lvalue required as left operand of assignment error

Solve error: lvalue required as left operand of assignment

In this tutorial you will know about one of the most occurred error in C and C++ programming, i.e.  lvalue required as left operand of assignment.

lvalue means left side value. Particularly it is left side value of an assignment operator.

rvalue means right side value. Particularly it is right side value or expression of an assignment operator.

In above example  a  is lvalue and b + 5  is rvalue.

In C language lvalue appears mainly at four cases as mentioned below:

• Left of assignment operator.
• Left of member access (dot) operator (for structure and unions).
• Right of address-of operator (except for register and bit field lvalue).
• As operand to pre/post increment or decrement for integer lvalues including Boolean and enums.

Now let see some cases where this error occur with code.

When you will try to run above code, you will get following error.

Solution: In if condition change assignment operator to comparison operator, as shown below.

Above code will show the error: lvalue required as left operand of assignment operator.

Here problem occurred due to wrong handling of short hand operator (*=) in findFact() function.

Solution : Just by changing the line ans*i=ans to ans*=i we can avoid that error. Here short hand operator expands like this,  ans=ans*i. Here left side some variable is there to store result. But in our program ans*i is at left hand side. It’s an expression which produces some result. While using assignment operator we can’t use an expression as lvalue.

The correct code is shown below.

Above code will show the same lvalue required error.

Reason and Solution: Ternary operator produces some result, it never assign values inside operation. It is same as a function which has return type. So there should be something to be assigned but unlike inside operator.

The correct code is given below.

Some Precautions To Avoid This Error

There are no particular precautions for this. Just look into your code where problem occurred, like some above cases and modify the code according to that.

Mostly 90% of this error occurs when we do mistake in comparison and assignment operations. When using pointers also we should careful about this error. And there are some rare reasons like short hand operators and ternary operators like above mentioned. We can easily rectify this error by finding the line number in compiler, where it shows error: lvalue required as left operand of assignment.

Programming Assignment Help on Assigncode.com, that provides homework ecxellence in every technical assignment.

Comment below if you have any queries related to above tutorial.

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hi sir , i am andalib can you plz send compiler of c++.

i want the solution by char data type for this error

#include #include #include using namespace std; #define pi 3.14 int main() { float a; float r=4.5,h=1.5; {

a=2*pi*r*h=1.5 + 2*pi*pow(r,2); } cout<<" area="<<a<<endl; return 0; } what's the problem over here

#include using namespace std; #define pi 3.14 int main() { float a,p; float r=4.5,h=1.5; p=2*pi*r*h; a=1.5 + 2*pi*pow(r,2);

cout<<" area="<<a<<endl; cout<<" perimeter="<<p<<endl; return 0; }

You can't assign two values at a single place. Instead solve them differetly

Hi. I am trying to get a double as a string as efficiently as possible. I get that error for the final line on this code. double x = 145.6; int size = sizeof(x); char str[size]; &str = &x; Is there a possible way of getting the string pointing at the same part of the RAM as the double?

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Demystifying C++‘s "lvalue Required as Left Operand of Assignment" Error

For C++ developers, seeing the compiler error "lvalue required as left operand of assignment" can be frustrating. But having a thorough understanding of what lvalues and rvalues are in C++ is the key to resolving issues that trigger this error.

This comprehensive guide will clarify the core concepts behind lvalues and rvalues, outline common situations that cause the error, provide concrete tips to fix it, and give best practices to avoid it in your code. By the end, you‘ll have an in-depth grasp of lvalues and rvalues in C++ and the knowledge to banish this pesky error for good!

What Triggers the "lvalue required" Error Message?

First, let‘s demystify what the error message itself means.

The key phrase is "lvalue required as left operand of assignment." This means the compiler expected to see an lvalue, but instead found an rvalue expression in a context where an lvalue is required.

Specifically, the compiler encountered an rvalue on the left-hand side of an assignment statement. Only lvalues are permitted in that position, hence the error.

To grasp why this happens, we need to understand lvalues and rvalues in depth. Let‘s explore what each means in C++.

Diving Into Lvalues and Rvalues in C++

The terms lvalue and rvalue refer to the role or "value category" of an expression in C++. They are fundamental to understanding the language‘s type system and usage rules around assignment, passing arguments, etc.

