# Modifying existing pipelines

### Existing pipeline

Let’s start where we left off in the Get started with pipeflow vignette, that is, we have the following pipeline

    pip
    #          step             depends                out keepOut      group  state
    #        <char>              <list>             <list>  <lgcl>     <char> <char>
    # 1:       data                     <data.frame[10x6]>   FALSE       data   Done
    # 2:  data_prep                data <data.frame[10x7]>   FALSE  data_prep   Done
    # 3:  model_fit           data_prep           <lm[13]>   FALSE  model_fit   Done
    # 4: model_plot model_fit,data_prep           <gg[11]>   FALSE model_plot   Done

with the following set data

    pip$get_data() |> head(3)
    #   Ozone Solar.R Wind Temp Month Day
    # 1    41     190  7.4   67     5   1
    # 2    36     118  8.0   72     5   2
    # 3    12     149 12.6   74     5   3

### Insert new step

Let’s say we want to insert a new step after the `data_prep` step that standardizes the y-variable.

    pip$insert_after(
        afterStep = "data_prep",
        step = "standardize",
        function(
            data = ~data_prep,
            yVar = "Ozone"
        ) {
            data[, yVar] <- scale(data[, yVar])
            data
        }
    )

    pip
    #           step             depends                out keepOut       group  state
    #         <char>              <list>             <list>  <lgcl>      <char> <char>
    # 1:        data                     <data.frame[10x6]>   FALSE        data   Done
    # 2:   data_prep                data <data.frame[10x7]>   FALSE   data_prep   Done
    # 3: standardize           data_prep             [NULL]   FALSE standardize    New
    # 4:   model_fit           data_prep           <lm[13]>   FALSE   model_fit   Done
    # 5:  model_plot model_fit,data_prep           <gg[11]>   FALSE  model_plot   Done

The `standardize` step is now part of the pipeline, but so far it is not used by any other step.

### Replace existing steps

Let’s revisit the function definition of the `model_fit` step

    pip$get_step("model_fit")[["fun"]]
    # [[1]]
    # function (data = ~data_prep, xVar = "Temp.Celsius")
    # {
    #     lm(paste("Ozone ~", xVar), data = data)
    # }

To use the standardized data, we need to change the data dependency such that it refers to the `standardize` step. Also instead of a fixed y-variable in the model, we want to pass it as a parameter.

    pip$replace_step(
        "model_fit",
        function(
            data = ~standardize,        # <- changed data reference
            xVar = "Temp.Celsius",
            yVar = "Ozone"              # <- new y-variable
        ) {
            lm(paste(yVar, "~", xVar), data = data)
        }
    )

The `model_plot` step needs to be updated in a similar way.

    pip$replace_step(
        "model_plot",
        function(
            model = ~model_fit,
            data = ~standardize,         # <- changed data reference
            xVar = "Temp.Celsius",
            yVar = "Ozone",              # <- new y-variable
            title = "Linear model fit"
        ) {
            coeffs <- coefficients(model)
            ggplot(data) +
                geom_point(aes(.data[[xVar]], .data[[yVar]])) +
                geom_abline(intercept = coeffs[1], slope = coeffs[2]) +
                labs(title = title)
        }
    )

The updated pipeline now looks as follows.

    pip
    #           step               depends                out keepOut       group  state
    #         <char>                <list>             <list>  <lgcl>      <char> <char>
    # 1:        data                       <data.frame[10x6]>   FALSE        data   Done
    # 2:   data_prep                  data <data.frame[10x7]>   FALSE   data_prep   Done
    # 3: standardize             data_prep             [NULL]   FALSE standardize    New
    # 4:   model_fit           standardize             [NULL]   FALSE   model_fit    New
    # 5:  model_plot model_fit,standardize             [NULL]   FALSE  model_plot    New

We see that the `model_fit` and `model_plot` steps now use (i.e., depend on) the standardized data. Let’s re-run the pipeline and inspect the output.

    pip$set_params(list(xVar = "Solar.R", yVar = "Wind"))
    pip$run()
    # INFO  [2025-07-26 20:25:23.487] Start run of 'my-pipeline' pipeline:
    # INFO  [2025-07-26 20:25:23.488] Step 1/5 data - skip 'done' step
    # INFO  [2025-07-26 20:25:23.489] Step 2/5 data_prep - skip 'done' step
    # INFO  [2025-07-26 20:25:23.490] Step 3/5 standardize
    # INFO  [2025-07-26 20:25:23.493] Step 4/5 model_fit
    # INFO  [2025-07-26 20:25:23.495] Step 5/5 model_plot
    # INFO  [2025-07-26 20:25:23.500] Finished execution of steps.
    # INFO  [2025-07-26 20:25:23.501] Done.

    pip$get_out("model_fit") |> coefficients()
    #  (Intercept)      Solar.R
    #  0.979672739 -0.006625601

    pip$get_out("model_plot")
    # Warning: Removed 2 rows containing missing values or values outside the scale range
    # (`geom_point()`).

### Removing steps

Let’s see the pipeline again.

    pip
    #           step               depends                out keepOut       group  state
    #         <char>                <list>             <list>  <lgcl>      <char> <char>
    # 1:        data                       <data.frame[10x6]>   FALSE        data   Done
    # 2:   data_prep                  data <data.frame[10x7]>   FALSE   data_prep   Done
    # 3: standardize             data_prep <data.frame[10x7]>   FALSE standardize   Done
    # 4:   model_fit           standardize           <lm[13]>   FALSE   model_fit   Done
    # 5:  model_plot model_fit,standardize           <gg[11]>   FALSE  model_plot   Done

When you are trying to remove a step, `pipeflow` by default checks if the step is used by any other step, and raises an error if removing the step would violate the integrity of the pipeline.

    try(pip$remove_step("standardize"))
    # Error : cannot remove step 'standardize' because the following steps depend on it: 'model_fit', 'model_plot'

To enforce removing a step together with all its downstream dependencies, you can use the `recursive` argument.

    pip$remove_step("standardize", recursive = TRUE)
    # Removing step 'standardize' and its downstream dependencies: 'model_fit', 'model_plot'

    pip
    #         step depends                out keepOut     group  state
    #       <char>  <list>             <list>  <lgcl>    <char> <char>
    # 1:      data         <data.frame[10x6]>   FALSE      data   Done
    # 2: data_prep    data <data.frame[10x7]>   FALSE data_prep   Done

Naturally, the last step never has any downstream dependencies, so it can be removed without any issues. There is another way to just remove the last step.

    pip$pop_step()
    # [1] "data_prep"

    pip
    #      step depends                out keepOut  group  state
    #    <char>  <list>             <list>  <lgcl> <char> <char>
    # 1:   data         <data.frame[10x6]>   FALSE   data   Done

Replacing steps in a pipeline as shown in this vignette will allow to re-use existing pipelines and adapt them programmatically to new requirements. Another way of re-using pipelines is to combine them, which is shown in the Combining pipelines vignette.