Retina Retina 0.8.2, 13 bytes
.+
$*
1
$`1
1
Try it online! Explanation: The first and last stages are just unary ⇔ decimal conversion. The middle stage replaces each 1 with the number of 1s to its left plus another 1 for the 1 itself, thus counting from 1 to n, summing the values implicitly.
Retina, 13 bytes
.+
$*
1
$`1
1
Try it online! Explanation: The first and last stages are just unary ⇔ decimal conversion. The middle stage replaces each 1 with the number of 1s to its left plus another 1 for the 1 itself, thus counting from 1 to n, summing the values implicitly.
Retina 0.8.2, 13 bytes
.+
$*
1
$`1
1
Try it online! Explanation: The first and last stages are just unary ⇔ decimal conversion. The middle stage replaces each 1 with the number of 1s to its left plus another 1 for the 1 itself, thus counting from 1 to n, summing the values implicitly.
Retina, 13 bytes
.+
$*
1
$`1
1
Try it online! Explanation: The first and last stages are just unary ⇔ decimal conversion. The middle stage replaces each 1 with the number of 1s to its left plus another 1 for the 1 itself, thus counting from 1 to n, summing the values implicitly.