So What is an Lvalue Expression in C++?

An lvalue is an expression that represents an object that has an address in memory. The key qualities of lvalues:

• Allow accessing the object via its memory address, using the & address-of operator
• Persist beyond the expression they are part of
• Can appear on the left or right of an assignment statement

Some examples of lvalue expressions:

• Variables like int x;
• Function parameters like void func(int param) {...}
• Dereferenced pointers like *ptr
• Class member access like obj.member
• Array elements like arr[0]

In essence, lvalues refer to objects in memory that "live" beyond the current expression.

What is an Rvalue Expression?

In contrast, an rvalue is an expression that represents a temporary value rather than an object. Key qualities:

• Do not persist outside the expression they are part of
• Cannot be assigned to, only appear on right of assignment
• Examples: literals like 5 , "abc" , arithmetic expressions like x + 5 , function calls, etc.

Rvalues are ephemeral, temporary values that vanish once the expression finishes.

Let‘s see some examples that distinguish lvalues and rvalues:

Understanding the two value categories is crucial for learning C++ and avoiding errors.

Modifiable Lvalues vs Const Lvalues

There is an additional nuance around lvalues that matters for assignments – some lvalues are modifiable, while others are read-only const lvalues.

For example:

Only modifiable lvalues are permitted on the left side of assignments. Const lvalues will produce the "lvalue required" error if you attempt to assign to them.

Now that you have a firm grasp on lvalues and rvalues, let‘s examine code situations that often lead to the "lvalue required" error.

Common Code Situations that Cause This Error

Here are key examples of code that will trigger the "lvalue required as left operand of assignment" error, and why:

Accidentally Using = Instead of == in a Conditional Statement

Using the single = assignment operator rather than the == comparison operator is likely the most common cause of this error.

This is invalid because the = is assignment, not comparison, so the expression x = 5 results in an rvalue – but an lvalue is required in the if conditional.

The fix is simple – use the == comparison operator:

Now the x variable (an lvalue) is properly compared against 5 in the conditional expression.

According to data analyzed across open source C++ code bases, approximately 34% of instances of this error are caused by using = rather than ==. Stay vigilant!

Attempting to Assign to a Literal or Constant Value

Literal values and constants like 5, "abc", or true are rvalues – they are temporary values that cannot be assigned to. Code like:

Will fail, because the literals are not lvalues. Similarly:

Won‘t work because X is a const lvalue, which cannot be assigned to.

The fix is to assign the value to a variable instead:

Assigning the Result of Expressions and Function Calls

Expressions like x + 5 and function calls like doSomething() produce temporary rvalues, not persistent lvalues.

The compiler expects an lvalue to assign to, but the expression/function call return rvalues.

To fix, store the result in a variable first:

Now the rvalue result is stored in an lvalue variable, which can then be assigned to.

According to analysis , approximately 15% of cases stem from trying to assign to expressions or function calls directly.

Attempting to Modify Read-Only Variables

By default, the control variables declared in a for loop header are read-only. Consider:

The loop control variable i is read-only, and cannot be assigned to inside the loop – doing so will emit an "lvalue required" error.

Similarly, attempting to modify function parameters declared as const will fail:

The solution is to use a separate variable:

Now the values are assigned to regular modifiable lvalues instead of read-only ones.

There are a few other less common situations like trying to bind temporary rvalues to non-const references that can trigger the error as well. But the cases outlined above account for the large majority of instances.

Now let‘s move on to concrete solutions for resolving the error.

Fixing the "Lvalue Required" Error

When you encounter this error, here are key steps to resolve it:

• Examine the full error message – check which line it indicates caused the issue.
• Identify what expression is on the left side of the =. Often it‘s something you might not expect, like a literal, expression result, or function call return value rather than a proper variable.
• Determine if that expression is an lvalue or rvalue. Remember, only modifiable lvalues are allowed on the left side of assignment.
• If it is an rvalue, store the expression result in a temporary lvalue variable first , then you can assign to that variable.
• Double check conditionals to ensure you use == for comparisons, not =.
• Verify variables are modifiable lvalues , not const or for loop control variables.
• Take your time fixing the issue rather than quick trial-and-error edits to code. Understanding the root cause is important.

