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roperators

Understanding the result of modulo operator: %%

I'm trying to understand how the %% operator works in R:

10 %% 10  # 0
20 %% 10  # 0

I'm not sure about these two results:

10 %% 20  # 10
2 %% 8  # 2

Can you help me understand the last two results? I'm a little confused.

Solution

  • Nothing wrong:

    10 = 1 * 10 + 0
20 = 2 * 10 + 0
10 = 0 * 20 + 10
2  = 0 * 8  + 2

    The modulo is the number after +.

    In general, for two numbers a and b, there is

    a = floor(a / b) * b + (a %% b)

    Let's write a toy function:

    foo <- function(a,b) c(quotient = floor(a / b), modulo = a %% b)

foo(10, 10)
#quotient   modulo 
#   1        0 

foo(20, 10)
#quotient   modulo 
#   2        0 

foo(10, 20)
#quotient   modulo 
#   0       10 

foo(2, 8)
#quotient   modulo 
#   0        2

    Update: Instead of using floor(a / b) to get quotient, we can also use a %/% b.