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In this video we will cover Loops in particular for loops and while loops.

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We will use many visual examples in this video.

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See the labs for examples with data.

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Before we talk about loops,

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let's go over the range function.

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The range function outputs and ordered sequence as a list I.

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If the input is a positive integer,

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the output is a sequence.

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The sequence contains the same number of elements as the input but starts at zero.

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For example, if the input is three the output is the sequence zero, one, two.

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If the range function has two inputs where

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the first input is smaller than the second input,

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the output is a sequence that starts at the first input.

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Then the sequence iterates up to but not including the second number.

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For the input 10 and 15 we get the following sequence.

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See the labs for more capabilities of the range function.

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Please note, if you use Python three,

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the range function will not generate a list explicitly like in Python two.

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In this section, we will cover for loops.

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We will focus on lists, but many of the procedures can be used on tuples.

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Loops perform a task over and over.

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Consider the group of colored squares.

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Let's say we would like to replace each colored square with a white square.

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Let's give each square a number to make things a little

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easier and refer to all the group of squares as squares.

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If we wanted to tell someone to replace squares zero with

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a white square, we would say equals replace square

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zero with a white square or we can say four squares

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zero in squares square zero equals white square.

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Similarly, for the next square we can say for square one in squares,

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square one equals white square.

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For the next square we can say for square two in squares,

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square two equals white square.

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We repeat the process for each square.

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The only thing that changes is the index of the square we are referring to.

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If we're going to perform a similar task in Python we cannot use actual squares.

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So let's use a list to represent the boxes.

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Each element in the list is a string representing the color.

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We want to change the name of the color in each element to white.

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Each element in the list has the following index.

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This is a syntax to perform a loop in Python.

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Notice the indent, the range function generates a list.

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The code will simply repeat everything in the indent five times.

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If you were to change the value to six it would do it 6 times.

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However, the value of I is incremented by one each time.

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In this segment we change the I element of the list to the string white.

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The value of I is set to zero.

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Each iteration of the loop starts at the beginning of the indent.

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We then run everything in the indent.

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The first element in the list is set to white.

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We then go to the start of the indent,

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we progress down each line.

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When we reach the line to change the value of the list,

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we set the value of index one to white.

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The value of I increases by one.

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We repeat the process for index two.

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The process continues for the next index,

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until we've reached the final element.

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We can also iterate through a list or tuple directly in python,

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we do not even need to use indices.

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Here is the list squares.

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Each iteration of the list we pass

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one element of the list squares to the variable square.

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Lets display the value of the variable square on this section.

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For the first iteration,

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the value of square is red,

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we then start the second iteration.

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For the second iteration,

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the value of square is yellow.

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We then start the third iteration. For the final iteration, the value of square is green,

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a useful function for iterating data is enumerate.

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It can be used to obtain the index and the element in the list.

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Let's use the box analogy with the numbers representing the index of each square.

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This is the syntax to iterate through a list and provide the index of each element.

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We use the list squares and use the names of the colors to represent the colored squares.

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The argument of the function enumerate is the list.

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In this case squares the variable I is

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the index and the variable square is the corresponding element in the list.

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Let's use the left part of the screen to

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display the different values of the variable square

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and I for the various iterations of the loop. For the first iteration,

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the value of the variable is red corresponding to

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the zeroth index, and the value for I is zero for the second iteration.

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The value of the variable square is yellow, and

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the value of I corresponds to its index i.e.

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1. We repeat the process for the last index.

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While loops are similar to for loops but instead of executing

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a statement a set number of times a while loop will only run if a condition is met.

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Let's say we would like to copy

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all the orange squares from the list squares to the list New squares.

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But we would like to stop if we encounter a non-orange square.

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We don't know the value of the squares beforehand.

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We would simply continue the process while the square is

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orange or see if the square equals orange.

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If not, we would stop. For the first example,

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we would check if the square was orange.

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It satisfies the conditions so we would copy the square.

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We repeat the process for the second square. The condition is met.

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So we copy the square. In the next iteration, we encounter a purple square.

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The condition is not met.

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So we stop the process.

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This is essentially what a while loop does.

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Let's use the figure on the left to represent the code. We will

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use a list with the names of the color to represent the different squares.

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We create an empty list of new squares.

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In reality the list is of indeterminate size.

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We start the index at zero the while statement will repeatedly

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execute the statements within the indent until the condition inside the bracket is false.

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We append the value of the first element of the list squares to the list new squares.

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We increase the value of I by one.

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We append the value of the second element of the list squares to the list new squares.

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We increment the value of I.

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Now the value in the array squares is purple;

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therefore, the condition for the while statement is false and we exit the loop.

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Check out the labs for more examples of loop many with real data.

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(Music)