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A for loop is simply a way of repeating a set of instructions multiple times. Anytime you find yourself doing something REMOVED then once, chances are you can use a loop, and for the purposes of this course those will be for loops. A for loop is constructed in the following manner:
for ITEM in LIST: instructions |
What the above tells JES to do, is to run the instructions one time for each item in the list (for a detailed discussion of lists, see below). For instance, the following:
CODE | RESULT |
myList = [1, 2, 3, 11] for item in myList: print "Hi!" | "Hi!" "Hi!" "Hi!" "Hi!" |
This happens because there are four items in myList. For each item, the line print "Hi!" is run. That's fine if we want to run -exactly- the same code each time...but many times, we need to run code that varies a little bit. For instance, what if we wanted to print out each item in myList? Well, the variable item can be referenced within the for loop. For instance:
CODE | RESULT |
myList = [1, 2, 3, 11] for item in myList: print item | 1 2 3 11 |
Note what is actually happening here. Each step of the loop, item becomes set equal to an item in the list. For the first iteration of the loop, item is set equal to the first item in myList, so it becomes equal to 1. The code inside the loop then runs, so item's value is printed out, hence, 1 is printed. The same happens with 2, then 3, then 11. Basically, the above loop is identical to the following in how it functions
CODE | RESULT |
myList = [1, 2, 3, 11] item = myList[0] print item item = myList[1] print item item = myList[2] print item item = myList[3] print item | 1 2 3 11 |
CODE | RESULT |
| range(0,6) range(20, 25) | [20, 21, 22, 23, 24] |
This can be used directly in a for loop
CODE | RESULT |
| for i in range(1,101): print i | 2 3 4 5 ... 99 100 |
JES includes some functions that return lists, that are especially useful in for loops. These include getPixels(), which returns a list consisting of every pixel in a picture, getSamples(), which returns a list of every sample from a sound, and a few others. An example of getPixels() is shown.
CODE | RESULT |
| pic = makePicture("C:\Program Files\Directory\filename.jpg") for item in getPixels(pic): setRed(item, 255) setBlue(item, 255) setREMOVED(item, 255) | This will turn every pixel in an image completely white, turning it into a blank canvas. |
Lastly, the idea of a nested for loop. You can place a for loop inside of another for loop. It can be hard to conceptualize what exactly this does, but the most common analogy is that of a table, of rows and columns. One loop controls which column you are in, the other determining what row you are in. So you enter row one, iterate through every column, then move on to row two, iterate through every column within it, etc. To give an example of this, consider the following grid
| A1 | B1 | C1 |
| A2 | B2 | C2 |
| A3 | B3 | C3 |
Now, if we wanted to iterate through these, left to right, top to bottom, printing out the contents of each cell, we could create the following code, with the following results.
CODE | RESULT |
list1 = ["A", "B", "C"] list2 = ["1", "2", "3"] for number in list2: for letter in list1: print letter + number | B1 C1 A2 B2 C2 A3 B3 C3 |
The key point in nested for loops, is to understand that the inside for loop runs through its entire list for each item in the outer loop.
A list is simply a collection of items. These can be integers, floats, booleans, strings, pictures, sounds...anything you can ascribe to a variable, can be placed into a list (even variables themselves). A list is expressed in the following way:
list = [item0, item1, item2, item3...] |
This would create a list containing the items as described above. The first item in the list has the index of 0, the second item in the list has the index of 1, etc. The easiest way to remember this, is if you want the nth item in a list, it's actually at index n-1.
You can pick specific items in a list, by the following syntax:
Where index is the numerical index of the item you want, so:
Code | Result |
| myList = ["Cat", "Dog", "Elephant", "Donkey", "Guerilla] myList[0] myList[2] myList[4] | "Cat" "Elephant" "Guerilla" |
Further uses for lists are covered in another section. This is all you need to understand to be able to use them in for loops.