how to define index in angular material table

Can you add index to let element; let i = index;" as you'd do with *ngFor?

<mat-row *matRowDef="let row; columns: displayedColumns; let i = index"></mat-row>

Or like so:

<ng-container matColumnDef="index">
  <mat-header-cell *matHeaderCellDef> Index </mat-header-cell>
  <mat-cell *matCellDef="let element; let i = index;">{{i}}</mat-cell>
</ng-container>

Working Example: https://stackblitz.com/edit/angular-acdxje?file=app/table-basic-example.html

For anyone who has set the multiTemplateDataRows property of mat-table to true, you can't use index. Instead you have use either dataIndex or renderIndex.

<mat-cell *matCellDef="let row; let i = dataIndex;">{{i}}</mat-cell>

See https://github.com/angular/material2/issues/12793

For those who are facing problem with keeping the right index when using pagination which table has more than 1 page. It can be especially important when you have editable element, thus you're using a routerLink to add/edit/delete selected elements.

<ng-container matColumnDef="title">
 <mat-header-cell *matHeaderCellDef mat-sort-header>Title</mat-header-cell>
 <mat-cell *matCellDef="let book; let i = index;" fxLayoutAlign.lt-md="center center">
  <button mat-button [routerLink]="[i + (paginator.pageIndex * paginator.pageSize)]" routerLinkActive="active"</button>
 </mat-cell>
</ng-container>

As well as

<mat-paginator #paginator [pageSize]="5" [pageSizeOptions]="[5, 10, 25, 100]"></mat-paginator>

In essence, i + (paginator.pageIndex * paginator.pageSize) is the solution, but it counts from 0. If you'd like to index from 1, simply make it (i+1) + (paginator.pageIndex * paginator.pageSize). Worth to note is that you really need the #paginator and [pageSize]="VALUE".