取模是在模运算的基础上求得的余数
b = mod(a,m)，这是matlab的帮助文档
b = mod(a,m) returns the remainder after division of a by m, where a is the dividend and m is the divisor. This function is often called the modulo operation, which can be expressed as b = a - m.*floor(a./m).
mod(a,0) = a
mod(-5,3) = -5 - 3*floor(-5/3) = -5 - 3* (-2) = 1
求余数 rem求余数是在除法的基础上取得的余数
r = rem(a,b)
r = rem(a,b) returns the remainder after division of a by b, where a is the dividend and b is the divisor. This function is often called the remainder operation, which can be expressed as r = a - b.*fix(a./b).
rem(a,0) = 无穷小量
rem(-5,3) = -5 - 3*fix(-5,3) = -5 - 3*(-1) = -2
此外1，经过测试，在C/C++, C#, JAVA,PHP这几门主流语言中，’%’运算符都是做取余运算，而在python中的’%’是做取模运算[1]。
此外2，余数在数学中的定义是始终大于等于0的，而对于某些编程语言的取余运算不是按照上面的定义来执行的，因此才会出现余数小于0的情况[1]。
因此，在代码移植和数值计算的时候要特别小心，这种bug一般是找不出来的。
参考：[1] ： https://blog.csdn.net/u012142247/article/details/79445820