How to change values between two values in an array

Question

Jake Jordan 2021-10-4

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回答： Swatantra Mahato 2021-10-7

I need to replace certain values within an array that fall between 2 and 4. The code should find where the array equals 2 and then find the following location where the array equals 4 and replace all values with 3. Then it should find where the array equals 4 and then find the following location where the array equals 2 and replaces all of those values with 1. And then loop through the array until all values are 1, 2, 3 or 4 and in order (repeats of values are OK).

Here is an example array:

transitions = [1285738, 2, 2, 2915260, 4, 4290129, 2, 5650291, 4, 8030363, 2, 9337983, 4, 12040647];

I need to:

change the values that fall between 2 and 4 to 3 (e.g., the 4th, 8th and 12th values should be 3)
change the values between 4 and 2 to 1 (e.g., the 6th and 10th values should be 1)

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DGM 2021-10-4

在 MATLAB Online 中打开

The requirements statement is still ambiguous.

"array equals 2 and ... array equals 4" should become 3

then

"array equals 4 and ... array equals 2" should become 1

If the first operation is performed, then the second condition is never true. If we're to assume that what you mean is that everything in some interval bounded by 2 and 4 is to be converted to 3, and all else is to be converted to 1, then it's still unclear whether 2 or 4 convert to 1 or 3. Your statements say both.

To add to the confusion,

"And then loop through the array until all values are 1, 2, 3 or 4 and in order"

Why would the result still contain 2 or 4? Why would it be in order? What order?

Let's say I assume the following:

values in the closed interval [2 4] become 3

else, they become 1

transitions = [1285738, 2, 2, 2915260, 4, 4290129, 2, 5650291, 4, 8030363, 2, 9337983, 4, 12040647];
ininterval = (transitions>=2) & (transitions<=4);
transitions(ininterval) = 3;
transitions(~ininterval) = 1;
transitions
transitions = 1×14
     1     3     3     1     3     1     3     1     3     1     3     1     3     1

If that's not what you're after, you'll have to clarify.

the cyclist 2021-10-4

I understood @Jake Jordan's rules to be:

If a 4 follows a 2, change every intervening value to 3, THEN
If a 2 follows a 4, change every intervening value to 1

DGM 2021-10-4

编辑：DGM 2021-10-4

... oh.

I guess "between" is one of those expressions like "in a row" that are sneakily ambiguous when talking about array structures.

EDIT: oof. I didn't even see OP's comment.

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Answer 1

Swatantra Mahato 2021-10-7

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1466489-how-to-change-values-between-two-values-in-an-array#answer_803316

在 MATLAB Online 中打开

Hi Jake,

I am assuming you want to replace values in the array that occur between a 2 and 4, and those that occur between a 4 and 2.

One way of doing this I can think of is using the "find" function to get the indices of all the 2s and 4s as demonstrated below

transitions = [1285738, 2, 2, 2915260, 4, 4290129, 2, 5650291, 4, 8030363, 2, 9337983, 4, 12040647];
t=find(transitions==2)
t = 1×4
     2     3     7    11
f=find(transitions==4)
f = 1×3
     5     9    13

you can then loop over 'f' and 't' and replace the values in between accordingly

you can find more information and how-to examples on the "find" function in the documentation

https://www.mathworks.com/help/matlab/ref/find.html

Hope this helps

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