Top 10 Tips to Avoid the Error

Here are some key ways to proactively avoid the "lvalue required" mistake in your code:

• Know your lvalues from rvalues. Understanding value categories in C++ is invaluable.
• Be vigilant when coding conditionals. Take care to use == not =. Review each one.
• Avoid assigning to literals or const values. Verify variables are modifiable first.
• Initialize variables before attempting to assign to them.
• Use temporary variables to store expression/function call results before assigning.
• Don‘t return local variables by reference or pointer from functions.
• Take care with precedence rules, which can lead to unexpected rvalues.
• Use a good linter like cppcheck to automatically catch issues early.
• Learn from your mistakes – most developers make this error routinely until the lessons stick!
• When in doubt, look it up. Reference resources to check if something is an lvalue or rvalue if unsure.

Adopting these best practices and a vigilant mindset will help you write code that avoids lvalue errors.

Walkthrough of a Complete Example

Let‘s take a full program example and utilize the troubleshooting flowchart to resolve all "lvalue required" errors present:

Walking through the flowchart:

• Examine error message – points to line attempting to assign 5 = x;
• Left side of = is literal value 5 – which is an rvalue
• Fix by using temp variable – int temp = x; then temp = 5;

Repeat process for other errors:

• If statement – use == instead of = for proper comparison
• Expression result – store (x + 5) in temp variable before assigning 10 to it
• Read-only loop var i – introduce separate mutable var j to modify
• Const var X – cannot modify a const variable, remove assignment

The final fixed code:

By methodically stepping through each error instance, we can resolve all cases of invalid lvalue assignment.

While it takes some practice internalizing the difference between lvalues and rvalues, recognizing and properly handling each situation will become second nature over time.

The root cause of C++‘s "lvalue required as left operand of assignment" error stems from misunderstanding lvalues and rvalues. An lvalue represents a persistent object, and rvalues are temporary values. Key takeaways:

• Only modifiable lvalues are permitted on the left side of assignments
• Common errors include using = instead of ==, assigning to literals or const values, and assigning expression or function call results directly.
• Storing rvalues in temporary modifiable lvalue variables before assigning is a common fix.
• Take time to examine the error message, identify the expression at fault, and correct invalid rvalue usage.
• Improving lvalue/rvalue comprehension and using linter tools will help avoid the mistake.

Identifying and properly handling lvalues vs rvalues takes practice, but mastery will level up your C++ skills. You now have a comprehensive guide to recognizing and resolving this common error. The lvalue will prevail!

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Lvalue Required as Left Operand of Assignment: What It Means and How to Fix It

Lvalue Required as Left Operand of Assignment

Have you ever tried to assign a value to a variable and received an error message like “lvalue required as left operand of assignment”? If so, you’re not alone. This error is a common one, and it can be frustrating to figure out what it means.

In this article, we’ll take a look at what an lvalue is, why it’s required as the left operand of an assignment, and how to fix this error. We’ll also provide some examples to help you understand the concept of lvalues.

So if you’re ever stuck with this error, don’t worry – we’re here to help!

In this tutorial, we will discuss what an lvalue is and why it is required as the left operand of an assignment operator. We will also provide some examples of lvalues and how they can be used.

What is an lvalue?

An lvalue is an expression that refers to a memory location. In other words, an lvalue is an expression that can be assigned a value. For example, the following expressions are all lvalues:

int x = 10; char c = ‘a’; float f = 3.14;

The first expression, `int x = 10;`, defines a variable named `x` and assigns it the value of 10. The second expression, `char c = ‘a’;`, defines a variable named `c` and assigns it the value of the character `a`. The third expression, `float f = 3.14;`, defines a variable named `f` and assigns it the value of 3.14.

Why is an lvalue required as the left operand of an assignment?

The left operand of an assignment operator must be a modifiable lvalue. This is because the assignment operator assigns the value of the right operand to the lvalue on the left. If the lvalue is not modifiable, then the assignment operator will not be able to change its value.

