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For Loops

For Loops

In this chapter, you will learn about two kinds of repetition structures in Python: for loops and while loops. This section describes for loops.

For Loops are a component of many programming languages. A For Loop is a repetition structure where a section of code runs a specified number of times.

Say we want to print out the statements:

Problem solving in teams
Problem solving in teams
Problem solving in teams

One way to accomplish this task is by coding three print statements in a row:

In [1]:
print('Problem solving in teams')
print('Problem solving in teams')
print('Problem solving in teams')

Problem solving in teams
Problem solving in teams
Problem solving in teams

Another way to accomplish the same task is to use a for loop. The basic structure of a for loop in Python is below:

for <var> in range(<num>):
    <code>

Where <var> can be any variable, range(<num>) is the number of times the for loop runs and <code> are the lines of code that execute each time the for loop runs. Note the for loop starts with the keyword for and includes a colon :. Both for and the colon : are required. Also, note <code> is indented. Each line of code that runs as part of the for loop needs to be indented the same number of spaces. Standard indentation in Python is four spaces.

The example above can be rewritten using a for loop:

In [2]:
for i in range(3):
    print('Problem solving in teams')

Problem solving in teams
Problem solving in teams
Problem solving in teams

range() function

Python's range() function returns an iterable list of values starting at zero and ending at n-1. For example, when range(3) is called, the values 0, 1, 2 are returned. Note 3 is not part of the output, even though the function input was range(3). We can be confirm the behavior of range() with a for loop:

In [3]:
for i in range(3):
    print(i)

0
1
2

Customizing range()

Python's range() function can be customized by supplying up to three arguments. The general format of the range function is below:

range(start,stop,step)

When range(3) is called, it produces the same output as range(0,3,1) (start=0,stop=3,step=1). Remember Python counting starts at 0 and ends at n-1. If only two arguments are supplied, as in range(0,3), a step=1 is assumed.

The table below includes examples of the Python's range() function and the associated output.

range() function output
range(3) 0, 1, 2
range(0,3) 0, 1, 2
range(0,3,1) 0, 1, 2
range(2,7,2) 2, 4, 6
range(0,-5,-1) 0, -1, -2, -3, -4
range(2,-3,1) (no output)

A code section that uses a for loop and range() with three arguments is below:

In [4]:
for i in range(5,9,1):
    print(i)

5
6
7
8

For loops with lists

For loops can also be run using lists. If a list is used, the loop will run as many times as there are items in the list. The general syntax is:

for <var> in <list>:
    <code>

Where <var> is a variable name assigned to the item in the list and <list> is the list object. Remember to include a colon : after the list. <code> is the programming code that runs for each item in the list.

An example of a list in a for loop is below:

In [5]:
my_list = ['electrical','civil','mechanical']
for item in my_list:
    print(item)

electrical
civil
mechanical

The loop ran three times because there are three items in the list. Each time through the loop, the variable item is set to one of the items in the list.

  • first time through the loop, item='electrical'
  • second time through the loop item='mechanical'
  • third time through the loop item='civil'.

For loops with strings

For loops can also be run using strings. In Python, strings can be indexed just like lists. A loop defined by a string runs as many times as there are characters in the string. The general structure a for loop using a string is:

for <char> in <string>:
    <code>

Where <char> is one of the characters in the string <string>. Just like for loops with range() and for loops with lists, make sure to include a colon : after the list. <code> is the programming code that runs for each character in the string. <code> needs to be indented

An example of a string in a for loop is below:

In [6]:
for letter in "Gabby":
    print(f"looping over letters in name: {letter}")

looping over letters in name: G
looping over letters in name: a
looping over letters in name: b
looping over letters in name: b
looping over letters in name: y