How to subtract each Country's value by year - r

Question

I have data for each Country's happiness (https://www.kaggle.com/unsdsn/world-happiness), and I made data for each year of the reports. Now, I don't know how to get the values for each year subtracted from each other e.g. how did happiness rank change from 2015 to 2017/2016 to 2017? I'd like to make a new df of differences for each.
I was able to bind the tables for columns in common and started to work on removing Countries that don't have data for all 3 years. I'm not sure if I'm going down a complicated path.
keepcols <- c("Country","Happiness.Rank","Economy..GDP.per.Capita.","Family","Health..Life.Expectancy.","Freedom","Trust..Government.Corruption.","Generosity","Dystopia.Residual","Year")
mydata2015 = read.csv("C:\\Users\\mmcgown\\Downloads\\2015.csv")
mydata2015$Year <- "2015"
data2015 <- subset(mydata2015, select = keepcols )
mydata2016 = read.csv("C:\\Users\\mmcgown\\Downloads\\2016.csv")
mydata2016$Year <- "2016"
data2016 <- subset(mydata2016, select = keepcols )
mydata2017 = read.csv("C:\\Users\\mmcgown\\Downloads\\2017.csv")
mydata2017$Year <- "2017"
data2017 <- subset(mydata2017, select = keepcols )
df <- rbind(data2015,data2016,data2017)
head(df, n=10)
tail(df, n=10)
df15 <- df[df['Year']=='2015',]
df16 <- df[df['Year']=='2016',]
df17 <- df[df['Year']=='2017',]
nocon <- rbind(setdiff(unique(df16['Country']),unique(df17['Country'])),setdiff(unique(df15['Country']),unique(df16['Country'])))
Don't have a clear path to accomplish what I want but it would look like
df16_to_17
Country Happiness.Rank ...(other columns)
Yemen (Yemen[Happiness Rank in 2017] - Yemen[Happiness Rank in 2016])
USA (USA[Happiness Rank in 2017] - USA[Happiness Rank in 2016])
(other countries)
df15_to_16
Country Happiness.Rank ...(other columns)
Yemen (Yemen[Happiness Rank in 2016] - Yemen[Happiness Rank in 2015])
USA (USA[Happiness Rank in 2016] - USA[Happiness Rank in 2015])
(other countries)

Answer 1

It's very straightforward with dplyr, and involves grouping by country and then finding the differences between consecutive values with base R's diff. Just make sure to use df and not df15, etc.:
library(dplyr)
rank_diff_df <- df %>%
group_by(Country) %>%
mutate(Rank.Diff = c(NA, diff(Happiness.Rank)))
The above assumes that the data are arranged by year, which they are in your case because of the way you combined the dataframes. If not, you'll need to call arrange(Year) before the call to mutate. Filtering out countries with missing year data isn't necessary, but can be done after group_by() with filter(n() == 3).
If you would like to view the differences it would make sense to drop some variables and rearrange the data:
rank_diff_df %>%
select(Year, Country, Happiness.Rank, Rank.Diff) %>%
arrange(Country)
Which returns:
# A tibble: 470 x 4
# Groups: Country [166]
Year Country Happiness.Rank Rank.Diff
<chr> <fct> <int> <int>
1 2015 Afghanistan 153 NA
2 2016 Afghanistan 154 1
3 2017 Afghanistan 141 -13
4 2015 Albania 95 NA
5 2016 Albania 109 14
6 2017 Albania 109 0
7 2015 Algeria 68 NA
8 2016 Algeria 38 -30
9 2017 Algeria 53 15
10 2015 Angola 137 NA
# … with 460 more rows
The above data frame will work well with ggplot2 if you are planning on plotting the results.
If you don't feel comfortable with dplyr you can use base R's merge to combine the dataframes, and then create a new dataframe with the differences as columns:
df_wide <- merge(merge(df15, df16, by = "Country"), df17, by = "Country")
rank_diff_df <- data.frame(Country = df_wide$Country,
Y2015.2016 = df_wide$Happiness.Rank.y -
df_wide$Happiness.Rank.x,
Y2016.2017 = df_wide$Happiness.Rank -
df_wide$Happiness.Rank.y
)
Which returns:
head(rank_diff_df, 10)
Country Y2015.2016 Y2016.2017
1 Afghanistan 1 -13
2 Albania 14 0
3 Algeria -30 15
4 Angola 4 -1
5 Argentina -4 -2
6 Armenia -6 0
7 Australia -1 1
8 Austria -1 1
9 Azerbaijan 1 4
10 Bahrain -7 -1

Answer 2

Assuming the three datasets are present in your environment with the name data2015, data2016 and data2017, we can add a year column with the respective year and keep the columns which are present in keepcols vector. arrange the data by Country and Year, group_by Country, keep only those countries which are present in all 3 years and then subtract the values from previous rows using lag or diff.
library(dplyr)
data2015$Year <- 2015
data2016$Year <- 2016
data2017$Year <- 2017
df <- bind_rows(data2015, data2016, data2017)
data <- df[keepcols]
data %>%
arrange(Country, Year) %>%
group_by(Country) %>%
filter(n() == 3) %>%
mutate_at(-1, ~. - lag(.)) #OR
#mutate_at(-1, ~c(NA, diff(.)))
# A tibble: 438 x 10
# Groups: Country [146]
# Country Happiness.Rank Economy..GDP.pe… Family Health..Life.Ex… Freedom
# <chr> <int> <dbl> <dbl> <dbl> <dbl>
# 1 Afghan… NA NA NA NA NA
# 2 Afghan… 1 0.0624 -0.192 -0.130 -0.0698
# 3 Afghan… -13 0.0192 0.471 0.00731 -0.0581
# 4 Albania NA NA NA NA NA
# 5 Albania 14 0.0766 -0.303 -0.0832 -0.0387
# 6 Albania 0 0.0409 0.302 0.00109 0.0628
# 7 Algeria NA NA NA NA NA
# 8 Algeria -30 0.113 -0.245 0.00038 -0.0757
# 9 Algeria 15 0.0392 0.313 -0.000455 0.0233
#10 Angola NA NA NA NA NA
# … with 428 more rows, and 4 more variables: Trust..Government.Corruption. <dbl>,
# Generosity <dbl>, Dystopia.Residual <dbl>, Year <dbl>
The value of first row for each Year would always be NA, rest of the values would be subtracted by it's previous values.