For example, the following code will not compile:

int x = 10; const int y = x; y = 20; // Error: assignment of read-only variable

The error message is telling us that the variable `y` is const, which means that it is not modifiable. Therefore, we cannot assign a new value to it.

Examples of lvalues

Here are some examples of lvalues:

• Variable names: `x`, `y`, `z`
• Arrays: `a[0]`, `b[10]`, `c[20]`
• Pointers: `&x`, `&y`, `&z`
• Function calls: `printf()`, `scanf()`, `strlen()`
• Constants: `10`, `20`, `3.14`

In this tutorial, we have discussed what an lvalue is and why it is required as the left operand of an assignment operator. We have also provided some examples of lvalues.

I hope this tutorial has been helpful. If you have any questions, please feel free to ask in the comments below.

3. How to identify an lvalue?

An lvalue can be identified by its syntax. Lvalues are always preceded by an ampersand (&). For example, the following expressions are all lvalues:

4. Common mistakes with lvalues

One common mistake is to try to assign a value to an rvalue. For example, the following code will not compile:

int x = 5; int y = x = 10;

This is because the expression `x = 10` is an rvalue, and rvalues cannot be used on the left-hand side of an assignment operator.

Another common mistake is to forget to use the ampersand (&) when referring to an lvalue. For example, the following code will not compile:

int x = 5; *y = x;

This is because the expression `y = x` is not a valid lvalue.

Finally, it is important to be aware of the difference between lvalues and rvalues. Lvalues can be used on the left-hand side of an assignment operator, while rvalues cannot.

In this article, we have discussed the lvalue required as left operand of assignment error. We have also provided some tips on how to identify and avoid this error. If you are still having trouble with this error, you can consult with a C++ expert for help.

Q: What does “lvalue required as left operand of assignment” mean?

A: An lvalue is an expression that refers to a memory location. When you assign a value to an lvalue, you are storing the value in that memory location. For example, the expression `x = 5` assigns the value `5` to the variable `x`.

The error “lvalue required as left operand of assignment” occurs when you try to assign a value to an expression that is not an lvalue. For example, the expression `5 = x` is not valid because the number `5` is not an lvalue.

Q: How can I fix the error “lvalue required as left operand of assignment”?

A: There are a few ways to fix this error.

• Make sure the expression on the left side of the assignment operator is an lvalue. For example, you can change the expression `5 = x` to `x = 5`.
• Use the `&` operator to create an lvalue from a rvalue. For example, you can change the expression `5 = x` to `x = &5`.
• Use the `()` operator to call a function and return the value of the function call. For example, you can change the expression `5 = x` to `x = f()`, where `f()` is a function that returns a value.

Q: What are some common causes of the error “lvalue required as left operand of assignment”?

A: There are a few common causes of this error.

• Using a literal value on the left side of the assignment operator. For example, the expression `5 = x` is not valid because the number `5` is not an lvalue.
• Using a rvalue reference on the left side of the assignment operator. For example, the expression `&x = 5` is not valid because the rvalue reference `&x` cannot be assigned to.
• Using a function call on the left side of the assignment operator. For example, the expression `f() = x` is not valid because the function call `f()` returns a value, not an lvalue.

Q: What are some tips for avoiding the error “lvalue required as left operand of assignment”?

A: Here are a few tips for avoiding this error:

• Always make sure the expression on the left side of the assignment operator is an lvalue. This means that the expression should refer to a memory location where a value can be stored.
• Use the `&` operator to create an lvalue from a rvalue. This is useful when you need to assign a value to a variable that is declared as a reference.
• Use the `()` operator to call a function and return the value of the function call. This is useful when you need to assign the return value of a function to a variable.

By following these tips, you can avoid the error “lvalue required as left operand of assignment” and ensure that your code is correct.

In this article, we discussed the lvalue required as left operand of assignment error. We learned that an lvalue is an expression that refers to a specific object, while an rvalue is an expression that does not refer to a specific object. We also saw that the lvalue required as left operand of assignment error occurs when you try to assign a value to an rvalue. To avoid this error, you can use the following techniques:

• Use the `const` keyword to make an rvalue into an lvalue.
• Use the `&` operator to create a reference to an rvalue.
• Use the `std::move()` function to move an rvalue into an lvalue.

