How to declare ngif value by concatenate in angular

OK, your solution is overcomplicated. Just store the index of clicked position

<p matLine (click)="hides(i)">{{ season }}</p>

  hiddenIndex = -1;

  hides(val: number) {
    this.hiddenIndex = val;
  }

and hide the element:

    <span class="bullet" *ngIf="hiddenIndex !== i"></span>

The groupid0 and the rest of the variables should go. Here's a modified stackblitz: https://stackblitz.com/edit/angular-8-material-starter-template-fkttxa?file=src/app/app.component.ts

I try to modify your data seasons and become like this

for app.component.html

<mat-list>
  <mat-list-item *ngFor="let season of seasons; index as i">
    <span class="bullet" *ngIf="season.bullet"></span>
    <p matLine (click)="hides(season)">{{ season.title }}</p>
  </mat-list-item>
</mat-list>

for app.component.ts

import { Component, OnInit } from '@angular/core';

export interface Season {
  title: string;
  bullet: boolean;
}

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css'],
})
export class AppComponent implements OnInit {
  name = 'Angular';
  seasons: any[] = [
    { title: 'Group id', bullet: true },
    { title: 'Group name', bullet: true },
    { title: 'Group value', bullet: true },
  ];

  constructor() {}
  ngOnInit() {}

  hides(val: any) {
    const index = this.seasons.findIndex((item) => item.title === val.title);
    val.bullet
      ? (this.seasons[index]['bullet'] = false)
      : (this.seasons[index]['bullet'] = true);
  }
}

More detail https://stackblitz.com/edit/angular-8-material-starter-template-pa4aus?file=src/app/app.component.html

You can't use ngIf like that in angular. Instead you can define a getter:

component.ts

class YourComponent {
    private _seasons: string[] = [ /* original seasons array */ ]

    public get seasons() {
        return this._seasons.map((s, i) => ({
            _original: s,
            get condition() {
                return this._original
                    .toLowerCase()
                    .split(' ')
                    .join('') + i
            }
        }))
    }
}

and component.html

<mat-list>
    <mat-list-item *ngFor="let season of seasons">
        <span *ngIf="season.condition" class="bullet"></span>
        <p (click)="hides(season)" matLine>{{ season }}</p>
    </mat-list-item>
</mat-list>