arduino invalid array assignment char

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Description

A data type used to store a character value. Character literals are written in single quotes, like this: 'A' (for multiple characters - strings - use double quotes: "ABC").

Characters are stored as numbers however. You can see the specific encoding in the ASCII chart . This means that it is possible to do arithmetic on characters, in which the ASCII value of the character is used (e.g. 'A' + 1 has the value 66, since the ASCII value of the capital letter A is 65). See Serial.println reference for more on how characters are translated to numbers.

The size of the char datatype is at least 8 bits. It’s recommended to only use char for storing characters. For an unsigned, one-byte (8 bit) data type, use the byte data type.

char var = val;

var : variable name. val : the value to assign to that variable.

Example Code

LANGUAGE Serial.println

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Compare if char array changed

Hello. I'm working on a project where I ask a device through serial, wait for response and then read the variables, then I'm gathering all of the variables by sprintf function to send them to my graph visualising software. For now I used fixed time of 100ms to send all of the variables, even if they didn't change yet but I want to send them as soon as anything changes. My plan was to check if "text" was different from "old_text" and then send it through Serial but I've got an error. Is there any easy way to check this without listing every variable because there will be almost 100 of them, and this sprintf function is one good place to check anything but supposedly I can't copy char arrays to check if it changed.

error: invalid array assignment

old_text=text;

Code snippet:

I'm thinking maybe if I sum all of the values and check if this value changed I can then send data but if there's any other way that I'm not aware of?

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invalid conversion from 'char' to 'char*' [-fpermissive]

Hi, can someone explain me why this code fails to compile?

edit: I'm using ARDUINO IDE, AVRISP mkll for generic ESP8266 module

I can't understand what I'm doing wrong. If you could give me some documentation to read and explain me the problem that would be the best.

EDIT: Now it seems to be ok. I've added the operator & before c . So it looks like this. buffer[length] = &c;            // save current char Can someone explain me what I just did? It would be really important to me to understand this thoroughly.

Furthermore I've another question: why I can't write something like this:

the compiler gives me this error: exit status 1, invalid array assignment. So what would be the correct way to handle this?

You declared buffer as holding pointers to characters (char *) and then attempted to put a char there. &c is a pointer to a character so of course that is allowed.

You probably intended to declare buffer as char and not as char* .

You cannot just copy arrays like that. You will need a loop or a function such as memcpy(...).

vaj4088: You declared buffer as holding pointers to characters (char *) and then attempted to put a char there. &c is a pointer to a character so of course that is allowed. You probably intended to declare buffer as char and not as char* .

mmh ok. So I ask you what are the differences between pointers and characters? Are there any advantages in using one instead of other?

vaj4088: You cannot just copy arrays like that. You will need a loop or a function such as memcpy(...).

Not like other languages. I thougth that if the arrays were been the same size, I would have been allowed to copy in that way. I was wrong. So i need to copy item by item.

There is a BIG difference between characters and pointers. If you need to ask the question, it is time to learn. Take a course or read some books about C or C++.

vaj4088: There is a BIG difference between characters and pointers. If you need to ask the question, it is time to learn. Take a course or read some books about C or C++.

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Assigning value to char array [duplicate]

I get no error when I type

But when I change it to the following, I get an error: Array type char is not assignable.

Why is array type char not assignable? Is it because the language is written that way on purpose or am I missing a point?

• For responses: Since this is tagged C++, is there a non -strcpy "more C++" way to do this while dealing with char[] ? –  user166390 Commented Feb 16, 2013 at 22:56
• A "more C++ way" would be to use string or vector<char> instead of char[]. By using char[] you're basically accepting all the C-ness that goes with it. –  Cogwheel Commented Feb 16, 2013 at 23:07
• Nominated to reopen because this is tagged c++ and the "duplicate" says "in C". I know the answer is the same either way, but they're still different languages and the answer is subject to change or have subtleties worth pointing out here. –  Jim Hunziker Commented Feb 16, 2018 at 15:31

7 Answers 7

An array is not a modifiable lvalue

The C++ way of doing this, as I commented above, would be to use std::string instead of char[] . That will give you the assignment behavior you're expecting.

That said, the reason you're only getting an error for the second case is that the = in these two lines mean different things:

The first is an initialization, the second is an assignment.

