

         days week
1  2004-09-19    3
2  2004-09-20    3
3  2004-09-21    3
4  2004-09-22    4
5  2004-09-23    4
6  2004-09-24    4
7  2004-09-25    4
8  2004-09-26    4
9  2004-09-27    4
10 2004-09-28    4
11 2004-09-29    4
12 2004-09-30    4
13 2004-10-01    1
14 2004-10-02    1
15 2004-10-03    1

As you can see df1 is longer than df2 and contains all the "dates" in df2. What I would like to get is a code that says: if the date is df1 is the same as the date in df2, then give me the weeknumber. I manage to do it individually:

if (df1$Date[1] == df2$days[1]) df2$week[1]

if (df1$Date[2] == df2$days[2]) df2$week[2] 

if (df1$Date[3] == df2$days[2]) df2$week[2] # this is the issue: df1 is longer than df2 since it repeats dates more than ones

if (df1$Date[4] == df2$days[2]) df2$week[2] # same as above
...

The problem is that when I try to loop for every element in df1 and df2 I fail since the two have different length since df1 repeats the same date n times. So I would need a loop that takes care of the repetions of dates in df1. Yet I got stuck here:

for (i in length(# don't know how to handle the different length)) {
if (df1$Date[1] == df2$days[1]) df2$week[1]
}

Can anyone help me sort this out?

Thanks!