vue2.0 computed instance of calculating the accumulated value after list loop

<template>
 <p class="hello">
  <h1>{{ msg }}</h1>
  <h2>Foo</h2>
  <p v-for ="(item, index) in list">
  <!--<p>{{item }}</p>-->
  <h1 v-show="false">{{a[index] = item.bb}}</h1>
   <!-- <h1>index:{{index}}</h1>-->
  </p>
  <h2>a:{{a}}</h2>
  <h2>{{cc}}</h2>
  <!--<button v-on:click="cc">点击</button>-->
 </p>
</template>
<script>
export default {
 name: &#39;foo&#39;,
 data () {
  return {
   msg: &#39;这儿是Foo&#39;,
   list:[{
     insertId: &#39;asfasf252&#39;,
     bb:29
   },{
    insertId: &#39;2652&#39;,
     bb:20
   },{
    insertId: &#39;996&#39;,
     bb:18
   }],
   a:[]
  }
 },
 computed:{
   cc: function(){
     var sum = 0;
     for(var i= 0 ;i< this.a.length; i++) {
        sum += parseInt(this.a[i]);
     }
     return sum;
   }
 }
}
</script>
<!-- Add "scoped" attribute to limit CSS to this component only -->
<style scoped>
h1, h2 {
 font-weight: normal;
}
ul {
 list-style-type: none;
 padding: 0;
}
li {
 display: inline-block;
 margin: 0 10px;
}
a {
 color: #42b983;
}
</style>