Can't seem to use Button Group due to it allways selecting the first button from the group.

So, I have a list of cards(like IDs) that can change through time, each card is a pdf. I have a menu that is supposed to have a button for each card, and upon clicking a button I navigate to a window that will load that pdf with react-native-pdf. My current problem is, my selectedIndex seems to not work at all. I got two possible problems, either it just comes as undefined, or it always selects the first button. My code for the selected index and button group is basically the same as the one in the webpage but I have this inside the button group.

<ButtonGroup
              onPress={() => {this.updateIndex;
                              this.props.navigation.navigate('Details', {
                              cardName: this.state.cardList[this.state.selectedIndex]})
                      }}
              selectedIndex={selectedIndex}
              buttons={buttons}
              containerStyle={{height: numberOfButtons, flexDirection: 'column'}}
            />

thanks in advance for any help

[MatiasN]

I ended up finding what I was doing wrong, thanks a lot for your help though! Really appreciate it

[iRoachie]

Can you create an example on https://snack.expo.io that reproduces this?