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Why is storing in an array the first time in solidity costs higher than the other times? The other times it has a constant cost which is less than the first time.

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(I'm sure this has come up before. If someone can find a previous answer then please duplicate... )

From the Yellow Paper, Appendix G, you'll see the following line:

Gsset - 20000 - Paid for an SSTORE operation when the storage value is set to non-zero from zero.

Meaning the first time you write a variable to storage, it's more expensive by 20,000 gas. Subsequent updates to the variable are cheaper.

Edit:

As JBrouwer has pointed out in the comments, for structs arrays you also need to store the length, so you're writing to two initially-zeroed storage slots rather than one, equating to 40,000 extra gas.

  • If you store an array you also have to store in the length slot (which is initially set to zero) and the value you want to store - this is hence at least 40k gas. – JBrouwer Nov 11 at 12:25
  • Ah, yeah, good point :-) If you add it as an answer I'll definitely upvote. – Richard Horrocks Nov 11 at 12:28
  • Include it in your answer =) You hence write to 2 slots not 1. – JBrouwer Nov 11 at 13:13
  • Thanks, done :-) – Richard Horrocks Nov 11 at 13:32
  • "For structs you also need to store the length" - I assume you mean array here instead of struct? Because struct x = {uint a}; can be stored in a single slot (like x someStructStorageValue) – JBrouwer Nov 12 at 16:39
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This is not unique to arrays, but to any state-variable in your contract.

Changing from zero to non-zero costs more gas than changing from non-zero to non-zero.

  • Thank you didn't know it – Haya Raed Nov 11 at 10:17

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