# Using Substring in the Change Value Script

**URL:** <https://forum.inductiveautomation.com/t/using-substring-in-the-change-value-script/65303>\
**Category:** Ignition\
**Created:** [October 17, 2022, 7:08pm UTC](https://forum.inductiveautomation.com/t/using-substring-in-the-change-value-script/65303 "2022-10-17T19:08:36Z")\
**Posts on this page:** 11\
**Page:** 1

<div class="post-metadata">

**Author:** ![John\_Marin](https://avatars.discourse-cdn.com/v4/letter/j/aca169/32.png) [@John\_Marin](https://forum.inductiveautomation.com/u/John_Marin)\
**Post date:** [October 17, 2022, 7:08pm UTC](https://forum.inductiveautomation.com/t/using-substring-in-the-change-value-script/65303/1 "2022-10-17T19:08:36Z")

</div>

Hello. I am trying to take the Substring of test2 and write that value to test3 but only when the value of tag1 changes. I tried in the Change Value tag script using the following:

if previousValue.value != currentValue.value:

```
	system.tag.writeBlocking("[~]Test2", currentValue.value)
	system.tag.writeBlocking("[~]Test3", substring("[~]Test2", 0, 2))

```

---

<div class="post-metadata">

**Author:** ![pascal.fragnoud](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pascal.fragnoud/32/24728_2.png) [@pascal.fragnoud](https://forum.inductiveautomation.com/u/pascal.fragnoud)\
**Post date:** [October 17, 2022, 7:13pm UTC](https://forum.inductiveautomation.com/t/using-substring-in-the-change-value-script/65303/2 "2022-10-17T19:13:04Z")

</div>

And ? What happens ?

ps: substring is not a python function

---

<div class="post-metadata">

**Author:** ![John\_Marin](https://avatars.discourse-cdn.com/v4/letter/j/aca169/32.png) [@John\_Marin](https://forum.inductiveautomation.com/u/John_Marin)\
**Post date:** [October 17, 2022, 7:13pm UTC](https://forum.inductiveautomation.com/t/using-substring-in-the-change-value-script/65303/3 "2022-10-17T19:13:49Z")

</div>

The value does not change. I want to pull the first 2 characters from a string tag and write them to a separate tag.

---

<div class="post-metadata">

**Author:** ![John\_Marin](https://avatars.discourse-cdn.com/v4/letter/j/aca169/32.png) [@John\_Marin](https://forum.inductiveautomation.com/u/John_Marin)\
**Post date:** [October 17, 2022, 7:15pm UTC](https://forum.inductiveautomation.com/t/using-substring-in-the-change-value-script/65303/4 "2022-10-17T19:15:15Z")

</div>

It makes sense if substring is not valid in script. New to ignition

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<div class="post-metadata">

**Author:** ![pascal.fragnoud](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pascal.fragnoud/32/24728_2.png) [@pascal.fragnoud](https://forum.inductiveautomation.com/u/pascal.fragnoud)\
**Post date:** [October 17, 2022, 7:15pm UTC](https://forum.inductiveautomation.com/t/using-substring-in-the-change-value-script/65303/5 "2022-10-17T19:15:25Z")

</div>

I was trying to hint that when you need help with something that doesn't work, providing the error is a good thing to do.  
I'm willing to bet that if you looked at the logs, you'd see something like `NameError: name "substring" is not defined.`

---

<div class="post-metadata">

**Author:** ![Transistor](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/transistor/32/3380_2.png) [@Transistor](https://forum.inductiveautomation.com/u/Transistor)\
**Post date:** [October 17, 2022, 7:15pm UTC](https://forum.inductiveautomation.com/t/using-substring-in-the-change-value-script/65303/6 "2022-10-17T19:15:50Z")

</div>

I'm not sure if this is the problem, but writeBlocking expects **lists**. The two arguements should be in `[...]`.

```auto
if previousValue.value != currentValue.value:
	system.tag.writeBlocking(["[~]Test2"], [currentValue.value])
	system.tag.writeBlocking(["[~]Test3"], [substring("[~]Test2", 0, 2)])

```

[https://docs.inductiveautomation.com/display/DOC80/system.tag.writeBlocking](https://docs.inductiveautomation.com/display/DOC80/system.tag.writeBlocking)

---

<div class="post-metadata">

**Author:** ![John\_Marin](https://avatars.discourse-cdn.com/v4/letter/j/aca169/32.png) [@John\_Marin](https://forum.inductiveautomation.com/u/John_Marin)\
**Post date:** [October 17, 2022, 7:17pm UTC](https://forum.inductiveautomation.com/t/using-substring-in-the-change-value-script/65303/7 "2022-10-17T19:17:00Z")

</div>

I think the main issue is with the substring instruction. I am able to write the full value to another tag without issue but I only want the first 2 characters. Thanks for the replies, and yes pascal that is the exact error I am getting.

---

<div class="post-metadata">

**Author:** ![pascal.fragnoud](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pascal.fragnoud/32/24728_2.png) [@pascal.fragnoud](https://forum.inductiveautomation.com/u/pascal.fragnoud)\
**Post date:** [October 17, 2022, 7:21pm UTC](https://forum.inductiveautomation.com/t/using-substring-in-the-change-value-script/65303/8 "2022-10-17T19:21:41Z")

</div>

I think the write and read functions actually do accept single tags now.

Though @John_Marin should really be using

```python
system.tag.writeBlocking(
    ["[~]Test2", "[~]Test3"],
    [currentValue.value, X]
)

```

About that X in there. I'm guessing you want the first 2 letters of `Test2`'s value. This, if `substring` worked, would return `[~`, the first the characters of the string you passed it.  
If you need the tag's value, you'll need to read it with `system.tag.readBlocking`

To get a substring in python, use slices.

```python
s = "foobarbaz"
s[:4] == "foo"
s[4:7] == "bar"
s[7:] == "baz"

```

---

<div class="post-metadata">

**Author:** ![John\_Marin](https://avatars.discourse-cdn.com/v4/letter/j/aca169/32.png) [@John\_Marin](https://forum.inductiveautomation.com/u/John_Marin)\
**Post date:** [October 17, 2022, 7:24pm UTC](https://forum.inductiveautomation.com/t/using-substring-in-the-change-value-script/65303/9 "2022-10-17T19:24:50Z")

</div>

Thank you very much! I will try this out.

---

<div class="post-metadata">

**Author:** ![John\_Marin](https://avatars.discourse-cdn.com/v4/letter/j/aca169/32.png) [@John\_Marin](https://forum.inductiveautomation.com/u/John_Marin)\
**Post date:** [October 17, 2022, 7:38pm UTC](https://forum.inductiveautomation.com/t/using-substring-in-the-change-value-script/65303/10 "2022-10-17T19:38:04Z")

</div>

It worked! Here is the final code, it takes the first 2 characters of the string to one tag and the remainder to the rest.

```auto
if previousValue.value != currentValue.value:
		system.tag.writeBlocking(
			["[~]Test2", "[~]Test3"],
			[currentValue.value[0:2], currentValue.value[2:]]
		)

```

Thanks again!

---

<div class="post-metadata">

**Author:** ![Transistor](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/transistor/32/3380_2.png) [@Transistor](https://forum.inductiveautomation.com/u/Transistor)\
**Post date:** [October 17, 2022, 7:39pm UTC](https://forum.inductiveautomation.com/t/using-substring-in-the-change-value-script/65303/11 "2022-10-17T19:39:49Z")

</div>

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