How to change array variable in c

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How to change array variable in c

By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Arduino Stack Exchange is a question and answer site for developers of open-source hardware and software that is compatible with Arduino. It only takes a minute to sign up. I'd like to make three arrays of the same length. Is there a way to variably determine array sizes, or do I just need to hardcode them?

I was taught hardcoding is bad and something to avoid at all costs. For that you will need dynamic allocation, which works on Arduino, but is generally not advised as this can cause the "heap" to become fragmented.

For more about problems with heap fragmentation, you can refer to this question.

how to change array variable in c

The size of the array must be known at compile time. Otherwise you should allocate memory dynamically using:.

Variable Length Arrays in C and C++

However if you just want to declare some arrays of the same size, you just have to declare the number a constant like this:. I think not hardcoding things only makes sense if you would reasonably expect the user to want to change the setting at some point. I don't know if that's the case. If you know the maximum length of array, just initialize the array to that length and use an integer to tell the program how much of that array to use.

If it's the difference between 7,10 bytes then you are not wasting that much memory allocation. I know I'm a little late here, but in theory regular arrays can't be created using a variable to define the amount of elements the array is going to have as in:. This will display an error since when declaring the array, the program expects for the value between the brackets to be a constant. Yet, there is a way in which you could create an array with a variable defining the amount of values this arrays is going to have through dynamic memory allocation for value sets this method has been tested with monodimensional arrays only, haven't tried for multidimensional yetand it goes something like this:.

After this, all that's left to do is assign a value for every element created in the instance myArray which is already an Array by now as you would for a normal array created as myArray[arrSize].

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The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. How can I declare an array of variable size Globally Ask Question. Asked 5 years, 8 months ago. Active 1 year, 2 months ago. Viewed k times. I had a similar problem and did this. I am learning too so can't say if it is valid solution or not but it worked. Active Oldest Votes.In this post, I want to write down the lesson learned about modifying array inside a function in C with an example from MAW 3.

In general, there are two cases when we need to use functions to work with array. Let's examine accordingly. If we take a look at what value hold the address, we can see that it's 1which is the first element of our int test[3] array. This leads to our very first important observation:. Since the size of int under my system is 4 bytes check by p sizeof int in gdband let's examine the four conseuctive bytes with starting address 0x7fffffffe :.

As you can see, this is integer 1. Now, let's start with the first iteration of the loop in change. Once we finish the iteration, i becomes 1 and let's see what change to our array :. We can see that the first element of our test array becomes 5 and the starting address of our array is still 0x7fffffffe In other words, the only thing changed is the value that address 0x7fffffffe holds. In addition, if you take a look at the array address output, you can see that before the function call, during the function call, and after the function call, the array address doesn't change at all: 0x7fffffffe This leads to our second observation:.

We can change the contents of array in the caller function i. This modification can be effective in the caller function without any return statement. However, doing so, we doesn't change the address of the array.

It seems that array is a local variable inside both caller function and callee function. However, this is wrong and the output confirms our observation above: array is local variable to the caller function and callee function, and when we pass a array into a function, the address is passed copied from caller to callee.

After that, address inside callee can reassign and will have no effect on the array address in caller. What if we want to resize the array to make it hold more values? Before we start to answer the above question. Let me clear out an important concept: "array on stack" and "array on heap". The array declared like this stays on the stack and local to the function calls. When we talk about resize the array, we mean the latter case. In other words, we can only change the array itself number of elements with dynamically allocated array in the heap.

In our case, The template sentence becomes "array is Now let's work out the " In our case, we start with "array" and go right, and nothing left with declaraiton. So, we must go left.

So now our template sentence becomes "array is pointer to Then we meet intwhich means all the symbol in the declaration is consumed and our sentence is complete: "array is pointer to pointer to int".

This means array variable itself is a pointer containing an address of a pointer, which holds an address of a int. The address holds by array is 0x7fffffffe We further examine the value holds by 0x7fffffffe and by our assumption, it should be another address and it is: 0x Then, we check the value hold by that address, which is expected 1 the first element of our test array.

From the picture we can see that we want to modify array inside change3 pointing to 5,5,5 and this change will persist to the test array in our caller function. In other words, we want both test and array no longer independent but want them "tie up" as the same pointer with different names.

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How do we do that?The usual way of declaring an array is to simply line up the type name, followed by a variable name, followed by a size in brackets, as in this line of code:. This code declares an array of 10 integers. The first element gets index 0, and the final element gets index 9. Remember, index refers to the position within the array, and size refers to the number of elements in the array. In certain situations, you can declare an array without putting a number in the brackets.

C - Arrays

For example, you can initialize an array without specifying the number of elements:. The compiler is smart enough to count how many elements you put inside the braces, and then the compiler makes that count the array size.

Specifying the array size helps decrease your chances of having bugs, bugs, everywhere bugs. Plus, it has the added benefit that, in the actual declaration, if the number in brackets does not match the number of elements inside braces, the compiler issues an error, at least if the number is smaller anyway.

The following. But if the number in brackets is greater than the number of elements, as in the following code, you will not get an error. So be careful! This technique is particularly powerful because the AddUp function can work for any size array. You can call the function like this:. But this way to do it is kind of annoying because you have to specify the size each time you call in to the function.

However, you can get around this problem. Look at this line of code:. With the array, the sizeof operator tells you how many bytes it uses. But the size of the array is usually the number of elements, not the number of bytes. So you divide the result of sizeof by 4 the size of each element. But now you have that magic number, 4, sitting there.

So a slightly better approach would be to enter this line:.Arrays a kind of data structure that can store a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.

