# Collecting output

Generally, one should keep pipeline steps as simple as possible, basically following the principle “one step, one task”. This means that usually a lot of pipeline steps are used to calculate intermediate results and only a few steps contain the final results that we are interested in. This vignette shows how to conveniently collect and possibly group the output of those final steps.

### Flag output steps

Output steps are flagged by settting the `keepOut` argument to `TRUE` when adding a step to the pipeline. In the following example, we will want to keep the output of the steps `data_summary`, `model_summary`, and `model_plot`.

    library(pipeflow)
    library(ggplot2)

    pip <- pipe_new(
            "my-pipeline",
            data = airquality
        ) |>

        pipe_add(
            "data_prep",
            function(data = ~data) {
                replace(data, "Temp.Celsius", (data[, "Temp"] - 32) * 5/9)
            }
        ) |>

        pipe_add(
            "data_summary",
            function(
                data = ~data_prep,
                xVar = "Temp.Celsius",
                yVar = "Ozone"
            ) {
                data[, c(xVar, yVar)]
            },
            keepOut = TRUE              # <- keep this
        ) |>

        pipe_add(
            "model_fit",
            function(
                data = ~data_prep,
                xVar = "Temp.Celsius",
                yVar = "Ozone"
            ) {
                lm(paste(yVar, "~", xVar), data = data)
            }
        ) |>

        pipe_add(
            "model_summary",
            function(
                fit = ~model_fit
            ) {
                summary(fit)
            },
            keepOut = TRUE              # <- keep this
        ) |>

        pipe_add(
            "model_plot",
            function(
                model = ~model_fit,
                data = ~data_prep,
                xVar = "Temp.Celsius",
                yVar = "Ozone",
                title = "Linear model fit"
            ) {
                coeffs <- coefficients(model)
                ggplot(data) +
                    geom_point(aes(.data[[xVar]], .data[["Ozone"]])) +
                    geom_abline(intercept = coeffs[1], slope = coeffs[2]) +
                    labs(title = title)
            },
            keepOut = TRUE              # <- keep this
        )

Looking at the pipeline, we see that the steps `data_summary`, `model-summary`, and `model_plot` have been flagged accordingly (see column `keepOut`).

    pip
    #             step             depends    out keepOut         group  state
    #           <char>              <list> <list>  <lgcl>        <char> <char>
    # 1:          data                     [NULL]   FALSE          data    New
    # 2:     data_prep                data [NULL]   FALSE     data_prep    New
    # 3:  data_summary           data_prep [NULL]    TRUE  data_summary    New
    # 4:     model_fit           data_prep [NULL]   FALSE     model_fit    New
    # 5: model_summary           model_fit [NULL]    TRUE model_summary    New
    # 6:    model_plot model_fit,data_prep [NULL]    TRUE    model_plot    New

Now let’s run and collect the output of the flagged steps using the `collect_out` method, which returns a list with the output of the flagged steps.

    pip$run()
    # INFO  [2025-07-26 20:25:27.949] Start run of 'my-pipeline' pipeline:
    # INFO  [2025-07-26 20:25:27.951] Step 1/6 data
    # INFO  [2025-07-26 20:25:27.954] Step 2/6 data_prep
    # INFO  [2025-07-26 20:25:27.956] Step 3/6 data_summary
    # INFO  [2025-07-26 20:25:27.958] Step 4/6 model_fit
    # INFO  [2025-07-26 20:25:27.961] Step 5/6 model_summary
    # INFO  [2025-07-26 20:25:27.967] Step 6/6 model_plot
    # INFO  [2025-07-26 20:25:27.972] Finished execution of steps.
    # INFO  [2025-07-26 20:25:27.973] Done.

    out <- pip$collect_out()

    names(out)
    # [1] "data_summary"  "model_summary" "model_plot"

As expected, the output list contains the output of the flagged steps.

    str(out, max.level = 1)
    # List of 3
    #  $ data_summary :'data.frame':    153 obs. of  2 variables:
    #  $ model_summary:List of 12
    #   ..- attr(*, "class")= chr "summary.lm"
    #  $ model_plot   :List of 11
    #   ..- attr(*, "class")= chr [1:2] "gg" "ggplot"

