Reduce to concatenate String Array? Faster solution?

Question

I use code like this to build one string from an string array:

class ActionCMD {   // "class" to avoid value copies when updating string members
    var cmds = [String]()   // simply the list of all strings to speed up inserting
    var cmd : String { return cmds.reduce("", +) }   // resulting string
}

But for 35.000 strings it needs 15 min. Is there a better (quicker) way to do the concatenating?

How do you get your input set of 35000 String? Perhaps some preprocessing can be made to speed things up — Kamil.S
– Kamil.S, Commented Nov 24, 2019 at 11:37
Is it definitely this code that takes 15 minutes and not the loading of the 35,000 strings? — Chris
– Chris, Commented Nov 24, 2019 at 11:54
Executing your code, the reduce, in a playground takes 0.0003 seconds for me on a few year old MacBook Pro so even though my test is far from precise I feel the issue must lay somewhere else for you — Joakim Danielson
– Joakim Danielson, Commented Nov 24, 2019 at 12:20

ielyamani · Accepted Answer · 2019-11-24 12:25:57Z

2

You should avoid creating intermediate strings. Instead, mutate the previously accumulated string, by using either:

cmds.reduce(into: "", { $0 += $1 })

or :

cmds.joined()

or simply :

var cmd = ""
for i in cmds.indices {
    cmd += cmds[i]
}

^{More on String concatenation here}

edited Nov 24, 2019 at 12:25

answered Nov 24, 2019 at 12:19

ielyamani

18.6k11 gold badges59 silver badges94 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Peter71 Over a year ago

Thanks, I will try "joined". I don't use a loop to do it by my own. Like to use library functions.

Peter71 Over a year ago

Its now fast! :-) I think 30 s instead of minutes! Thanks a lot.

ielyamani Over a year ago

@Peter71 That's quite a difference! Glad I could be of help :)

Collectives™ on Stack Overflow

Reduce to concatenate String Array? Faster solution?

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related