The correct use of findfirst

DOCS> “You can search for the index of a particular character
using the findfirst and findlast functions.”

julia> findfirst(isequal('('), "André(m, n)")

julia> findfirst(isequal('('), "Andre(m, n)")

It seems clear that this can lead to mistakes.
What’s the way out?

1 Like

In this case, findfirst is giving you precisely the index you would use to extract that character:

julia> s = "André(m, n)"
"André(m, n)"

julia> s[7]
'(': ASCII/Unicode U+0028 (category Ps: Punctuation, open)

Is your actual goal something other than this? Do you want to count the number of letters that occur before the ( character?


Is your actual goal something other than this?

For example consider:

function extractName(s::String)::String
    i = findfirst(isequal('('), s)
    return s[1:(i-1)]

Here I expect

extractName("Andre(m, n)") |> println

to return “Andre”, which is what it does. Similarly I expect

extractName("André(m, n)") |> println

to return “André” but I get

ERROR: LoadError: StringIndexError("André(m, n)", 6)

Have you read through

Not all indices into a string are valid, since some indices might not lie exactly at the start of a character (and some characters span multiple bytes). Instead, you can use prevind to extract the previous valid index:

julia> s = "André(m, n)"
"André(m, n)"

julia> i = findfirst(isequal('('), s)

julia> s[1:prevind(s, i)]

Although in this case a regular expression might be more appropriate:

julia> m = match(r"(.*)\(", s)
RegexMatch("André(", 1="André")

julia> m.captures[1]