Display a json object into an ionic list grouping by an item

Hi dear all , i need to display list of student for my app , i did a web service using php to get student names and encode it as a json , i got the result correctly then in my provider ( reddit-service) like shown below provider%20function
i got a list of students wich contains buttons , each student is in an item , i tried to declare an array and get the data stored in it with the *ngfor and that worked perfectly , but i need to get this data from the json i got from that url , i’m just a beginner , i did a research but got nothing useful , my html is this

do u have any idea please , i need to fix this as soon as possible