突然未定义数组js

Question

I am messing with a genetic algorithm for curiosity.

I was doing the generation switch part when I hit a wall. I removed the parents for the next generation from the previous actors array, pushed them in a parents array and was proceding to remove random parent pairs from the parents array and store them in progen1 and progen2 variables, but for some reason the inner array cromossomo of progen1 was undefined, event though I could access its elements when it progen1 whas stored in parents array.

This code:

var n1=Math.floor((Math.random(Math.floor(parents.length))));
console.log(parents[n1].cromossomo[0]);
var progen1=parents.splice(n1);
console.log(progen1.cromossomo[0]);
var n2=Math.floor((Math.random(Math.floor(parents.length))));
var progen2=parents.splice(n2);

produces:

{dx: -4.805923044269889, dy: -2.447397585132567}dx: -4.805923044269889dy: -2.447397585132567__proto__: Object
scripts.js:94 Uncaught TypeError: Cannot read property '0' of undefined
at switchGeneration (scripts.js:94)
at run (scripts.js:140)

and I am going crazy not knowing why this is happening!

Answer 1

splice() returns an array of all the elements that were removed from the array. So progen1 is an array of all the elements of parents starting from n1 , not a single object. The array doesn't have a cromossomo property, so progen1.cromossomo[0] tries to get the 0 index of undefined .

If your intent is to just remove 1 element from parents , you need to specify number of elements to remove when you call splice() . Then you can index the value to get that element.

progen1 = parents.splice(n1, 1)[0];

Answer 2

The error message reads: Cannot read property '0' of undefined

Looking at your snippet, I find cromossomo[0] .

If cromossomo is not present in either parents[n1] or progen1 , it is undefined , which leads to the error.

So maybe you have a look at these first and verify each of them has a cromosso .