# For Loops {% embed url="" %} [Control Flow](http://python.swaroopch.com/control_flow.html) ## The For Loop For loops are a mechanism to repeat something *for* a certain number of times. For loops provide us with a different way of thinking about looping. We get to define a fixed end as part of the statement. Just like a `while` loop and an if statement, `for` loops are blocks. They begin with the `for` **keyword** and are followed by a **variable declaration**. There is then the `in` **keyword** and then a **list**. It is completed with a **colon** and then the body of the loop is **indented**. The built-in `range()` function helps us by creating a list of numbers to loop over. ```python for num in range(1,11,1): print(num) ``` One thing to note from this loop is that the variable declaration `num` that we made in the loop can be used **within** the block. Python lets you use it **outside** of the block but in my opinion this is bad. The reason why is because what if you had a loop like this: ```python for num in range(0, 0, 0): print(num) print(num) ``` The print statement that is outside of the block would fail because we never initialize the `num` variable since there were no iterations. It would work if you had any iterations (eg. `range(0, 1, 1)`). This is inconsistent and something that most other programming languages disallow. You should never expect the variables that you initialize inside of a block to exist because if you never actually looped, then they won't. *Try running the* *`for`* *loops above with both of the range functions I have shown! One should work and the other should fail!* ## The Range Function The `range()` function returns a list of numbers and takes three arguments: 1. the starting number in the list 2. the terminating number that all numbers in the list will be less than 3. the step number. For example `range(1, 10, 2)` would return `[1, 3, 5, 7, 9]`. And `range(0, 10, 1)` would return `[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]`. We use this function in the `for` loop to give us a list of items to [iterate](http://dictionary.reference.com/browse/iterate). *Try writing a loop that prints all of the multiples of 3 less than 50!* ## For vs While Notice that if we were to do the same thing as our example `for` loop using a `while` loop it would look like: ```python num = 1 while num < 11: print(num) num += 1 ``` The `for` loop is a much better fit because we have a fixed *number* of times we wish to [iterate](http://dictionary.reference.com/browse/iterate). This doesn't mean that a `while` loop isn't as good. They simply are better at different things. You might recall that in the `while` loop lesson I challenged you to create a program that continued to ask `"Why?"` until the user answered with `"just because!"`. That kind of loop isn't really possible with a `for` loop. To know when to use a certain kind of loop try saying aloud how you want to loop. If you end up saying something like, "I want to loop **for** as many times as there are ..." then you probably want a `for` loop. If you say something like, "I want to keep printing "You're wrong" **until** they get the right answer, or **while** their answer isn't right" then you probably want a `while` loop. ## Infinite Loops `for` loops don't typically have the same problem with infinity as `while` loops. The reason is because your terminating condition is often better defined. Infinite loops can still happen and that is when you add to the loop you are iterating while you are looping over it. Imagine I have a list: `[1, 2, 3, 4, 5]` that I am iterating. Before the first iteration I have 5 numbers to go through. If I add `6` to the list during the first iteration then my list will be: `[1, 2, 3, 4, 5, 6]`. I just used `1` so the remaining number of items in my list is 5... Uh oh! We still have the same number of iterations. If we were to continue adding numbers in every iteration, then we would never reach 0. This is an infinite loop. Since you are going to be using the `range()` function for all of your `for` loops you won't run in to this problem but remember when we do learn about lists that you shouldn't add to the list you are looping over. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://programming.reedcwilson.com/lesson-3/for-loops.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.