I was recently skimming my Twitter feed for new Go articles, libraries, etc and I stumbled across a tweet with this message:. Regardless, this struck me as a big cry for help.

In JSON there are effectively two types of null. Unfortunately or fortunately? That field will always be present, and the value of that field will be nil or a valid value.

This leaves us with two options for PublishedAt - either it will reference an integer, or it will be nil. Where this does come into play is when we are doing partial updates. But what happens when the user does want to update this field? So how to we handle this in Go? The first thing to note is that we can customize how JSON gets unmarshalled for any type by implementing the Unmarshaler interface.

That means we need to make sure we DO NOT use a pointer if we need to determine if a key has been set. Putting those two tidbits together, we can create a custom JSONInt type that will be used to determine if a value has been set, and whether or not it is null.

Specifically, we can set a few flags to dictate whether or not the key was provided. If you want to see this in action, we can test it out with a few JSON strings using the code below, or you can run it on the Go Playground.

While there are likely better API designs to avoid this issue, if you do ever run into a situation where you need to support both types of null for a JSON key you can leverage this technique to make it happen. Simply replace the Value field with whatever type you need to support and you should be set.

Golang working with JSON: Maps vs. Structs

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golang check if struct field exists

Jon Calhoun is a full stack web developer who also teaches about Go, web development, algorithms, and anything programming related. He also consults for other companies who have development needs. If you need some development work done, get in touch!

Jon is a co-founder of EasyPost, a shipping API that many fortune companies use to power their shipping infrastructure, and prior to founding EasyPost he worked at google as a software engineer. Jon's latest progress update: Writing Course Notes.Welcome to tutorial no. A struct is a user-defined type that represents a collection of fields.

golang check if struct field exists

It can be used in places where it makes sense to group the data into a single unit rather than having each of them as separate values. For instance, an employee has a firstName, lastName and age. It makes sense to group these three properties into a single struct named Employee.

The above snippet declares a struct type Employee with fields firstNamelastName and age.

golang check if struct field exists

The above Employee struct is called a named struct because it creates a new data type named Employee using which Employee structs can be created. This struct can also be made more compact by declaring fields that belong to the same type in a single line followed by the type name. In the above struct firstName and lastName belong to the same type string and hence the struct can be rewritten as. Although the above syntax saves a few lines of code, it doesn't make the field declarations explicit.

Please refrain from using the above syntax. In line no. The order of the fields need not necessarily be the same as that of the order of the field names while declaring the struct type. In this case. This will work without any problems. In line In this case, it is necessary to maintain the order of fields to be the same as specified in the struct declaration. Please refrain from using this syntax since it makes it difficult to figure out which value is for which field.

We specified this format here just to understand that this is also a valid syntax :. It is possible to declare structs without creating a new data type. These types of structs are called anonymous structs. In line no 8. As we have already mentioned, this struct is called anonymous because it only creates a new struct variable emp3 and does not define any new struct type like named structs.

This program prints. When a struct is defined and it is not explicitly initialized with any value, the fields of the struct are assigned their zero values by default. The above program defines emp4 but it is not initialized with any value.

Hence firstName and lastName are assigned the zero values of string which is an empty string "" and agesalary are assigned the zero values of int which is 0. It is also possible to specify values for some fields and ignore the rest. In this case, the ignored fields are assigned zero values.Before learning Go, I used to write most of my applications in Python.

Why not use maps, which are equivalent to a Python dictionary. What were the benefits of using structs vs. The truth is, it all depends on how we use the JSON data; in some cases, maps work better than structs.

In most cases, structs are safer than maps. In today's article, I will explore the benefits of these techniques and highlight where one might be more useful than the other. To start breaking down these techniques, let's first establish the differences between parsing a JSON into a map and parsing into a struct.

We can use the following JSON as an example throughout our article. This nesting will help highlight differences in the parsing methods of maps and structs. We will first start parsing our JSON into a map. Once we've parsed the JSON, we can begin breaking down the code and explaining how to access our data once it is within a map.

So far, this seems pretty easy and straightforward. As far as parsing goes, the nice thing about this method is we don't need to know much about the JSON structure to parse it. The way we print that field, though, is very unsafe.

The first problem is, by just tossing data["name"] into the Printf function, we are assuming that the field is actually within the JSON.

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But what if it isn't? The bad news is, our program would panic because we tried to access a value that doesn't exist. If it does, the ok value will be true ; if not, the ok value will be false. Our code above uses type assertion to declare that the name value is a string type.

