Why does the model matrix function ignore creating a dummy for all categories?

Why does it ignore creating the dummy for females?

suppressMessages(library(tidyverse))

data(titanic_train, package = "titanic")

titanicTib <- as_tibble(titanic_train) %>%
mutate_at(.vars = c("Survived", "Sex", "Pclass"), .funs = factor) %>%
mutate(FamSize = SibSp + Parch) %>%
select(Pclass, Sex, Age, Fare, FamSize)

X <-model.matrix(~ -1 + Pclass + Sex + Age + Fare + FamSize, data=titanicTib)

head(X, 5)

>Solution :

To have all levels of variable Sex in the model matrix, add

contrasts.arg = list(Sex = contrasts(titanicTib$Sex, contrasts = FALSE)) 

to the model.matrix call.

X <- model.matrix(~ -1 + Pclass + Sex + Age + Fare + FamSize, data = titanicTib,
                  contrasts.arg = list(Sex = contrasts(titanicTib$Sex, contrasts = FALSE)))

head(X, 5)
#>   Pclass1 Pclass2 Pclass3 Sexfemale Sexmale Age    Fare FamSize
#> 1       0       0       1         0       1  22  7.2500       1
#> 2       1       0       0         1       0  38 71.2833       1
#> 3       0       0       1         1       0  26  7.9250       0
#> 4       1       0       0         1       0  35 53.1000       1
#> 5       0       0       1         0       1  35  8.0500       0

^{Created on 2023-02-21 with reprex v2.0.2}