如何计算因子列的水平发生率?[副本]

df <- data.frame(
    cola = c('1','b','c','1','1','e','1',NA,'c','d'),
    colb = c("A",NA,"C","D",'a','b','c','d','c','d'),
    colc = c('a','b','c','d','a','b','c','d','c','d'),stringsAsFactors = TRUE)

df$cola 是 1 b c 1 1 e 1 <NA> c d
我想知道有多少 1 在这个专栏里(答案是4),怎么做?