tf.assign to variable slice doesn't work inside tf.while_loop

Question

What is wrong with the following code? The tf.assign op works just fine when applied to a slice of a tf.Variable if it happens outside of a loop.

Answer 1

I was executing this a few times and it isn't consistent. But variable slices do work inside the while loop.

Tried to split the graph inside body because the results are incorrect sometimes.

The correct answer (11, array([ 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89])) is returned sometimes but not always.

import tensorflow as tf

v = [1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0]
n = len(v)
a1 = tf.Variable(v, name = 'a')

def cond(i, _):
    return i < n

s = tf.InteractiveSession()
s.run(tf.global_variables_initializer())

def body( i, _):
    x = a1[i-1]
    y = a1[i-2]
    z = tf.add(x,y)
    op = a1[i].assign( z )
    with tf.control_dependencies([op]): #Edit This fixed the inconsistency.
       increment = tf.add(i, 1)
    return increment, op

print(s.run(tf.while_loop(cond, body, [2, a1])))