### Grouping output steps

Often certain output steps are related and should be grouped together. This can be achieved conveniently by setting the `group` argument when adding a step to the pipeline. Let’s illustrate this by slightly modifying the previous example.

    pip <- Pipeline$new("my-pipeline", data = airquality) |>

        pipe_add(
            "data_prep",
            function(data = ~data) {
                replace(data, "Temp.Celsius", (data[, "Temp"] - 32) * 5/9)
            }
        ) |>

        pipe_add(
            "used_data",
            function(
                data = ~data_prep,
                xVar = "Temp.Celsius",
                yVar = "Ozone"
            ) {
                data[, c(xVar, yVar)]
            },
            keepOut = TRUE,
            group = "Data"                 # <- define 'Data' group here
        ) |>

        pipe_add(
            "model_fit",
            function(
                data = ~data_prep,
                xVar = "Temp.Celsius",
                yVar = "Ozone"
            ) {
                lm(paste(yVar, "~", xVar), data = data)
            }
        ) |>

        pipe_add(
            "model_summary",
            function(
                fit = ~model_fit
            ) {
                summary(fit)
            },
            keepOut = TRUE,
            group = "Model"                # <- define 'Model' group here
        ) |>

        pipe_add(
            "model_plot",
            function(
                model = ~model_fit,
                data = ~data_prep,
                xVar = "Temp.Celsius",
                yVar = "Ozone",
                title = "Linear model fit"
            ) {
                coeffs <- coefficients(model)
                ggplot(data) +
                    geom_point(aes(.data[[xVar]], .data[["Ozone"]])) +
                    geom_abline(intercept = coeffs[1], slope = coeffs[2]) +
                    labs(title = title)
            },
            keepOut = TRUE,
            group = "Model"                # <- define 'Model' group here
        )

Looking at the pipeline, the defined groups are shown in the `group` column.

    pip
    #             step             depends    out keepOut     group  state
    #           <char>              <list> <list>  <lgcl>    <char> <char>
    # 1:          data                     [NULL]   FALSE      data    New
    # 2:     data_prep                data [NULL]   FALSE data_prep    New
    # 3:     used_data           data_prep [NULL]    TRUE      Data    New
    # 4:     model_fit           data_prep [NULL]   FALSE model_fit    New
    # 5: model_summary           model_fit [NULL]    TRUE     Model    New
    # 6:    model_plot model_fit,data_prep [NULL]    TRUE     Model    New

As you see, by default, the group is identical to the step name, that is, each step represents the trivial case of a one-sized group. Again, we run the pipeline and collect the output.

    pip$run()
    # INFO  [2025-07-26 20:25:28.137] Start run of 'my-pipeline' pipeline:
    # INFO  [2025-07-26 20:25:28.138] Step 1/6 data
    # INFO  [2025-07-26 20:25:28.141] Step 2/6 data_prep
    # INFO  [2025-07-26 20:25:28.145] Step 3/6 used_data
    # INFO  [2025-07-26 20:25:28.147] Step 4/6 model_fit
    # INFO  [2025-07-26 20:25:28.151] Step 5/6 model_summary
    # INFO  [2025-07-26 20:25:28.153] Step 6/6 model_plot
    # INFO  [2025-07-26 20:25:28.160] Finished execution of steps.
    # INFO  [2025-07-26 20:25:28.161] Done.

    out <- pip$collect_out()

    names(out)
    # [1] "Data"  "Model"

As we can see, the output related to the modelling has been grouped into one sublist named `Model`.

    str(out, max.level = 2)
    # List of 2
    #  $ Data :'data.frame':    153 obs. of  2 variables:
    #   ..$ Temp.Celsius: num [1:153] 19.4 22.2 23.3 16.7 13.3 ...
    #   ..$ Ozone       : int [1:153] 41 36 12 18 NA 28 23 19 8 NA ...
    #  $ Model:List of 2
    #   ..$ model_summary:List of 12
    #   .. ..- attr(*, "class")= chr "summary.lm"
    #   ..$ model_plot   :List of 11
    #   .. ..- attr(*, "class")= chr [1:2] "gg" "ggplot"