How do you remove multiples of certain numbers from a row vector?

Question

Caroline F 2022-3-7

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1665774-how-do-you-remove-multiples-of-certain-numbers-from-a-row-vector

评论： Stephen23 2022-3-16

I am trying to create a for loop that finds all values in the range [1,100] that are also multiple of 3 or 5, but not 3 AND 5, and save these in a row vector labeled M1. It was suggested that I use mod()/rem() functions. I am struggling on how to remove the numbers that are multiples of 3 and 5.

M1=[];
for x = 1:100
    if mod(x,3)==0||mod(x,5)==0
      M1=[M1,x];
    end 
    if mod(x,3)==0 & mod(x,5)==0
      M1(x)=[]
    end
end
M1

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Max Alger-Meyer 2022-3-7

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1665774-how-do-you-remove-multiples-of-certain-numbers-from-a-row-vector#answer_912164

在 MATLAB Online 中打开

The easiest way to do this is with is just adding additional conditions to your first if statement, and deleting the second if statement alltogether. We do this by placing the "or" conditions in parantheses, and having a third condition that checks that the sum of the mod(x,3) and mod(x,5) isn't zero. If mod(x,3) and mod(x,5) sum to zero, then we know that the index must be wholly divisible by both.

M1=[];
for x = 1:100
    if (mod(x,3) == 0||mod(x,5) == 0) && sum(mod(x,3) + mod(x,5)) ~= 0
      M1=[M1,x];
    end 
end
M1
M1 = 1×41
     3     5     6     9    10    12    18    20    21    24    25    27    33    35    36    39    40    42    48    50    51    54    55    57    63    65    66    69    70    72

2 个评论
显示无隐藏无

Garrett Viton 2022-3-14

How would one go about doing this if it was "3 and 5" instead of "3 or 5"?

Max Alger-Meyer 2022-3-15

在 MATLAB Online 中打开

So if I understand you correctly, you want to remove only numbers that are divisible by both 3 and 5? If that's the case, you could just use the second condition in the if statement:

M1=[];
for x = 1:100
    if sum(mod(x,3) + mod(x,5)) ~= 0
      M1=[M1,x];
    end 
end
M1
M1 = 1×94
     1     2     3     4     5     6     7     8     9    10    11    12    13    14    16    17    18    19    20    21    22    23    24    25    26    27    28    29    31    32

Alternatively, we can simplify this a little further by thinking about what is really happening. This didn't cross my mind the first time, but if you looks at the outputs from my original answer, you can see that the only numbers that are divisible by both 3 and 5 are just going to be multiple of 15, so we can use mod(x,15) instead of checking the sum of the mods of 3 and 5. That leaves us with:

M1=[];
for x = 1:100
    if mod(x,15) ~= 0
      M1=[M1,x];
    end 
end
M1
M1 = 1×94
     1     2     3     4     5     6     7     8     9    10    11    12    13    14    16    17    18    19    20    21    22    23    24    25    26    27    28    29    31    32

请先登录，再进行评论。

Answer 2

Jan 2022-3-15

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1665774-how-do-you-remove-multiples-of-certain-numbers-from-a-row-vector#answer_918119

编辑：Jan 2022-3-16

在 MATLAB Online 中打开

You do not need a loop.

n = 100;
x = false(1, n);
x(3:3:n)   = true;
x(5:5:n)   = true;
x(15:15:n) = false;
Result     = find(x)
Result = 1×41
     3     5     6     9    10    12    18    20    21    24    25    27    33    35    36    39    40    42    48    50    51    54    55    57    63    65    66    69    70    72
% Or:
x      = 1:100;
Result = x((mod(x, 3) == 0 | mod(x, 5) == 0) & mod(x, 15) ~= 0)
Result = 1×41
     3     5     6     9    10    12    18    20    21    24    25    27    33    35    36    39    40    42    48    50    51    54    55    57    63    65    66    69    70    72
% Shorter:
Result = x(~(mod(x, 3) & mod(x, 5)) & mod(x, 15))
Result = 1×41
     3     5     6     9    10    12    18    20    21    24    25    27    33    35    36    39    40    42    48    50    51    54    55    57    63    65    66    69    70    72