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[No Audio]

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This tutorial is about for loops, which are

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used when we know the number of times a loop

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will be executed. One of the key uses of the

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for loop, is to iterate or go through the

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elements of array or vector. Let me declare a vector.

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[No Audio]

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Next to print the elements of the

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array one by one, I will use a for loop.

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In this case now, I know in advance that there

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are 6 elements in the vector. So I will use

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a for loop using the keyword of for, and

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then I will mention the iterating variable

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which will be incremented in each of the

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iteration itself. Programmers by convention

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use the variable i in connection with a for loop.

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[No Audio]

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Next, we will mention the values

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variable I will be assuming. The syntax for

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that is to write i in, and then the range of

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values which, in this case will be from 0 to 5.

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[No Audio]

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Now in the first iteration, the

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variable i will have a value of 0, and then

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in subsequent iterations, its value will be

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incremented by one each time.

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Inside the body of the loop,

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I will simply print the values

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of the variable i.

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[No Audio]

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Let us execute this.

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[No Audio]

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During each iteration of

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the loop, we have the value of the variable

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i being printed on the terminal, starting

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from the value of 0 and ending at a value of 5.

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Let us now use the variable i to

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print the values of the vector.

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[No Audio]

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In this case, the first placeholder will have

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the values of the variable i, and the second

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place holder will have the ith element of the vector.

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When the loop executes for the

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first time, the variable i will have a value of 0.

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And therefore the print statement

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will display, the first value inside the

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vector, having a value of 45.

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In subsequent iterations,

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the values will be displayed in

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sequential order the same way.

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Let us execute this.

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[No Audio]

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We can also iterate through all the elements

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in and understand. Instead of mentioning the

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values of 0..5, we can write

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something like, for i in some_vec.

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And then inside the print statement, I will update

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the print command, and we'll print the values of i.

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[No Audio]

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In this case now, the variable i will be

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assuming values from the vector in sequential

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order, let us cargo run this to confirm.

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Now, here is an important point to note

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before we go further, the value of some_vec

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are being consumed inside the

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loop, and if we want to access the vector

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outside the loop, the compiler will give us

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an error. Let me add a print statement, where I

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will access the vector.

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[No Audio]

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You may note a red line, it says borrowed

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of a moved value. In fact, when we write for

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i in some_vec, so the values

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are moved from the vector to the variable i,

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and therefore changes the ownership.

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Moreover, the scope of the variable i is also

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limited to the body of the for loop. And if I

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write a variable i instead of some_vec

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in the print statement, it will also

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complain, saying that cannot find i in the

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scope. This means that we have to be very

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careful when using this syntax.

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If we do not want this to happen, but rather we want a

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variable i to use the borrowed values, we

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will use the iter function. This is quite a

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handy function and it has many useful usages,

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which will become more evident to us, when we

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cover the iterators in the upcoming section.

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We can call this function on the vector, and

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it will enable the borrowing of each element

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of the collection through each iteration,

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without changing the ownership. If we include

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now the println statement afterwards,

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the compiler won't complain, because the ownership

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remains with some_vec variable.

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However, please note that the variable i is

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in the scope of the loop only, and is not

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known outside the body of the loop.

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[No Audio]

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You may recall from the previous section of the

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course, that we can use the & sign also for

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referencing and borrowing of elements.

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This means that instead of using the .iter

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function, which basically borrows the

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elements one by one, we may also use the & sign

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which will exactly do the same job.

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[No Audio]

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Let us cargo run to confirm.

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[No Audio]

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Please note that, when I write some_vec

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without the & sign, so the type of the

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variable i is that of i32, which

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suggests actual owned value and not references.

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When I change it to some_vec.iter,

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then the type of the variable i is

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changed to a reference to i32, that is an

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i32 with an & sign. This updation of

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the types of variables will become very handy to us,

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as we tend to move along in Rust programming.

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In the previous section of the

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course, we covered two types of references

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called immutable references and mutable references.

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The syntax that we have just seen,

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allows for immutable references of vector values.

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What if we want to have mutable

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references to all the values of the vector?

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To have mutable references, we will use

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iter_mut function, it allows for

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mutable references of the vector. This means

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that, we can now use the variable i to update

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the values of the vector inside the loop,

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inside the for loop, without transferring the ownership.

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Let me update all the values inside the loop.

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[No Audio]

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Okay, let us stop here for a while, because we

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have encountered something that is unusual.

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Let me explain the meaning of the star in

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this case. The star is called a deref

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operator, and when we use a variable as a

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mutable variable, so the value itself is like

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a pointer and not the actual value. So, to

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update the actual value of the variable which

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it is pointing to, we will use a star

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operator. In summary for updating, or

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accessing a value of a variable which is

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behind a mutable reference, we will always

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use the star. Please note again, that in the

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print command, we are not doing any

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processing, there is updation or computation

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on the values therefore the Rust syntax do

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not require the deref operator in this

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case, let us cargo run this.

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[No Audio]

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The array has been updated, and the values are

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being printed. Please note that the variable

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i type, in this case, is set to a mutable

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reference of type i32. This gives us a

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useful hint that, instead of using this

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function, we may write something like &mut

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some_vec, and the code will still

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work in the very same way. Because &mut

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before the some_vec, creates the

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immutable reference for the vector.

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[No Audio]

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Let us execute this to check.

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[No Audio]

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Okay, last but not the least, the for loop

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can be used to iterate through the

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elements of other types of collections also,

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such as HashMaps, HashSets, and others. The

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details of those will come later on, as we

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progress in the course. That brings us to the

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end of this tutorial. There are very few

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topics left in this section of the course,

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after which we will be covering a bit more

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advanced topics in Rust. I hope you will be

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enjoying the course. So do come back again

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with more fun and exciting tutorials. And

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until next tutorial, happy Rust programming.

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[No Audio]