The first line allocates enough space on the stack to hold 10 characters, and initializes the first three of those characters to be 'H', 'i', and '\0'. From this point on, all a does is refer to the position of the the array on the stack. Because the array is just a place on the stack, a is never allowed to change. If you want a different location on the stack to hold a different value, you need a different variable.

The second (invalid) line, on the other hand, tries to change a to refer to a (technically different) incantation of "Hi" . That's not allowed for the reasons stated above. Once you have an initialized array, the only thing you can do with it is read values from it and write values to it. You can't change its location or size. That's what an assignment would try to do in this case.

• 1 Would you mind including that you could use a std::string and get the behavior the OP wants? I think that would be a good addition to a good answer. –  NathanOliver Commented Mar 11, 2016 at 16:25
• 1 Yeah, I usually try to give two tier answers to questions like this (one direct, the other second-guessing the intent). Since the asker had seen my comment and this thread got closed as dupe, I guess I didn't bother. –  Cogwheel Commented Mar 11, 2016 at 17:29

The language does not allow assigning string literals to character arrays. You should use strcpy() instead:

• 1 Correct -- for a situation like this you should use strcpy . Forgetting you ever even heard of strncpy would probably be no loss at all -- it's almost as useless as gets . –  Jerry Coffin Commented Feb 17, 2013 at 0:37
• Related Difference between 'strcpy' and 'strcpy_s'? –  Quazi Irfan Commented Mar 3, 2017 at 5:45

a is a pointer to the array, not the array itself. It cannot be reassigned.

You tagged with C++ BTW. For that case better use std::string. It's probably more what you're expecting.

• 1 a is the array itself, not a pointer to the array. It can be implicitly converted to a pointer to the first element. –  Joseph Mansfield Commented Feb 16, 2013 at 22:48

Simple, the

is a little "extra feature", as it cannot be done like that on run-time.

But that's the reason for C/C++ standard libraries.

This comes from the C's standard library. If using C++ you should use std::string, unless you really want to suck all the possible performance from your destination PC.

this is because initialization is not an assignment. the first thing which works is an initialization, and the second one, which does not work, as expected, is assignment. you simply cant assign values to arrays you should use sth like strcpy or memcpy . or you can alternatively use std::copy from <algorithm>

It is so simple,(=) have two different mean assignment and initialization. You can also write your code like that

in this code you have no need to write a difficult code or function and even no need of string.h

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Error: invalid types 'int[int]' for array subscript

I am trying to get an Arduino to work with some Adafruit Motor shields. Everything seemed to work fine, but when I tried to create a function, I get the error in the subject. As far as I can tell, everything was declared properly (the array is initialized and is indexed elsewhere with without issue). Only the function seems to cause an issue.

Would anyone have a guess at what's wrong?

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• 2 Please post the full error message from the compiler. And the code where it occurs. –  Mikael Patel Commented Feb 11, 2016 at 20:04

2 Answers 2

Take a close look at this line:

Looks like it should be:

Otherwise it is an illegal index (as there is only one shield).

The next error is in the function switchMotor(). Again, have a close look at the definition of "motor". There is a local variable with that name but you want to also access the global array with the same name.

My guess is that this is where the compiler give you the error message!

• To be able to help you must add the compiler error output with line, file, and the code. It is too much guessing otherwise. –  Mikael Patel Commented Feb 11, 2016 at 21:00
• Changing the int motor = and the j == motor to xmotor instead of motor allows it to compile perfectly fine for me. Just get rid of that variable naming conflict and it should be fine. –  Majenko Commented Feb 11, 2016 at 22:56
• Thanks Mikael, the motor naming conflict was the problem! –  Pedro Almada Commented Feb 15, 2016 at 12:23

I get a warning flag in UECIDE for the line:

‣ operation on 'address' may be undefined [-Wsequence-point]

Increment address, and then assign the new value from address into address. That seems a bit odd to me. I think you just meant to increment:

Also it flags up the 1 instead of i that Mikael found:

‣ array subscript is above array bounds [-Warray-bounds]

And of course there is the (somewhat hard to spot - Kudos to Mikael for finding it - it took me a bit to see it myself) naming conflict:

‣ invalid types 'int[int]' for array subscript

It was only when I used the advanced editing facilities of UECIDE to highlight all occurrences of motor that I found there was extra ones that shouldn't have been there causing the problem.

So just changing the integer variable name from motor to something else fixes it:

• Cheers, that was the issue! –  Pedro Almada Commented Feb 15, 2016 at 12:24

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