title: Adding a single number to a column in a julia dataframe?
body: <p>I want to add the number 5 to a column in my julia dataframe. How do I do that?</p>
  
  <pre><code>julia&gt; using DataFrames, CSV
  
  julia&gt; iris = CSV.read(joinpath(Pkg.dir("DataFrames"), "test/data/iris.csv"));
  
  julia&gt; head(iris)
  6×5 DataFrame
  │ Row │ SepalLength │ SepalWidth │ PetalLength │ PetalWidth │ Species │
  ├─────┼─────────────┼────────────┼─────────────┼────────────┼─────────┤
  │ 1   │ 5.1         │ 3.5        │ 1.4         │ 0.2        │ setosa  │
  │ 2   │ 4.9         │ 3.0        │ 1.4         │ 0.2        │ setosa  │
  │ 3   │ 4.7         │ 3.2        │ 1.3         │ 0.2        │ setosa  │
  │ 4   │ 4.6         │ 3.1        │ 1.5         │ 0.2        │ setosa  │
  │ 5   │ 5.0         │ 3.6        │ 1.4         │ 0.2        │ setosa  │
  │ 6   │ 5.4         │ 3.9        │ 1.7         │ 0.4        │ setosa  │
  
  julia&gt; iris[:SepalLength] += 5
  
  ERROR: MethodError: no method matching +(::Array{Union{Missing, Float64},1}, ::Int64)
  Closest candidates are:
    +(::Any, ::Any, ::Any, ::Any...) at operators.jl:502
    +(::Complex{Bool}, ::Real) at complex.jl:292
    +(::Missing, ::Number) at missing.jl:93
    ...
  Stacktrace:
   [1] top-level scope at none:0
  
  julia&gt; map(iris[2], x -&gt; x + 5)
  ERROR: MethodError: no method matching iterate(::getfield(Main, Symbol("##33#34")))
  Closest candidates are:
    iterate(::Core.SimpleVector) at essentials.jl:578
    iterate(::Core.SimpleVector, ::Any) at essentials.jl:578
    iterate(::ExponentialBackOff) at error.jl:171
    ...
  Stacktrace:
   [1] iterate at ./generator.jl:44 [inlined]
   [2] collect(::Base.Generator{getfield(Main, Symbol("##33#34")),Array{Union{Missing, Float64},1}}) at ./array.jl:619
   [3] map(::Array{Union{Missing, Float64},1}, ::Function) at ./abstractarray.jl:2013
   [4] top-level scope at none:0
  </code></pre>