[R] a little question about R

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Hello,sir: Here's a little question about R which needs your help.Thanks in advance. 
If I wanna make a sequence just like a,b,c,d (In other words,a vector consists of 4 characters :a,b,c,d ).How can I do it in a shortcut manner? Yes,I can do it as following: c("a","b","c","d") and the result is:[1] "a" "b" "c" "d".
But I remember there's a shortcut manner to do the same thing,something like 
c("a":"d").But the system said:"Error in "a":"d" : NA/NaN argument" So I wonder the correct method to do it(I remember the expression is very similar with
c("a":"d")). Thanks from the bottom of my heart. My best regards!