# Index error when using new ODE interface

**URL:** <https://discourse.mc-stan.org/t/index-error-when-using-new-ode-interface/21427>\
**Category:** Modeling\
**Created:** [March 22, 2021, 5:53pm UTC](https://discourse.mc-stan.org/t/index-error-when-using-new-ode-interface/21427 "2021-03-22T17:53:38Z")\
**Posts on this page:** 1\
**Showing post:** 2

<div class="post-metadata">

**Author:** ![rok\_cesnovar](https://avatars.discourse-cdn.com/v4/letter/r/7bcc69/32.png) [@rok\_cesnovar](https://discourse.mc-stan.org/u/rok_cesnovar)\
**Post date:** [March 22, 2021, 6:05pm UTC](https://discourse.mc-stan.org/t/index-error-when-using-new-ode-interface/21427/2 "2021-03-22T18:05:53Z")

</div>

In the old interface you had

`real y[n_days, 4];`

which is an array of 4-element arrays. The first dimension has n\_days.

In the new interface you wrote

`vector[n_days] y[4];`

which is a 4-element array of vectors the size of n\_days.

What you want is

`vector[4] y[n_days];`

or the equivalent but nicer way

`array[n_days] vector[4]`

---

_[View the full topic](https://discourse.mc-stan.org/t/index-error-when-using-new-ode-interface/21427)._