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For Loops

3:15 with Jeremy McLain

Some while loops can be written more succinctly as for loops.

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    This pattern of using an integer that increases each time the loop runs, is so 0:00
    common that there's a special kind of loop just for that. 0:04
    It's called a for loop. 0:08
    I'll write what that looks like lower existing loops so we can compare the two. 0:10
    This is doing 0:15
    exactly the same 0:22
    thing the while loop, up here does. 0:32
    We can break our while loop up here into for parts. 0:38
    First, we declare index as an integer and initialize it to 0. 0:41
    Second, we check to see if we’ve looped through all the indexes of the array. 0:46
    Third, we do something with each of the array items Fourth, we increment index so 0:51
    that the next time through the loop we're getting the next item in the array. 0:57
    For loops also have the same for parts only there written more concisely. 1:03
    Here's where we declare index as an integer and initialize it to zero. 1:09
    Here's where we check to see if we've loop through all the indexes of the array. 1:13
    The body of the loop is used for doing something with the array item. 1:18
    And here's where we increment index so 1:22
    that the next time through the loop we're getting the next item in the array. 1:25
    Just think of for loops as condensed while loops. 1:30
    Now that we have this for loop we don't need this while loop anymore. 1:34
    For loops aren't only used for looping through a race. 1:39
    They're also really handy when we want to count from one number to another. 1:42
    We can set the starting number here and it can be any number we want. 1:47
    We decide where to stop counting here. 1:52
    This can be any condition that eventually stops the loop by evaluating to false. 1:55
    Each time the loop runs, index will count up by one. 2:00
    In fact, this variable is often called a loop counter. 2:05
    We could also count by more than one by using plus equals like this. 2:09
    This will count up by two. 2:14
    We want to count by one so I'll change this back to index plus plus. 2:17
    Because this variable index is being declared right here, 2:22
    its scope is limited to inside of the loop. 2:26
    That means that it can only be used inside the 4 loop. 2:30
    If we want to access the loop counter variable after the 4 loop has ended, 2:34
    we'd have to move the declaration up here above the 4 loop somewhere. 2:38
    [PAUSE] It may look strange, but this is perfectly valid code. 2:42
    When reading code, you'll notice that most for loops use the single letter I 2:52
    as the name of the loop counter variable. 2:57
    This tradition goes all the way back to the first programming languages created. 3:00
    You can name it whatever makes sense for your code though. 3:05
    Just to maintain tradition I'll shorten index to I for us. 3:08

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