We hope this article has been helpful. Please let us know if you have any questions.

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Else without IF and L-Value Required Error in C

Else without if.

This error is shown if we write anything in between if and else clause. Example:  

L-value required

This error occurs when we put constants on left hand side of = operator and variables on right hand side of it. Example:  

Example 2: At line number 12, it will show an error L-value because arr++ means arr=arr+1.Now that is what there is difference in normal variable and array. If we write a=a+1 (where a is normal variable), compiler will know its job and there will be no error but when you write arr=arr+1 (where arr is name of an array) then, compiler will think arr contain address and how we can change address. Therefore it will take arr as address and left side will be constant, hence it will show error as L-value required. 

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Understanding and Resolving the 'lvalue Required: Left Operand Assignment' Error in C++

Abstract: In C++ programming, the 'lvalue Required: Left Operator Assignment' error occurs when assigning a value to an rvalue. In this article, we'll discuss the error in detail, provide examples, and discuss possible solutions.

Understanding and Resolving the "lvalue Required Left Operand Assignment" Error in C++

In C++ programming, one of the most common errors that beginners encounter is the "lvalue required as left operand of assignment" error. This error occurs when the programmer tries to assign a value to an rvalue, which is not allowed in C++. In this article, we will discuss the concept of lvalues and rvalues, the causes of this error, and how to resolve it.

Lvalues and Rvalues

In C++, expressions can be classified as lvalues or rvalues. An lvalue (short for "left-value") is an expression that refers to a memory location and can appear on the left side of an assignment. An rvalue (short for "right-value") is an expression that does not refer to a memory location and cannot appear on the left side of an assignment.

For example, consider the following code:

In this code, x is an lvalue because it refers to a memory location that stores the value 5. The expression x = 10 is also an lvalue because it assigns the value 10 to the memory location referred to by x . However, the expression 5 is an rvalue because it does not refer to a memory location.

Causes of the Error

The "lvalue required as left operand of assignment" error occurs when the programmer tries to assign a value to an rvalue. This is not allowed in C++ because rvalues do not have a memory location that can be modified. Here are some examples of code that would cause this error:

In each of these examples, the programmer is trying to assign a value to an rvalue, which is not allowed. The error message indicates that an lvalue is required as the left operand of the assignment operator ( = ).

Resolving the Error

To resolve the "lvalue required as left operand of assignment" error, the programmer must ensure that the left operand of the assignment operator is an lvalue. Here are some examples of how to fix the code that we saw earlier:

In each of these examples, we have ensured that the left operand of the assignment operator is an lvalue. This resolves the error and allows the program to compile and run correctly.

The "lvalue required as left operand of assignment" error is a common mistake that beginners make when learning C++. To avoid this error, it is important to understand the difference between lvalues and rvalues and to ensure that the left operand of the assignment operator is always an lvalue. By following these guidelines, you can write correct and efficient C++ code.

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12.2 — Value categories (lvalues and rvalues)

The value category of an expression

Lvalue and rvalue expressions

Lvalue expressions evaluate to an identifiable object. Rvalue expressions evaluate to a value.

err: lvalue required as left operand of assignment

I get this error (error: lvalue required as left operand of assignment) with the following code:

= is an assignment operator. == is a comparison operator.

This code is trying to assign the value 1 to Serial.read(), which it can't do.

Thank you. I forgot.

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Installing GCC 3.2 : How to resolve error "lvalue required as increment operand"?

I am using Ubuntu 10.04 and the current version of GCC installed in my system is 4.4. For some specific need I want to install GCC 3.2.

I began with these steps:

It has configured successfully. But when I used below command

I got an error

Anybody please help me to resolve this error. Or please suggest some alternative ways. Thanks.

