Confusion when appending to slice-CodePudding

Can someone explain why in the first example if appending to y changes also x but in the second example it doesnt?

    x := make([]int, 0, 4)
    x = append(x, 1, 2, 3, 4) //len(x)=4 and cap(x)=4
    y := x[:2]                //Len(y)=2 and cap(y)=4

    y = append(y, 30, 40)

    fmt.Println("x:", x)
    fmt.Println("y:", y)

    //x: [1 2 30 40]
    //y: [1 2 30 40]

    x := make([]int, 0, 4)
    x = append(x, 1, 2, 3, 4) //len(x)=4 and cap(x)=4
    y := x[:2]                //Len(y)=2 and cap(y)=4

    y = append(y, 30, 40,50)

    fmt.Println("x:", x)
    fmt.Println("y:", y)

    //x: [1 2 3 4]
    //y: [1 2 30 40 50]

And how is this affected by the capacity? For example if I change the capacity to 5

x := make([]int, 0, 5)
x = append(x, 1, 2, 3, 4) 
y := x[:2]             

y = append(y, 30, 40, 50)

fmt.Println("x:", x)
fmt.Println("y:", y)

//x: [1 2 30 40]
//y: [1 2 30 40 50]

then it seems to work but then I would assume that x also contains 50?

CodePudding user response：

In the first case y has the same underlying array as x and as you append to y and everything fits into that array x is also affected.

In the second case y has the same underlying array as x as first, but when you try to append new values the underlying array is not enough so a new one is allocated and x is unaffacted.

In the third case it's similar to the first one - when you append things to y it affects x as well. When you print the contents of x the len of x is took into consideration so only the first 4 values are printed (x has a length of 4, even though it has the capacity of 5).