How to remove items from a slice while ranging over it?

Question

What is the best way to remove items from a slice while ranging over it?

For example:

type MultiDataPoint []*DataPoint  func (m MultiDataPoint) Json() ([]byte, error) {     for i, d := range m {         err := d.clean()         if ( err != nil ) {             //Remove the DP from m         }     }     return json.Marshal(m) } 

Answer 1

As you have mentioned elsewhere, you can allocate new memory block and copy only valid elements to it. However, if you want to avoid the allocation, you can rewrite your slice in-place:

i := 0 // output index for _, x := range s {     if isValid(x) {         // copy and increment index         s[i] = x         i++     } } // Prevent memory leak by erasing truncated values  // (not needed if values don't contain pointers, directly or indirectly) for j := i; j < len(s); j++ {     s[j] = nil } s = s[:i] 

Full example: http://play.golang.org/p/FNDFswPeDJ

Note this will leave old values after index i in the underlying array, so this will leak memory until the slice itself is garbage collected, if values are or contain pointers. You can solve this by setting all values to nil or the zero value from i until the end of the slice before truncating it.