Check if a key exists in a map in Go

2022-11-06T19:08:58.026Z

A map is a data structure and an unordered collection of key-value pairs where keys are the unique identifiers. If you are from a Python background, this is similar to a dictionary.

The main advantage of using the map is retrieving the value based on the key. However, if you try to access the key that does not exist on the map, you will get zero as the output.

Let us take an example to demonstrate the issue.

// Program to retrieve a key-value from the map

package main

import "fmt"

func main() {

    // create a map
    cars := map[string]int{
        "Audi":       90000,
        "Ford":       50000,
        "Volkswagen": 70000,
        "BMW":        40000}

    fmt.Println("Adui Car Price:", cars["Audi"])
    fmt.Println("Maruti Car Price:", cars["Maruti"])

}

Output

Adui Car Price: 90000
Maruti Car Price: 0

Check if the map has a key in Go

We can solve the problem by using the index expression.

Syntax

if val, ok := cars["Maruti"]; ok {
    //do something here
}

Shorthand Syntax

This is a shorthand syntax if you just want to check if the key exists in the map.

_, exists := dict[key_name]

In Go, if statements can be used for conditional checks and initialization statements. If we look at the above example:

We have initialized two variables, val and ok.
The val variable will receive the value of the Maruti from the map; otherwise, it receives zero if the value is empty.
If the Maruti key exists in the map, the ok variable will receive true; otherwise, false.
Next, we evaluate if the variable ok is true, and then the code block inside if statement gets executed.

Let us now implement this in our example and retrieve the value from the map for a given key.

// Program to retrieve a key-value from the map

package main

import "fmt"

func main() {

    // create a map
    cars := map[string]int{
        "Audi":       90000,
        "Ford":       50000,
        "Volkswagen": 70000,
        "BMW":        40000}

    if val, ok := cars["Audi"]; ok {
        fmt.Println("Adui Car Price:", val)
    } else {
        fmt.Println("Audi Car Doesnt exist")
    }

    if val, ok := cars["Maruti"]; ok {
        fmt.Println("Maruti Car Price:", val)
    } else {
        fmt.Println("Maruti Car Doesn't exist")
    }

}

Output

Adui Car Price: 90000
Maruti Car Doesn't exist

Note: The value and ok variables are only available inside the if block or else block or else if block (local scope). So you cannot access these variables outside this block.

If you want to access variables outside the if block, we can re-write the example as shown below.

// Program to retrieve a key-value from the map

package main

import "fmt"

func main() {

    // create a map
    cars := map[string]int{
        "Audi":       90000,
        "Ford":       50000,
        "Volkswagen": 70000,
        "BMW":        40000}

    val, ok := cars["Audi"]
    if ok {
        fmt.Println("Adui Car Price:", val)
    } else {
        fmt.Println("Audi Car Doesn't exist")
    }

}

Adui Car Price: 90000

Note: Apart from the variable access there is no other change in both the examples. The first example is more compact when compared the the second example.