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Go Pass a Slice to Function
Last Updated : July 21, 2025
In Go, slices are passed to functions by reference, meaning that any modification made to the slice within the function will affect the original slice.
Passing a Slice to a Function
You can pass a slice as a parameter to a function just like any other data type. Since slices internally contain a pointer to the underlying array, the function receives a copy of the slice header but points to the same underlying data.
Example
This example demonstrates how changes inside a function affect the original slice:
package main
import "fmt"
func modifySlice(s []int) {
s[0] = 100
s = append(s, 999) // Appending creates a new underlying array if capacity exceeds
fmt.Println("Inside function:", s)
}
func main() {
nums := []int{1, 2, 3}
modifySlice(nums)
fmt.Println("In main:", nums)
}
When you run the above code, the output will be:
Inside function: [100 2 3 999]
In main: [100 2 3]
Explanation
- The value at index
0 is changed to 100 and is reflected outside the function.
- The
append() may allocate a new array if needed, so the new element 999 is not seen in main.
Appending to Slice Inside Function
If you want to reflect appended elements outside the function, return the modified slice and assign it back.
Example
package main
import "fmt"
func addElement(s []int) []int {
s = append(s, 10)
return s
}
func main() {
data := []int{1, 2, 3}
data = addElement(data)
fmt.Println("Updated slice:", data)
}
When you run the above code, the output will be:
Updated slice: [1 2 3 10]
Difference Between Array and Slice Parameters
Unlike arrays, slices allow dynamic resizing and naturally refer to shared data. Arrays, when passed to functions, behave like value types (copied entirely).
Key Points
- Slices are reference types – changes to elements inside a function are visible outside.
- Appending inside a function may create a new array – changes won’t affect the original slice unless reassigned.
- Use return values if you want to keep structural changes like appended elements.
Go Pass a Slice to Function Exercise
Select the correct option to complete each statement about passing slices to functions in Go.
- When you pass a slice to a function, it:
- If you append to a slice inside a function, the change:
- Which of the following is true?
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