How to convert string into float value in the dataframe

Question

We are facing an error when we have a column which have datatype as string and the value like col1 col2 1 .89

So, when we are using

[self.wormhole listenForMessageWithIdentifier:@"messageIdentifier" 
 listener:^(id messageObject) {
    // Do Something
}];

After this we are getting error as cannt convert non int into type float