What's the reason that I can not successfully create an URL by using a string from a function i-CodePudding

I am a newbie in IOS development and recently take one course online. For one fetchWeather(cityName: String) function, I am trying to create an URL by a string that has the name of a weather forecast website, my API key and the city name. However, I find that if I use the cityName from the function, the URL fails. The wired thing is if I do not use the argument value, but create a new local variable city_name in the function that has the same value with cityName, which is the argument value, the URL succeed. Here are my two test cases:

func fetchWeather(cityName: String) {
        // The first case that use the argument of the function
        print("First test case: ")
        print(cityName)
        let weatherURL = "https://api.openweathermap.org/data/2.5/weather?appid=2b5e3c1ae10223238e8bbf73b814e563&units=metric&q="
        let first_urlString = weatherURL cityName
        guard let first_url = URL(string:first_urlString)  else{
            print("First_URL fails")
        
        // The second case that directly combine two strings
            print("\nSecond test case: ")
            let city_name = "London"
            let second_url_String = weatherURL   city_name
            guard let second_url = URL(string:second_url_String)  else{
                print("Second_URL fails")
                return
            }
            print("Second_URL succeed")
            
            print("\n" first_urlString)
            print(second_url_String)
            print("The first string is equals to the second string: "   String(first_urlString==second_url_String))
            
            return
        }
        print("First_URL succeed")
        
        
    }

And here's the output:

First test case: 
London 
First_URL fails

Second test case: 
Second_URL succeed

https://api.openweathermap.org/data/2.5/weather?appid=2b5e3c1ae10223238e8bbf73b814e563&units=metric&q=London 
https://api.openweathermap.org/data/2.5/weather?appid=2b5e3c1ae10223238e8bbf73b814e563&units=metric&q=London
The first string is equals to the second string: false

Those two url_strings look exactly the same, so why they are different? I believe that the difference between those two url_strings is the problem that my first URL fails and the second one succeed.

CodePudding user response：

Good job actually cutting and pasting your exact output into your question. That's how I was able to diagnose this.

The cityName you're passing to fetchWeather has a U 0020 SPACE at the end.

00000000  4c 6f 6e 64 6f 6e 20                              |London |
00000007

The URL parser doesn't like that. Try this instead:

let first_urlString = weatherURL   cityName.trimmingCharacters(in: .whitespacesAndNewlines)