# Looping in Solidity

In solidity, mapping is very useful to store token value of an address. We have seen it in many contracts and they are normally defined this way:

`mapping (address => uint) public users;`

Since the mapping is public, we get a free getter and we can get the value of myAddress by using a simple

`users(myAddress);`

Solidity mapping might look similar to an associative array at first glance but its not. It doesn’t have indices, making it hard to loop through all addresses. It is still possible though but not straight forward.

Arrays are much easier to manage,

`address[] public addressIndices;`
`// start adding address in arrayaddressIndices.push(newAddress);...`
`// We know the length of the arrayuint arrayLength = addressIndices.length;`
`for (uint i=0; i<arrayLength; i++) {  // do something}`

Let’s say we want to calculate the total value of all addresses, having an array of addresses can really be helpful.

`mapping (address => uint) public mappedUsers;address[] public addressIndices;`
`// start adding address in arrayaddressIndices.push(newAddress);`
`// define mappedUsers as wellmappedUsers[newAddress] = someValue;...`
`// We know the length of the arrayuint arrayLength = addressIndices.length;`
`// totalValue auto init to 0uint totalValue;`
`for (uint i=0; i<arrayLength; i++) {  totalValue += mappedUsers[addressIndices[i]];}`

What if we want to delete the array efficiently? We have to move the array’s last position to the deleted position.

`uint indexToBeDeleted;mapping (address => uint) public mappedUsers;address[] public addressIndices;uint arrayLength = addressIndices.length;`
`for (uint i=0; i<arrayLength; i++) {  if (addressIndices[i] == addressToBeDeleted) {    indexToBeDeleted = i;    break;  }}`
`// if index to be deleted is not the last index, swap position.if (indexToBeDeleted < arrayLength-1) {  mappedUsers[indexToBeDeleted] = mappedUsers[arrayLength-1];}// we can now reduce the array length by 1addressIndices--;`

Referring to the code above, assuming we don’t want a for-loop to look for the index of the address to be deleted, we need to record the index of the item in a struct. It gets a bit more complicated if we want to do a proper CRUD.

A sample of the full code might look something like this.

Happy coding!

Like what you read? Give Bernard Peh a round of applause.

From a quick cheer to a standing ovation, clap to show how much you enjoyed this story.