# Counting number of variants for each interval
**URL:** https://discuss.hail.is/t/counting-number-of-variants-for-each-interval/2842
**Category:** Hail Query & hailctl
**Created:** [September 21, 2022, 8:33am UTC](https://discuss.hail.is/t/counting-number-of-variants-for-each-interval/2842 "2022-09-21T08:33:06Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Ofer](https://discuss.hail.is/u/Ofer)
#### Post date: [September 21, 2022, 8:33am UTC](https://discuss.hail.is/t/counting-number-of-variants-for-each-interval/2842/1 "2022-09-21T08:33:06Z")
Hi,
I have a list of intervals (the rows in cm\_in\_genes dataframe), and I’m trying to calculate for each row the number of variants that are found in t. Therefore, I’m iterating over the cm\_in\_genes table, filtering according to the rows and counting the number of rows after the filtering.
After some iterations, I’m getting this error:
Is there another efficient way to do this?
I will appreciate your help 🙂
Thanks in advance,
Ofer
---
### Author:  [@danking](https://discuss.hail.is/u/danking)
#### Post date: [September 21, 2022, 2:31pm UTC](https://discuss.hail.is/t/counting-number-of-variants-for-each-interval/2842/2 "2022-09-21T14:31:01Z")
Hey @Ofer ,
You’re creating a _huge_ Hail expression which our compiler is failing to parse. That’s almost certainly the cause of the StackOverflowError [1]. In each loop, you mutate the `t` variable, so the last iteration is actually applying _all_ your filters. I think what you actually want is this [2]:
```python
for index, row in cm_in_genes.iterrows():
temp = t.filter(...)
cm_in_genes.at[index, 'trio_001...'] = temp.count()
```
The above will only apply the one, correct, filter. However, Hail is not like pandas: the methods on Tables and MatrixTables build a recipe. They do not eagerly compute things. All the computation happens when you call an action function like `count`, `write`, or `export`.
In general, you should fully embrace Hail and express your entire analysis within Hail. That allows Hail to 1) optimize your code across the whole analysis and 2) process the data as few times as possible. Here’s how you can count variants per interval in a more efficient manner:
```python
i = hl.Table.from_pandas(cm_in_genes)
i = i.key_by(interval = hl.locus_interval(
hl.literal('chr') + hl.str(i.chrom),
i.CM_start,
i.CM_end
))
# key_by is a "shuffle" so we need to write and read after it.
i.write("gs://.../my_intervals.ht")
i = hl.read_table("gs://.../my_intervals.ht")
# Hail annoyingly throws away the key after a join, so we
# copy the interval into another field for later use
i = i.annotate(interval_copy = i.interval)
t = t.annotate(the_interval_or_NA = i[t.locus].interval_copy)
t = t.filter(hl.is_defined(t.the_interval_or_NA))
t = t.annotate(the_interval = t.the_interval_or_NA)
t = t.group_by(t.the_interval).aggregate(hl.agg.count())
# save all that hard work to a file
t = t.write("gs://.../count_by_interval.ht")
# peek at the results
t = hl.read_table("gs://.../count_by_inteval.ht")
t.show()
```
* * *
[1] In the future, it’s helpful to include the full stack trace as text in your post, so that we can verify assumptions like this.
[2] In the future, it’s helpful to include the code as text because we can copy it and modify it as necessary.
---
### Author:  [@Ofer](https://discuss.hail.is/u/Ofer)
#### Post date: [September 22, 2022, 9:42am UTC](https://discuss.hail.is/t/counting-number-of-variants-for-each-interval/2842/3 "2022-09-22T09:42:01Z")
Thanks!
