for-loops

This lesson is based on Dr. David Whipp’s material from the python-for-geo-people -course and the Software Carpentry group’s lessons on Programming with Python.

Examples from the class

Often we need to repeat a task, such as greeting people:

# Print greetings line-by line
print("Tervetuloa Iida")
print("Tervetuloa Elina")
print("Tervetuloa Virpi")
print("Tervetuloa Linda")

The general pattern in the above commands contains three main elements; the print-command, the greeting, and the name of the person. If we list the names we want to greet, we could say that, “for each name in the list, print out a greeting:”

#Names in a list
namelist = ["Iida", "Elina", "Virpi", "Linda"]

# For each name in the list print greetings:
for name in namelist:
    print("Tervetuloa", name)

We can also add a counter of how many people we have greeted

greeting_cnt = 0

for name in namelist:
    print("Tervetuloa", name)
    Tervehdys_cnt += 1
    print("I have greeted: " + str(greeting_cnt) + "/" + str(len(nimilista)) + "people")

Basics of for loops

Loops allow parts of code to be repeated over some number of times.

1. Let’s consider an example. Suppose we want to take a word and print out each letter of the word separately. We could do the following:

   >>> word = 'rock'
   >>> print(word[0])
   r
   >>> print(word[1])
   o
   >>> print(word[2])
   c
   >>> print(word[3])
   k
   

   But this is a bad idea. Why? Well there are two reasons. First, it does not scale nicely for long strings, and will take forever to type in. Second, it won’t work if the word is not 4 characters long.

   >>> word = 'ore'
   >>> print(word[0])
   o
   >>> print(word[1])
   r
   >>> print(word[2])
   e
   >>> print(word[3])
   ---------------------------------------------------------------------------
   IndexError                                Traceback (most recent call last)
   <ipython-input-14-e3303df6f566> in <module>()
   ----> 1 print(word[3])
   
   IndexError: string index out of range
   

2. We could do a much better job by using a for loop.

   word = 'rock'
   for char in word:
       print(char)
   

The previous command will print out:

r
o
c
k

Note here that the ... is displayed in the IPython window when entering code in a loop and you do not need to type in the .... Not only is this shorter, but it is also more flexible. Try out a different word such as granite. Still works, right?

3. for loops in Python have the general form below.

   for variable in collection:
       do things with variable
   

   The variable can be any name you like, and the statement of the for loop must end with a :. The code that should be executed as part of the loop must be indented beneath the for loop, and the typical indentation is 4 spaces. There is not additional special word needed to end the loop, just change the indentation back to normal.

4. Let’s consider another example.

   >>> length = 0
   >>> for letter in 'earthquake':
   ...    length = length + 1
   ...
   >>> print('There are', length, 'letters')
   There are 10 letters
   

   Can you follow what happens in this loop?

5. Note that the variable used in the loop, letter in the case above is just a normal variable and still exists after the loop has completed with the final value given to letter.

   >>> letter = 'x'
   >>> for letter in 'fault':
   ...     print(letter)
   ...
   f
   a
   u
   l
   t
   >>> print('After the loop, letter is', letter)
   t
   

6. A loop can be used to iterate over any list of values in Python. So far we have considered only character strings, but we could also write a loop that performs a calculation a specified number of times.

   >>> for number in range(5):
   ...     print(number)
   ...
   0
   1
   2
   3
   4
   

   What happens here? Well, in this case, we use a special function called range() to give us a list of 5 numbers [0, 1, 2, 3, 4] and then print each number in the list to the screen. When given a integer (whole number) as an argument, range() will produce a list of numbers with a length equal to the specified number. The list starts at zero and ends with number-1. You can learn a bit more about range by typing

   >>> help(range)
   

7. Often when you use for loops, you are looping over the values in a list and either calculating a new value or modifying the existing values. Let’s consider an example.

   >>> mylist = [0.0, 1.0, 2.0, 3.0, 4.0, 5.0]
   >>> print(mylist)
   [0.0, 1.0, 2.0, 3.0, 4.0, 5.0]
   >>> for i in range(6):
   ...     mylist[i] = mylist[i] + i
   ...
   >>> print(mylist)
   [0.0, 2.0, 4.0, 6.0, 8.0, 10.0]
   

   So, what happened? We first create a list of 6 numbers. Then, we loop over 6 values using the range() function and add each value to the existing location in mylist. What would happen if we ran this for loop a second time?

8. One of the drawbacks in the example above is that we need to know the length of the list before running that for loop example. However, we already know how to find the length of a list using the len() function, and we can take advantage of this knowledge to make our for loop more flexible.

   >>> for i in range(len(mylist)):
   ...     mylist[i] = mylist[i] + i
   ...
   >>> print(mylist)
   [0.0, 3.0, 6.0, 9.0, 12.0, 15.0]
   

   We’ve done exactly what we had done in the previous example, but replaced the known length of the list 6 with use of the len() function to provide the list length. Now if we add or remove values in mylist, our code will still work as expected.

   >>> mylist.append(18.0)
   >>> mylist.append(21.0)
   >>> print(mylist)
   [0.0, 3.0, 6.0, 9.0, 12.0, 15.0, 18.0, 21.0]
   >>> for i in range(len(mylist)):
   ...     mylist[i] = mylist[i] + i
   ...
   >>> print(mylist)
   [0.0, 4.0, 8.0, 12.0, 16.0, 20.0, 24.0, 28.0]
   

   Using the len() function with range() to perform calcluations using list or array values is an extremely common operation in Python.