title: Increment Number Property in AWS DynamoDB
body: <p>How do I increment a number in AWS Dynamodb? </p>
  
  <p>The guide says when saving an item to simply resave it:
  <a href="http://docs.aws.amazon.com/mobile/sdkforios/developerguide/dynamodb_om.html" rel="nofollow">http://docs.aws.amazon.com/mobile/sdkforios/developerguide/dynamodb_om.html</a></p>
  
  <p>However I am trying to use a counter where many users may be updating at the same time.</p>
  
  <p>Other documentation has told me to use and UpdateItem operation but I cannot find a good example to do so.</p>
  
  <p><a href="http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.Modifying.html" rel="nofollow">http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.Modifying.html</a></p>
  
  <p>However, I cannot find a method to implement the expression. In the future I will be adding values to arrays and maps. Will this be the same? My code is in Obj C</p>
  
  <p>Currently my code looks like:</p>
  
  <pre><code>AWSDynamoDBUpdateItemInput *updateItemInput = [AWSDynamoDBUpdateItemInput new];
          updateItemInput.tableName = @"TableName";
          updateItemInput.key = @{
                                  UniqueItemKey:@"KeyValue"
                                  };
          updateItemInput.updateExpression = @"SET counter = counter + :val";
          updateItemInput.expressionAttributeValues =@{
                                                       @":val":@1
                                                       };
  </code></pre>