# Per-row operations and modes

## Per-row operations

Per-row operations produce one output record for each input record. They can
be combined with each other, but not with summary operations in the same
command.

### Fields

| Operation | Result |
| --- | --- |
| `cut FIELDS`, `echo FIELDS` | Print the selected fields of each record |

```console
$ printf 'a\tb\tc\n' | fastmash cut 3,1
c	a
```

### Numbers

| Operation | Result |
| --- | --- |
| `round` | Nearest integer, halves away from zero |
| `floor`, `ceil` | Round down, round up |
| `trunc` | Round toward zero |
| `frac` | Fractional part, keeping the sign |
| `bin[:WIDTH]` | Lower bound of the bucket of width `WIDTH` (default 100) containing the value |
| `strbin[:COUNT]` | Hash the field's bytes into a bucket number from 0 to `COUNT`−1 (default 10) |
| `getnum[:TYPE]` | Extract the first number found in a text field |

`getnum` types: `n` digits, `i` signed integer, `p` digits and dots (the
default), `d` signed digits and dots, `h` hexadecimal, `o` octal. Text with no
number gives `0`.

```console
$ printf 'item-42.5kg\n' | fastmash getnum 1
42.5
$ printf '17\n183\n' | fastmash bin:50 1
0
150
```

### Text

| Operation | Result |
| --- | --- |
| `base64`, `debase64` | Encode or decode the field as Base64 |
| `md5`, `sha1`, `sha224`, `sha256`, `sha384`, `sha512` | Lowercase hexadecimal checksum of the field's bytes |
| `dirname`, `basename` | Directory part, or last component, of a path |
| `extname`, `barename` | File extension, or file name without it |

MD5 and SHA-1 are provided for compatibility and checksums, not for security.

Path operations work on the text alone; they don't touch the filesystem.

## Field modes

| Mode | Effect |
| --- | --- |
| `reverse` | Reverse the order of fields in each record |
| `noop`, `nop` | Read the input and print nothing (useful for validation); with `--full`, print each record |

```console
$ printf 'a\tb\tc\n' | fastmash reverse
c	b	a
```

## Table modes

| Mode | Effect |
| --- | --- |
| `transpose` | Swap rows and columns |
| `check [N lines] [N fields]` | Verify every record has the same number of fields, and optionally the expected counts |
| `rmdup FIELD`, `dedup FIELD` | Keep the first record for each distinct value of one key field |
| `health [CONTROLS]` | Inspect structural issues, missing values and lexical types, optionally validating supplied rules |

```console
$ printf 'a\tb\n1\t2\n' | fastmash transpose
a	1
b	2
$ printf 'x\t1\ny\t2\nx\t3\n' | fastmash rmdup 1
x	1
y	2
```

`transpose` and `reverse` require every record to have the same number of
fields. With `--no-strict` they accept ragged input, and `transpose` fills
missing cells with the `--filler` text (default `N/A`).

`check` prints a short confirmation and exits with status 0 when the table is
consistent, or reports the first inconsistent record and exits with status 1.

[Table health reports](table-health.md) inspect the complete accepted input,
with readable or schema-version-1 TSV output and bounded examples. Inferred
findings are advisory; `validate` fails only for violations of supplied type,
presence or width rules. Health accepts ordinary text input, not quoted CSV.

## Selecting complete records

`top:N FIELD` and `bottom:N FIELD` keep the highest or lowest N records by one
numeric field. They copy every field in rank order, with earlier input records
winning ties. Add `-g` for a separate cutoff per group and `-s` for interleaved
keys. Text and quoted CSV, headers and sorted spill are supported. See
[Highest and lowest records](top-n-records.md).

## Comparing two datasets

`compare BEFORE AFTER OPERATION FIELDS` summarizes two raw datasets and reports
before, after, signed difference and percentage for each result. `-g` aligns
keys globally even when they are interleaved, and named fields bind independently
to each input header. Optional ranking highlights the largest changes while
keeping added, removed and unavailable keys visible. See
[Dataset comparison](dataset-comparison.md) for report columns and limits.