Combining Two Data Frames of Different Lengths

Merge dataframes, different lengths

You could add a join variable to dat2 then using merge:

dat2$variable <- rownames(dat2)
merge(dat1, dat2)
   variable ID value concreteness familiarity typicality
1    amoeba  1     0         3.60        1.30       1.71
2    amoeba  2     0         3.60        1.30       1.71
3    amoeba  3    NA         3.60        1.30       1.71
4 bacterium  1     0         3.82        3.48       2.13
5 bacterium  2     0         3.82        3.48       2.13
6 bacterium  3     0         3.82        3.48       2.13
7     leech  1     1         5.71        1.83       4.50
8     leech  2     1         5.71        1.83       4.50
9     leech  3     0         5.71        1.83       4.50

How can I combine two dataframes with different lengths in R?

Try using left_join in the dplyr package.

library(dplyr)

# make fake data
df1 <- data.frame(id = c("A", "B", "C", "D", "E"), val = rpois(5, 5))
df2 <- data.frame(id = c("A", "B", "C", "E"), val = rpois(4, 20))

# use left_join
df3 <- left_join(df1, df2, by = "id")

# rename and set NAs to 0
names(df3) <- c("id", "val", "val")
df3[is.na(df3)] <- 0

How to merge two dataframes with different lengths in python

I assume your output should look somewhat like this:

Week	Coeff1	Coeff2
1	-0.456662	0.571707
1	-0.533774	0.086152
1	-0.432871	0.824832
2	3	3
2	NaN	3

how to combine two data frames of different lengths?

This is too long for a comment, but really just need to demonstrate that the solution I gave in comments does work. If you are having problems with getting merge to work, then there must be some other issue with your data, which we cannot diagnose because you did not provide a dput of your data.frames

df1 = read.table(text = 
"Date    Duration
6/27/2014   10.00
6/30/2014   20.00
7/11/2014   15.00",
header = T)

df2 = read.table(text = 
"Date    Percent_Removal
6/27/2014   20.39
6/30/2014   27.01
7/7/2014    49.84
7/11/2014   59.48
7/17/2014   99.04",
header = T)

df1$Date <- as.Date (df1$Date, format= "%m/%d/%Y")
df2$Date <- as.Date (df2$Date, format= "%m/%d/%Y")

df3 = merge(df1,df2)
#         Date Duration Percent_Removal
# 1 2014-06-27       10           20.39
# 2 2014-06-30       20           27.01
# 3 2014-07-11       15           59.48

Note that no additional options need to be specified in the merge statement because

The default value by = is the column names that are common to both data frames. In this case, only Date is shared.
the default values of all.x, all.y and all give the desired behaviour where only the rows that are in both data frames are kept.

Merging data frames of different lengths without unique keys

dplyr::bind_rows(df1,df2)
# Warning in bind_rows_(x, .id) :
#   Unequal factor levels: coercing to character
# Warning in bind_rows_(x, .id) :
#   binding character and factor vector, coercing into character vector
# Warning in bind_rows_(x, .id) :
#   binding character and factor vector, coercing into character vector
#     Name Age School
# 1  Steve  10   <NA>
# 2  Peter  20   <NA>
# 3  Jason  NA    xyz
# 4 Nelson  NA    abc

You can alleviate some of this by pre-assigning unrecognized columns, which also works well with base R:

df2 <- cbind(df2, df1[NA,setdiff(names(df1), names(df2)),drop=FALSE])
df1 <- cbind(df1, df2[NA,setdiff(names(df2), names(df1)),drop=FALSE])
df1
#       Name Age School
# NA   Steve  10   <NA>
# NA.1 Peter  20   <NA>
df2
#        Name School Age
# NA    Jason    xyz  NA
# NA.1 Nelson    abc  NA

# ensure we use the same column order for both frames
nms <- names(df1)
rbind(df1[,nms], df2[,nms])
#         Name Age School
# NA     Steve  10   <NA>
# NA.1   Peter  20   <NA>
# NA1    Jason  NA    xyz
# NA.11 Nelson  NA    abc

Merge two python pandas data frames of different length but keep all rows in output data frame

You can read the documentation here: http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.merge.html

What you are looking for is a left join. The default option is an inner join. You can change this behavior by passing a different how argument:

df1.merge(df2,how='left', left_on='Column1', right_on='ColumnA')

How can I merge two data frames with different length and with two conditions in R?

You can use the left_join with by to join by multiple columns. You can use the following code:

library(dplyr)
df3 <- left_join(df2, df1, by = c("From" = "Country", "Year" = "Year"))

Output:

  Year From Vote  To Score
1    1   NE    1 Ger   0.8
2    1   NE    2   I   0.8
3    1   UK    2 Ger   0.9
4    1   UK    3   I   0.9
5    2   NE    2 Ger   0.7
6    2   NE    2   I   0.7
7    2   UK    4 Ger   1.0
8    2   UK    2   I   1.0