Instead of declaring individual variables, such as number0, number1, A specific element in an array is accessed by an index. All arrays consist of contiguous memory locations. The lowest address corresponds to the first element and the highest address to the last element.

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This is called a single-dimensional array. The arraySize must be an integer constant greater than zero and type can be any valid C data type. If you omit the size of the array, an array just big enough to hold the initialization is created.

You will create exactly the same array as you did in the previous example. The above statement assigns the 5 th element in the array with a value of All arrays have 0 as the index of their first element which is also called the base index and the last index of an array will be total size of the array minus 1. An element is accessed by indexing the array name. This is done by placing the index of the element within square brackets after the name of the array.

The above statement will take the 10 th element from the array and assign the value to salary variable. The following example Shows how to use all the three above mentioned concepts viz. Arrays are important to C and should need a lot more attention.

C supports multidimensional arrays. The simplest form of the multidimensional array is the two-dimensional array. You can pass to the function a pointer to an array by specifying the array's name without an index. You can generate a pointer to the first element of an array by simply specifying the array name, without any index.

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how to change array variable in c

Multi-dimensional arrays C supports multidimensional arrays. Passing arrays to functions You can pass to the function a pointer to an array by specifying the array's name without an index. Return array from a function C allows a function to return an array.

Pointer to an array You can generate a pointer to the first element of an array by simply specifying the array name, without any index.Arrays and pointers are intimately linked in C. To use arrays effectively, you have to know how to use pointers with them. Fully understanding the relationship between the two probably requires several days of study and experimentation, but it is well worth the effort.

how to change array variable in c

Enter this code and try to compile it. You will find that C will not compile it. If you want to copy a into byou have to enter something like the following instead:. Arrays in C are unusual in that variables a and b are not, technically, arrays themselves. Instead they are permanent pointers to arrays.

Since they are permanent pointers you cannot change their addresses. Because a and b are pointers, you can do several interesting things with pointers and arrays. For example, the following code works:. Technically, a points to the address of the 0th element of the actual array. This element is an integer, so a is a pointer to a single integer. Therefore, declaring p as a pointer to an integer and setting it equal to a works.

Now that p is pointing at the 0th element of ayou can do some rather strange things with it. The a variable is a permanent pointer and can not be changed, but p is not subject to such restrictions.

C++ Tutorial 27 - Dynamic Arrays

C actually encourages you to move it around using pointer arithmetic. C takes care of the details of element size. You can copy the array a into b using pointers as well. What if you go beyond the end of the array a or b with the pointers p or q? C does not care -- it blithely goes along incrementing p and qcopying away over other variables with abandon.

You need to be careful when indexing into arrays in C, because C assumes that you know what you are doing. You can pass an array such as a or b to a function in two different ways. Imagine a function dump that accepts an array of integers as a parameter and prints the contents of the array to stdout.

There are two ways to code dump :. Note that only a pointer to the array, rather than the contents of the array, is passed to the function.

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Also note that C functions can accept variable-size arrays as parameters.Sometimes in C programming, a variable must be like cellular phone service: available everywhere. The variable also can be used by any function at any time.

This type of variable could be called a universal variable. It could be called a worldwide variable. Global variables solve specific problems by making the variable declaration universal. That way, any function anywhere in the program can access the variable. Tossing Your Age Around shows how a global variable is declared and used. The global variables age and float are affected by both functions.

They can be passed to those functions, but both values cannot be returned. C functions return only one value. Therefore, the global variable is used as a solution. Line 6 declares the global int variable age and the float variable feet.

The variables are then used in every function. Their values can be accessed throughout the code. Even when those values are changed in the twice function, the main function uses the new values.

Be aware that two printf statements in the main function wrap their text in Tossing Your Age Around. Exercise 1 : Type the source code for Tossing Your Age Around into your editor, creating a new program.

Build and run. If you can pass a value to a function, do so! In that case, consider making those variables global instead.

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A better example of using a global variable, and a situation where global variables are completely necessary, is when passing a structure to a function. In that case, you must declare the structure as global so that all functions can access variables of that structure type.

To pass a structure to a function, the structure must be declared globally, which happens between Lines 7 and That has to happen even before the function is prototyped, which takes place at Line The initialize function runs from Lines 30 through The structure is passed to the function and returned. Note that the structure variable must be fully defined as the argument. On Line 30, the function is given the variable name b inside the function.

The return statement at Line 40 passes the structure back to the calling function. Indeed, the initialize function is defined as a structure bot type of function. Exercise 2 : Screw your courage to the sticking place, and type all those lines of source code from Passing a Structure to a Function into your editor.

The output demonstrates how the structure array was passed one element at a time to a function, modified in the function, and then returned.

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Dan Gookin wrote the original For Dummies book in Now, with more than 11 million copies in print, his many books have been translated into 32 languages. Visit him at wambooli. Basics of Global Variables in C Programming.Whenever we use const qualifier with variable name, it becomes a read-only variable and get stored in. In the below program, a read-only variable declared using the const qualifier is tried to modify:. The variables declared using const keyword, get stored in.

By assigning the address of the variable to a non-constant pointer, We are casting a constant variable to a non-constant pointer. The compiler will give warning while typecasting and will discard the const qualifier. Compiler optimization is different for variables and pointers. That is why we are able to change the value of a constant variable through a non-constant pointer. Note : If we try to change the value through constant pointer then we will get an error because we are trying to change the read-only segment.

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Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below. Writing code in comment? Please use ide. How to modify a const variable in C? Internal static variable vs. Check out this Author's contributed articles. Load Comments.


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