I’m trying to implement the first line -
```python
i = hl.Table.from_pandas(cm_in_genes)
```
But I’m getting this error:
```python
---------------------------------------------------------------------------
ExpressionException Traceback (most recent call last)
/cs/labs/michall/ofer.feinstein/my_env/lib/python3.7/site-packages/hail/expr/expressions/base_expression.py in raise_for_holes(t)
239 try:
--> 240 raise_for_holes(vt)
241 except ExpressionException as exc:
/cs/labs/michall/ofer.feinstein/my_env/lib/python3.7/site-packages/hail/expr/expressions/base_expression.py in raise_for_holes(t)
231 if t is None:
--> 232 raise ExpressionException("Hail cannot impute type")
233 if t in (tbool, tint32, tint64, tfloat32, tfloat64, tstr, tcall):
ExpressionException: Hail cannot impute type
The above exception was the direct cause of the following exception:
ExpressionException Traceback (most recent call last)
/cs/labs/michall/ofer.feinstein/my_env/lib/python3.7/site-packages/hail/expr/expressions/base_expression.py in raise_for_holes(t)
252 try:
--> 253 raise_for_holes(t.element_type)
254 except ExpressionException as exc:
/cs/labs/michall/ofer.feinstein/my_env/lib/python3.7/site-packages/hail/expr/expressions/base_expression.py in raise_for_holes(t)
241 except ExpressionException as exc:
--> 242 raise ExpressionException(f'cannot impute field {k}') from exc
243 return
ExpressionException: cannot impute field chrom
The above exception was the direct cause of the following exception:
ExpressionException Traceback (most recent call last)
/tmp/ipykernel_13276/3652694852.py in
----> 1 i = hl.Table.from_pandas(cm_in_genes)
2 # i = i.key_by(interval = hl.locus_interval(
3 # hl.literal('chr') + hl.str(i.chrom),
4 # i.CM_start,
5 # i.CM_end
in from_pandas(df, key)
/cs/labs/michall/ofer.feinstein/my_env/lib/python3.7/site-packages/hail/typecheck/check.py in wrapper(__original_func, *args, **kwargs)
575 def wrapper(__original_func, *args, **kwargs):
576 args_, kwargs_ = check_all(__original_func, args, kwargs, checkers, is_method=is_method)
--> 577 return __original_func(*args_, **kwargs_)
578
579 return wrapper
/cs/labs/michall/ofer.feinstein/my_env/lib/python3.7/site-packages/hail/table.py in from_pandas(df, key)
3406 hl_type_hints[field] = type_hint
3407
-> 3408 new_table = hl.Table.parallelize(data, partial_type=hl_type_hints)
3409 return new_table if not key else new_table.key_by(*key)
3410
in parallelize(cls, rows, schema, key, n_partitions, partial_type)
/cs/labs/michall/ofer.feinstein/my_env/lib/python3.7/site-packages/hail/typecheck/check.py in wrapper(__original_func, *args, **kwargs)
575 def wrapper(__original_func, *args, **kwargs):
576 args_, kwargs_ = check_all(__original_func, args, kwargs, checkers, is_method=is_method)
--> 577 return __original_func(*args_, **kwargs_)
578
579 return wrapper
/cs/labs/michall/ofer.feinstein/my_env/lib/python3.7/site-packages/hail/table.py in parallelize(cls, rows, schema, key, n_partitions, partial_type)
534 if partial_type is not None:
535 partial_type = hl.tarray(hl.tstruct(**partial_type))
--> 536 rows = to_expr(rows, dtype=dtype, partial_type=partial_type)
537 if not isinstance(rows.dtype.element_type, tstruct):
538 raise TypeError("'parallelize' expects an array with element type 'struct', found '{}'"
/cs/labs/michall/ofer.feinstein/my_env/lib/python3.7/site-packages/hail/expr/expressions/base_expression.py in to_expr(e, dtype, partial_type)
273 raise TypeError("expected expression of type '{}', found expression of type '{}'".format(dtype, e.dtype))
274 return e
--> 275 return cast_expr(e, dtype, partial_type)
276
277
/cs/labs/michall/ofer.feinstein/my_env/lib/python3.7/site-packages/hail/expr/expressions/base_expression.py in cast_expr(e, dtype, partial_type)
279 assert dtype is None or partial_type is None
280 if not dtype:
--> 281 dtype = impute_type(e, partial_type)
282 x = _to_expr(e, dtype)
283 if isinstance(x, Expression):
/cs/labs/michall/ofer.feinstein/my_env/lib/python3.7/site-packages/hail/expr/expressions/base_expression.py in impute_type(x, partial_type)
128 def impute_type(x, partial_type=None):
129 t = _impute_type(x, partial_type=partial_type)
--> 130 raise_for_holes(t)
131 return t
132
/cs/labs/michall/ofer.feinstein/my_env/lib/python3.7/site-packages/hail/expr/expressions/base_expression.py in raise_for_holes(t)
253 raise_for_holes(t.element_type)
254 except ExpressionException as exc:
--> 255 raise ExpressionException('cannot impute array elements') from exc
256 return
257 if isinstance(t, tdict):
ExpressionException: cannot impute array elements
```
I’m using Hail version 0.2.95-513139587f5…
Do you know the reason for this error?
Thanks!
---
### Author:  [@danking](https://discuss.hail.is/u/danking)
#### Post date: [September 22, 2022, 4:23pm UTC](https://discuss.hail.is/t/counting-number-of-variants-for-each-interval/2842/4 "2022-09-22T16:23:07Z")
This means that Hail cannot guess the type of some column of the cm\_in\_genes pandas DataFrame. Probably a column with only NAs?
If you have this interval list as a UCSC BED file, you can use Hail’s [`hl.import_bed`](https://hail.is/docs/0.2/methods/impex.html#hail.methods.import_bed) directly. If it’s some other file, you could try [`hl.import_table`](https://hail.is/docs/0.2/methods/impex.html#hail.methods.import_table).
Or you could remove the pandas column that are all NAs, NaNs, or empty lists so that Hail can guess the type correctly. If you share the first few rows of the pandas data frame here I can probably identify the problematic column.