But what if it's not a string? JSON allows for many different types. When parsing with maps, the values are not type enforced. Meaning, everything is an interface until you tell it otherwise. Suppose we were to parse a JSON where the name key held a boolean instead of a string. Our code, as it stands today, would once again panic. It is unsafe to assume you know the type of value a JSON map is going to hold.

Now our code is finally safe to access the name data without any panics. But that took a lot of work for just one field. What does it take for a more complex field like our numbers list? Let's take a look. Our numbers are a lot more complicated because we have an array within our JSON. Any array is parsed and placed into the map as an array of interfaces.

But our assertions don't stop there; now we need to loop through the values and type assert each value individually. Where this gets tricky is when our values are not numbers. Technically, our JSON could have mixed types in this array; it could provide us with integers and strings.

As we check each value individually, we also need to put in logic that verifies the whole list's integrity.

justforfunc #29: dependency injection in a code review

When it comes to complicated JSON's like this, maps can be very problematic when using the data. The reason is that you have to account for all sorts of variations in your code. If you forget to type assert one value or forget to check if a key exists, your program will crash.Join Stack Overflow to learn, share knowledge, and build your career. Connect and share knowledge within a single location that is structured and easy to search.

While I don't need help with the actual validation, I would like know how to validate usernames only if it is included as part of the JSON object. At the moment, if a username isn't included then User. Username will still exist but be empty i. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams?

Learn more. Validate struct field if it exists Ask Question. Asked 5 years, 8 months ago. Active 5 years, 8 months ago. Viewed 13k times. NewDecoder req. I'm not sure I understand, why not check if len user. If a user object is posted without username then len user. Username will still equal 0. I want to conditionally validate - that is, I want to validate the username only if it was included in the POST object.

Sorry if it's unclear! So I need to be able to distinguish between a User object than contains a username with value "" i. Add a comment. Active Oldest Votes. Println "username set:", u. Username json. Println "username set:", u2. Interesting, I didn't know it could be done like this. I'm going to test in the morning and will tick then if it's all good! I've got this going.Sign in to comment. Sign in to answer this question.

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How to determine if a JSON key has been set to null or not provided

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You are now following this question You will see updates in your activity feed. You may receive emails, depending on your notification preferences. How to check the existance of a struct field at a given index? Show older comments.

How to check the existance of a struct field at a given index?

Vote 0. Commented: madhan ravi on 16 Oct Accepted Answer: madhan ravi. I want to check existance of p 2. How to do? Cancel Copy to Clipboard. Stephen Cobeldick on 16 Oct Every element of a structure array has the same fields. So if you have defined this:. Then by definition of a non-scalar structure the field p 2. If you have not allocated it a value, then it will be initialized to the empty array.

This means it is not clear what your question relates to:. Note that exist is totally irrelevant to this problem. Accepted Answer. Read the below link:. I think, this cannot help to me, because I dond wnat to check the hole filed, just the existance of the value of thet filed at a given index, as in the example above!A map in Golang is a collection of unordered pairs of key-value.

golang check if struct field exists

It is widely used because it provides fast lookups and values that can retrieve, update or delete with the help of keys. In maps, most of the data types can be used as a key like int, string, float64, rune, etc. Maps also allow structs to be used as keys. These structs should be compared with each other. A structure or struct in Golang is a user-defined type that allows to combine fields of different types into a single type.

Println str. Skip to content. Related Articles. Last Updated : 10 May, Name string. Pincode int. Println "True". Println "False". Println sample. Println str, val. Println "Original map was"mp. Println "The remaining map after deletion:". Println "Is the key present:"check. Println "Value of the key:"value. Println "Is the key present:"check2. Recommended Articles. Article Contributed By :.ContainsAny function is used to check whether any Unicode code points in chars are present in the string.

It is an inbuilt function used to find if a specified string exists in the string if found it will return true or else false. Here, the first parameter is the original string and the second parameter is the substring or a set of characters which is to be found within the string. Even if one of the characters in the substring is found in the string, the function returns true. The function returns a boolean value i. Writing code in comment? Please use ide. Skip to content.

Related Articles. Last Updated : 08 Jun, Syntax: func ContainsAny str, charstr string bool Here, the first parameter is the original string and the second parameter is the substring or a set of characters which is to be found within the string. ContainsAny str1, "Geeks". ContainsAny str2, "GFG". ContainsAny "GeeksforGeeks""Gz". ContainsAny "GeeksforGeeks""ue". ContainsAny "GeeksforGeeks"" ". Println "Result 2: "res2. Println "Result 3: "res3. Println "Result 4: "res4. Println "Result 5: "res5.

Geometric in Python.

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