Paste variable name in mutate (dplyr)

dplyr 0.7.0 onwards does not require use of mutate_. Here is a solution using := to dynamically assign variable names and helper functions quo name.

It will be helpful to read vignette("programming", "dplyr") for more info. See also Use dynamic variable names in `dplyr` for older versions of dplyr.

df <- df %>%
  group_by(segment) %>%
  mutate( !!paste0('MeanSum',quo_name(nameVarPeriod1)) := 
mean(!!as.name(paste0('Sum',quo_name(nameVarPeriod1)))))

dplyr 1.0.0 alternative:

Using the new across function in dplyr 1.0.0 we can set names using glue style syntax and can include the function name and original column as part of the name:

my_fn <- function(nameVarPeriod1 = 'A2'){
  col_list <- paste0('Sum',nameVarPeriod1)
  df %>% 
    group_by(segment) %>%
    mutate(across(col_list, list(mean=mean), .names = "{fn}{col}"))
}

my_fn()
#   segment   SumA2 meanSumA2
#   <fct>     <dbl>     <dbl>
# 1 Seg5    107585.   107585.
# 2 Seg1    127344.    82080.
# 3 Seg4    205810.   205810.
# 4 Seg2    138453.    81528.
# 5 Seg2     24603.    81528.
# 6 Seg14    44444.    54422.
# 7 Seg11   103672    103672 
# 8 Seg6     88696.    88696.
# 9 Seg14    64400     54422.
#10 Seg1     36816.    82080.

Not sure what is your purpose of renaming summarized column name with original column names. But if you are looking for a solution where you want to have sum of multiple columns and hence wants to rename those then dplyr::mutate_at does it for you.

library(dplyr)
df %>% group_by(segment) %>%
  mutate(SumA3 = SumA2) %>%     #Added another column to demonstrate 
  mutate_at(vars(starts_with("SumA")), funs(mean = "mean"))

#  segment  SumA2  SumA3 SumA2_mean SumA3_mean
# <fctr>   <dbl>  <dbl>      <dbl>      <dbl>
# 1 Seg5    107585 107585     107585     107585
# 2 Seg1    127344 127344      82080      82080
# 3 Seg4    205810 205810     205810     205810
# 4 Seg2    138453 138453      81528      81528
# 5 Seg2     24603  24603      81528      81528
# 6 Seg14    44444  44444      54422      54422
# 7 Seg11   103672 103672     103672     103672
# 8 Seg6     88696  88696      88696      88696
# 9 Seg14    64400  64400      54422      54422
# 10 Seg1     36816  36816      82080      82080