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When processing data downloaded from popular survey engines, it’s not uncommon for multiple choice questions to be represented as one column per possible response coded as 0/1. So, a question with just two responses might be downloaded as part of a CSV with one column for q1_1 and another for q1_2. If the responses are mutually exclusive, then (q1_1 == 0 iff q1_2 == 1) and (q1_1 == 1 iff q1_2 == 0). If the responses are part of a “choose all that apply” question, then it’s possible to have multiple 1s.

How can these individual binary indicator variables be reassembled into a single response variable?

First, let’s simulate some response data for non-mutually exclusive questions—each row represents one respondent’s choices:

```
df <- data.frame(
q1_1 = round(runif(5), 0),
q1_2 = round(runif(5), 0),
q1_3 = round(runif(5), 0),
q2_1 = round(runif(5), 0),
q2_2 = round(runif(5), 0),
q3_1 = round(runif(5), 0),
q3_2 = round(runif(5), 0),
q3_3 = round(runif(5), 0),
q3_4 = round(runif(5), 0)
)
df
```

```
q1_1 q1_2 q1_3 q2_1 q2_2 q3_1 q3_2 q3_3 q3_4
1 1 0 0 1 0 1 1 0 1
2 0 1 0 0 1 1 1 0 1
3 1 1 1 1 1 0 0 0 1
4 0 1 0 1 1 1 1 0 1
5 0 0 1 1 0 1 0 0 0
```

R’s dplyr package offers the coalesce function, which doesn’t suit my needs when the data contains 0s for non-selected response choices. Notice below in row 2, for example, that q1 and q2 select the first non-NA values, which is 0:

```
library(dplyr)
df %>%
mutate(q1 = coalesce(q1_1, q1_2, q1_3)) %>%
mutate(q2 = coalesce(q2_1, q2_2)) %>%
mutate(q3 = coalesce(q3_1, q3_2, q3_3, q3_4))
```

```
q1_1 q1_2 q1_3 q2_1 q2_2 q3_1 q3_2 q3_3 q3_4 q1 q2 q3
1 1 0 0 1 0 1 1 0 1 1 1 1
2 0 1 0 0 1 1 1 0 1 0 0 1
3 1 1 1 1 1 0 0 0 1 1 1 1
4 0 1 0 1 1 1 1 0 1 0 1 1
5 0 0 1 1 0 1 0 0 0 0 1 1
```

If you replace all 0s with NA, you can get closer to what you need:

```
df %>%
mutate_all(~ifelse(. == 0, NA_real_, .)) %>%
mutate(q1 = coalesce(q1_1, q1_2, q1_3)) %>%
mutate(q2 = coalesce(q2_1, q2_2)) %>%
mutate(q3 = coalesce(q3_1, q3_2, q3_3, q3_4))
```

```
q1_1 q1_2 q1_3 q2_1 q2_2 q3_1 q3_2 q3_3 q3_4 q1 q2 q3
1 1 NA NA 1 NA 1 1 NA 1 1 1 1
2 NA 1 NA NA 1 1 1 NA 1 1 1 1
3 1 1 1 1 1 NA NA NA 1 1 1 1
4 NA 1 NA 1 1 1 1 NA 1 1 1 1
5 NA NA 1 1 NA 1 NA NA NA 1 1 1
```

Unfortunately, the q1 vector here only tells us that there was *some* response by each respondent, not which response they gave for q1.

It would be nice to have a version of coalesce that gathered not the first non-NA *value*, but the *column name* of the first non-NA value. Here, I’ll use the structure of dplyr’s coalesce as a model:

```
coalesce_colname <-
function(...) {
if (missing(..1)) {
abort("At least one argument must be supplied")
}
colnames <- as.character(as.list(match.call()))[-1]
values <- list(...)
x <- values[[1]]
x[!is.na(x)] <- colnames[1]
values <- values[-1]
colnames <- colnames[-1]
for (i in seq_along(values)) {
x <- ifelse(is.na(x) & !is.na(values[[i]]), colnames[i], x)
}
x
}
```

With this, you have a drop-in replacement for coalesce that captures the column name:

```
df %>%
mutate_all(~ifelse(. == 0, NA_real_, .)) %>%
mutate(q1 = coalesce_colname(q1_1, q1_2, q1_3)) %>%
mutate(q2 = coalesce_colname(q2_1, q2_2)) %>%
mutate(q3 = coalesce_colname(q3_1, q3_2, q3_3, q3_4))
```

```
q1_1 q1_2 q1_3 q2_1 q2_2 q3_1 q3_2 q3_3 q3_4 q1 q2 q3
1 1 NA NA 1 NA 1 1 NA 1 q1_1 q2_1 q3_1
2 NA 1 NA NA 1 1 1 NA 1 q1_2 q2_2 q3_1
3 1 1 1 1 1 NA NA NA 1 q1_1 q2_1 q3_4
4 NA 1 NA 1 1 1 1 NA 1 q1_2 q2_1 q3_1
5 NA NA 1 1 NA 1 NA NA NA q1_3 q2_1 q3_1
```

and with a little effort, you can wrangle the column name to extract the response value:

```
df %>%
mutate_all(~ifelse(. == 0, NA_real_, .)) %>%
mutate(q1 = coalesce_colname(q1_1, q1_2, q1_3)) %>%
mutate(q2 = coalesce_colname(q2_1, q2_2)) %>%
mutate(q3 = coalesce_colname(q3_1, q3_2, q3_3, q3_4)) %>%
mutate_at(c("q1", "q2", "q3"), ~stringr::str_extract(., "\\d+$"))
```

```
q1_1 q1_2 q1_3 q2_1 q2_2 q3_1 q3_2 q3_3 q3_4 q1 q2 q3
1 1 NA NA 1 NA 1 1 NA 1 1 1 1
2 NA 1 NA NA 1 1 1 NA 1 2 2 1
3 1 1 1 1 1 NA NA NA 1 1 1 4
4 NA 1 NA 1 1 1 1 NA 1 2 1 1
5 NA NA 1 1 NA 1 NA NA NA 3 1 1
```

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**R – William Doane**.

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