string not passing to post method nsurlconnection

Question

I am new to IOS i created nsurlconnection using POST method i passed a parameter "str".In nslog str value become null how to pass my string "str" in post method.

str coding: str string contain branchid1,currencyid1,currencyid2 are taken from picker view did select row.

str = [NSString stringWithFormat:@"branch_id=%@&from_curr=%@&to_curr=%@&value=%@",branchid1,currencyid1,currencyid2,fourthstr];

Picker view coding:

- (void) pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component

{
    if(pickerView.tag ==2){
    txtText.text = (NSString *)[arrMsg objectAtIndex:row];
        branchid1 = (NSString *)[arrmsg1 objectAtIndex:row];

        NSLog([arrmsg1 objectAtIndex:row]);

    }else if(pickerView.tag ==1){
    currency1.text = (NSString *)[currencyname1 objectAtIndex:row];
        currencyid1 = (NSString *)[id1 objectAtIndex:row];
        NSLog([id1 objectAtIndex:row]);


    }
    else
    {
        currency2.text = (NSString *)[from_currency objectAtIndex:row];
        currencyid2 = (NSString *)[id2 objectAtIndex:row];

        NSLog(@"%@",currencyid2);
        NSLog([id2 objectAtIndex:row]);
    }

button action:

}

post method:

-(void) sendDataToServer : (NSString *) method params:(NSString *)str{


    NSData *postData = [str dataUsingEncoding:NSASCIIStringEncoding allowLossyConversion:YES];
    NSString *postLength = [NSString stringWithFormat:@"%lu", (unsigned long)[str length]];

    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:URL]];
    NSLog(@"%@",str);

    [request setHTTPMethod:@"POST"];
    [request setValue:postLength forHTTPHeaderField:@"Content-Length"];
    [request setHTTPBody:postData];

    NSURLConnection *theConnection = [NSURLConnection connectionWithRequest:request delegate:self];

    if( theConnection ){

        mutableData = [[NSMutableData alloc]init];
    }
}

button action:

- (IBAction)btnact:(id)sender {

        if (branchides.length!=0 && currencyide1.length!=0 && currencyid2.length!=0 && fourthstr.length!=0)
    {
        str = [NSString stringWithFormat:@"branch_id=%@&from_curr=%@&to_curr=%@&value=%@",branchides,currencyide1,currencyid2,fourthstr];
        NSLog(@"%@",str);

        //[self sendDataToServer :@"POST" params:str];
    }
    else
    {
        // show some alert for empty
    }

    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Value"
                                                    message:final
                                                   delegate:self
                                          cancelButtonTitle:@"Cancel"
                                          otherButtonTitles:@"ok", nil];

    alert.tag = 100;
    [alert show];
    [self sendDataToServer :@"POST" params:str];
}