multiply the digits in the odd-numbered positions by 3 and add them together in matlab

Assuming the input number is stored as a string, the following code can be used to multiply the digits in the odd-numbered positions by 3 and add them together:

input_str = '123456789'; odd_digits = input_str(1:2:end); odd_digit_sum = sum(arrayfun(@(x) str2double(x)*3, odd_digits));

First, we extract the odd-numbered digits using indexing. We then use the arrayfun function to apply a lambda function that multiplies each odd digit by 3 and casts it as a double. Finally, we take the sum of the resulting array to get the sum of the odd digits multiplied by 3.

Note that we assume the input string only contains digits. If it contains non-digit characters, this code will throw an error.