What is the zero value for a string?

Question

How to verify that a string, or a struct with only string properties, is initialised or not, inside of a mapping?

According to the docs, it means checking that the element of the mapping is at its 0-value, which is unclear to me when talking about strings.

contract C {
    mapping (address => string) m1;
    mapping (address => StringStruct) m2;
struct StringStruct {
    string someString; // Always defined when initialising this struct
    // There could be other string properties here...
}

function amIInBothMappings() returns (bool) {
     // Check that both m1[msg.sender] and m2[msg.sender].someString are non-0
}

}

In the example: what is the best way to implement amIInBothMappings that checks that msg.sender is un-initialised in both mappings m1 and m2?

It seems that string comparison is still manual: https://forum.ethereum.org/discussion/3238/string-compare-in-solidity . Compare the first string to the string attribute in the struct. The zero string is just '' . — Sebi, Jun 05 '16 at 20:36

score 31 · Accepted Answer · edited Apr 13 '17 at 13:01

31

One way is to check for the length of a String:

if (bytes(m1[msg.sender]).length != 0 && bytes(m2[msg.sender].someString).length != 0)
    // do your thing

See the answer I posted here

edited Apr 13 '17 at 13:01

Community

1

answered Jun 06 '16 at 00:29

dbryson

6,403
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1

indeed, the bytes().length trick seems to work well! – lajarre Jun 06 '16 at 12:55

Miguel Mota · Answer 2 · 2018-01-19T23:31:26.923

0

Another way is to compare the sha3 hashes of the strings

if (sha3(myVariable) != sha3("")) {
 // is not empty string
}

However, this method is probably less efficient.

edited Jan 19 '18 at 23:31

answered Oct 03 '17 at 18:39

Miguel Mota

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47

What is the zero value for a string?

2 Answers2

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