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This tutorial is about Function Pointer types,

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which are used to refer to a function

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whose identity is not necessarily known at

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compile time. As opposed to referencing a

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data value, a function pointer points to

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executable code within a memory. The function

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type allows us to define a variable which

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will be of type function which means that, it

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will be pointing to some function and then we

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can change the pointer in runtime to change

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it to, point to some other function. The topic

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is not much complex, and simple and therefore,

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this tutorial will be a bit short. With this,

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let's start to learn about Function Types.

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We will first define a couple of functions, I

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will define a max and min function, which will

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compute, and return the maximum and minimum of

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the two input values.

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First, we will define the max function.

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Next, I will define our main function

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in the very same way.

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Let us use these functions now

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using function pointers.

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I will first define

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a variable of type function, and we'll

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initialize it from the function of main.

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You may note that, the type of the variable is

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being automatically set by the Rust compiler

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to that of the function signature, containing

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the information of the inputs and outputs.

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We can now use the same variable to call a

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function, I will call the function in the

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print statement with some inputs.

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The Rust will interpret the variable of f as

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a function and the value inside the

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parentheses as input to the function.

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Let us execute the code.

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It displays correctly the

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minimum of the two values, now let us change

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the value of the variable to

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that of max and execute again.

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You may note that the function of maximum is

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now being executed at the maximum of the two

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values is being displayed to us. This way, we

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can change the value of the function variable

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to refer to different functions using a

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single variable. Okay now let us see some of

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the uses of the function types. We will first

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look at the case, where we can use the

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function pointer as an argument or parameter

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to another function. I will first create a

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simple function for printing an input string.

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Next, I will define another function called

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fn print_full_info.

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The first parameter of this function will be a function type.

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The second input will be a string

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and the last input will be the age.

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Inside the function, we will first call the

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fn prints_name using the

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function pointer which is f, and we'll pass

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the input string of some_one to it.

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Next, I will add an additional print statement

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for printing the age of the person.

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I will now call these functions from the main.

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I will first define a couple of variables

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containing the name and age.

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Next, I will call the fn prints_full_info with suitable inputs.

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Let us cargo run this to see the outputs.

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The fn print_full_info first calls the function of

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prints_name using the functional pointer of f  n

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and then it next prints my name.

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Let us cover some more examples and details

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about function pointers. In this example we

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will define a function called add_one and

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we'll call it from another function called twice.

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First, I will define the add_one

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function, which will do nothing but add one to the

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value that is being passed to it as an input.

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Next, I will define another function

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which will call this function twice. So I

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will call this function as do_twice.

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The input to the function will be a

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functional pointer in i32 value and the

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output is i32 value.

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Inside the body of the function, I will call

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the fn add_one twice, and

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we'll add the values together using the

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function pointer.

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Now, any value that is

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being passed to this function, will be added

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one and then the values will be added.

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For instance, if I pass a value of let's say 10.

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And then a value of 1 will be added which

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will make its value 11. And then 11 will be

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added to itself, which will make the value equal to 22.

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Let us now use these functions

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in the main program now, I will use a

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variable which will hold the output value

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from the function do_twice.

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The function name add_one will

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serve as a type of functional pointer for the

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variable f inside the parameter list, of the

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do_twice function. Next I will

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add a simple print statement, which will

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print and have the value of the answer.

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The answer in this case will be having a

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value of 12. Because a value of 1 will be

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added to the value of 5, which will make

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its value equal to 6 and then 6 will be

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added to itself. So the final answer will be 12.

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Let us cargo run this.

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We obtained the required answer. The function pointers

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are very useful in some advanced topics such

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as communicating with other languages and

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external API's. Okay, that brings us to the

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end of this tutorial, we have learned about

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the functional pointers which provides a nice

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way for using the functions inside other

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functions are having a pointer to a function code.

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In the upcoming tutorial, we will be

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looking at the concept of iterator, so do

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come back for learning that and until next

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tutorial, happy Rust programming.

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