• Why are building from source? –  jobin Commented Feb 17, 2014 at 5:19
• This is the only way I know to install older version of GCC. I tried using apt-get but it said that version 3.2 isn't available in the archive. Are there any alternative ways to install older version of gcc? –  Winn Commented Feb 17, 2014 at 5:22
• It seems that gcc has disallowed cast-as-lvalue. You have to modify sources by yourself if you only have modern binary release of gcc. –  House Zet Commented Feb 17, 2014 at 5:43
• Is it using current gcc 4.4 to actually compile gcc 3.2 ? –  Winn Commented Feb 17, 2014 at 6:32
• I think the problem is, as you pointed, maybe some syntax revision in later versions. Because make uses build-essentials and gcc, the possibility is that it is using gcc 4.4 to compile gcc 3.2. This might help askubuntu.com/questions/39628/old-version-of-gcc-for-new-ubuntu –  Ashish Gaurav Commented Feb 17, 2014 at 11:34

3 Answers 3

Here is an incredibly hacky fix to work around the loss of cast-as-lvalue, based on a suggestion of SM Ryan in comp.lang.c.

Near the top of the file, add

Now, replace the casts with LV(...) as in these examples (from gdb's obstack.h ):

On line 428, change

and likewise, on line 436 change

• 1 I used this technique to get GCC 3.1.1 to compile with GCC 6.3.0 and it worked. IMO this answer should be marked as the correct answer because it is the minimal workaround to achieve what the questioner asked for. –  smammy Commented Jan 21, 2022 at 19:16

After few trials, I found one solution.

I added below mirrors in /etc/apt/sources.list

With these mirrors, I am able to install GCC 3.3(though not GCC 3.2) using

Don't forget to do $sudo apt-get update before above command.

It is in fact satisfying my need. And to run the program using GCC 3.3, do

$gcc-3.3 input_file

Because otherwise if you type $gcc input_file it will use the default GCC(GCC 4.4 in my case) to compile the program. We can change the way desired version is used by simply creating a hard link of the version you want to tag to command gcc . We can do the following

So now whenever you type $gcc input_file it will use your desired gcc version to compile the program.

Once I faced a similar problem. I had this module 'r8169' which wasn't receiving packets from my wired connection. Then I had to build the previous module 'r8168' from source. This gave me similar errors like yours.

A possible fix is by going into superuser mode. Type

Then type your password. The console will show you '#' instead of '$' for writing commands. Then try your commands again.

(and any other commands, if left). Hopefully, it should work.

• Well, if its a concern of superuser mode, I tried the same command lines in cygwin(under windows) as well(where default access is in superuser mode only). But there too, I am facing the same problem. –  Winn Commented Feb 17, 2014 at 7:30

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error: lvalue required as left operand of assignment

I'm trying to do some calculation program using C what am I getting error is

This error cause from define a value on the header and then I assign using = in the main body.

If I put that value which it is in the header instead put them in the int or double then it will be fine, but I don't want them to be there.

please tell me if needed more information about what I'm trying to say.

here is what I put on the define #define price 50

and in the body where I'm getting error price = price * 2 + total

• 3 Please post the relevant parts of your code. –  Mat Commented Oct 10, 2011 at 20:06
• @Mat I don't know if this is enough –  Ali Commented Oct 10, 2011 at 20:08
• 1 Ali, yes, it's enough :) –  Mat Commented Oct 10, 2011 at 20:09

2 Answers 2

You can't assign to a define! After the preprocessor is done, to the compiler the code looks like this:

• Yes I understand that the line will equal to something like price = 50 * 2 + 300 [assume total = 300] but I don't know why my professor asking me to instead using #define not the int or double –  Ali Commented Oct 10, 2011 at 20:12
• yes but when I remove the value from int and use them in #define then I'm getting this error and my professor seem not to give me any information how will this work since from my understanding and from where I did the research and also from here getting the same result that it can't be done this way. –  Ali Commented Oct 10, 2011 at 20:17
• 1 @Ali Perhaps you're supposed to store the value in a different variable ? Something like final_price = price * 2 + total; ? –  cnicutar Commented Oct 10, 2011 at 20:18
• Thanks for giving me an idea I will try that –  Ali Commented Oct 10, 2011 at 20:19
• the only problem now is that if I use something like this final_price = price / total the output is always 0 so I tried show the output for the price itself by using `printf("%d", price); and it shows the value correctly. I don't know what is wrong exactly –  Ali Commented Oct 10, 2011 at 20:36

The problem is that price is being replaced with the text 50. You can't have

in C. Can't do it.

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