# Group By

This is used to group items in an array by an expression.

## Group by field
Given a sample.yml file of:
```yaml
[{foo: 1, bar: 10}, {foo: 3, bar: 100}, {foo: 1, bar: 1}]
```
then
```bash
yq 'group_by(.foo)' sample.yml
```
will output
```yaml
- - [{foo: 1, bar: 10}, {foo: 3, bar: 100}, {foo: 1, bar: 1}]
  - [{foo: 1, bar: 10}, {foo: 3, bar: 100}, {foo: 1, bar: 1}]
- - [{foo: 1, bar: 10}, {foo: 3, bar: 100}, {foo: 1, bar: 1}]
```

## Group by field, with nuls
Given a sample.yml file of:
```yaml
[{cat: dog}, {foo: 1, bar: 10}, {foo: 3, bar: 100}, {no: foo for you}, {foo: 1, bar: 1}]
```
then
```bash
yq 'group_by(.foo)' sample.yml
```
will output
```yaml
- - [{cat: dog}, {foo: 1, bar: 10}, {foo: 3, bar: 100}, {no: foo for you}, {foo: 1, bar: 1}]
  - [{cat: dog}, {foo: 1, bar: 10}, {foo: 3, bar: 100}, {no: foo for you}, {foo: 1, bar: 1}]
- - [{cat: dog}, {foo: 1, bar: 10}, {foo: 3, bar: 100}, {no: foo for you}, {foo: 1, bar: 1}]
  - [{cat: dog}, {foo: 1, bar: 10}, {foo: 3, bar: 100}, {no: foo for you}, {foo: 1, bar: 1}]
- - [{cat: dog}, {foo: 1, bar: 10}, {foo: 3, bar: 100}, {no: foo for you}, {foo: 1, bar: 1}]
```