Using Reduce() function in IOS9 swift2.0

Question

I am in process of learning swift and web services.

When I tried to compile the following code in IOS 9, I get

reduce is unavailable: call the reduce() method on the sequence

private class func urlWithName(name: String, var args: [String: String]) -> NSURL
{
    args["username"] = "ijoshsmith"
    let
    baseURL      = "http://api.geonames.org/",
    queryString  = queryWithArgs(args),
    absolutePath = baseURL + name + "?" + queryString
    return NSURL(string: absolutePath)!
}

private class func queryWithArgs(args: [String: String]) -> String
{
    let parts: [String] = reduce(args, [])
        {
            result, pair in
            let
            key   = pair.0,
            value = pair.1,
            part  = "\(key)=\(value)"
            return result + [part]
    }
    return (parts as NSArray).componentsJoinedByString("&")
}

So, I understand that I should covert reduce(args, []) to something like args.reduce(initial: T, combine:(T, Self.Generator.Element) throws -> T). Can someone help me in explaining and converting it.

thanks.

The error the compiler gave means that - You can use the reduce method on arrays(sequence).The high order functions like, map(), reduce() & filter() are meant to be used with Sequence / Array. — iamyogish
– iamyogish, Commented Oct 13, 2015 at 6:30
Also Please follow this link : weheartswift.com/… to learn more about the higher order functions. Trust me they're very useful. :) HTH — iamyogish
– iamyogish, Commented Oct 13, 2015 at 6:31

Caleb · Accepted Answer · 2015-10-13 07:29:13Z

2

let parts: [String] = reduce(args, [])

You're trying to call reduce as a standalone function, but it's actually a method in the SequenceType protocol, which is adopted by Array. Assuming that args is the array to which you want to apply reduce, you'd call it like:

foo = args.reduce(t, f)

where t is the initial value and f is the combining function.

edited Oct 13, 2015 at 7:29

answered Oct 13, 2015 at 6:29

Caleb

125k19 gold badges189 silver badges282 bronze badges

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

6 Comments

iamyogish Over a year ago

Just to add a point, the result of the reduce() will be of type 't'

Aaron Brager Over a year ago

A minor clarification: it's not a member function of arrays, it's part of SequenceType.

Caleb Over a year ago

@AaronBrager SequenceType is the protocol that declares reduce(), but Array still has to provide the implementation. It's a good point to mention SequenceType, though, and I'll add that. Thanks.

Aaron Brager Over a year ago

If I recall correctly, it's implemented as a default protocol implementation, not on Array, according to a stack trace I looked at. I might be remembering wrong though.

Aaron Brager Over a year ago

Here's the stack trace I was thinking of - my interpretation might be wrong though. Let me know what you think

|

Aaron Brager · Accepted Answer · 2015-10-13 06:29:08Z

1

The correct syntax looks like this:

private class func queryWithArgs(args: [String: String]) -> String
{
    let parts: [String] = args.reduce([]) {
            result, pair in
            let
            key   = pair.0,
            value = pair.1,
            part  = "\(key)=\(value)"
            return result + [part]
    }
    return parts.joinWithSeparator("&")
}

I also updated your return value to avoid an NSArray cast.

You might also want to see Alamofire's ParameterEncoding.swift, which has a more thorough approach to parameter encoding already written.

answered Oct 13, 2015 at 6:29

Aaron Brager

66.4k21 gold badges169 silver badges295 bronze badges

Collectives™ on Stack Overflow

Using Reduce() function in IOS9 swift2.0

2 Answers 2

6 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related