for loop is the first Python tool for iteration that we will explore.
A for loop is typically used for definite iteration.
Definite iteration is the process of repeating a specific task with a specific data set.
for loop begins it can usually be determined exactly how many times it will execute:
once for each item in the data set.
for Loop Syntax¶
We have already seen the basic syntax of a
1 2 3 4
for num in range(51): print(num) print("Not in the loop!")
This program prints the integers 0 through 50, one number per line. In the language of definite iteration, we say that the loop has a data set of 0-50, and its action is to print a value to the console.
Let’s break down this syntax piece by piece, so we can begin to understand how
loops are structured.
- In line 1,
numis called the loop variable. Each time the loop executes,
numgets assigned a new value based on the number in the
range. In this example,
numwill be assigned a value 51 times, starting with 0 and ending at 50. More on that later.
- Line 2 begins the loop body. The loop body is ALWAYS indented.
The indentation determines exactly what statements are “in the loop”.
In this example, we are printing each value of
numas we loop through the set range.
- The first unindented line after the
forstatement marks the end of the loop body. In this example, line 4 is not part of the loop.
- The loop body can contain any number of statements.
- The number of times the loop body runs depends on the value in
184.108.40.206. Line By Line¶
Let’s modify the code just a little to follow the operation of a
1 2 3 4 5
for num in range(4): print(num) print("Hello" * num) print("Done!")
0 1 Hello 2 HelloHello 3 HelloHelloHello Done!
- The first time Python executes the
forstatement in line 1,
numis assigned a value of
- Next, Python checks if the value of
numis less than the value inside
num < 4evaluates to
True, the loop body executes.
- Line 2 prints the current value of
- Line 3 prints the string
- Python reaches the end of the loop body (the indented lines). At this point,
it increases the value of
numby 1 and then MOVES BACK TO THE
forSTATEMENT (line 1).
- The new value of
1) gets compared to the
num < 4still returns
True, the loop body executes again.
- Lines 2 and 3 run with the new value of
num, so we see
Helloprinted to the console.
- Python again reaches the end of the loop body, increases the value of
numand moves back up to the
- This process continues until the value of
numreaches the end of the specified
range. Once the comparison
num < 4returns
False, the loop ends. Since Python adds 1 after each iteration, this occurs when
numis 4 (so
4 < 4is
False). At that point, the loop body will have run exactly 4 times, with
numtaking the values 0, 1, 2, and 3.
- Once the loop finishes, Python proceeds to line 5 and prints
We can use a picture to show the flow of execution of this
Notice that even though line 1 uses
range(4), the value
4 is NOT
included in the output. Why?
220.127.116.11. Begin Counting at 0¶
Iterating a certain number of times is a very common thing to do, and Python
gives us the built-in
range keyword to provide a set of values for the loop
variable to use.
The sequence provided by
range always starts with
0. If you ask for
range(4), then you will get 4 values starting with 0. In other words, 0, 1,
2, and finally 3. Notice that 4 is not included since we started with 0.
range(10) provides 10 values, 0 through 9. Starting a count at 0
instead of at 1 is called zero-based indexing and is very common in
Programmers like to count from 0!
range(n), the loop variable will take each integer value from 0 up
to BUT NOT INCLUDING
7.2.2. Check Your Understanding¶
How does Python know what lines are contained in the loop body?
- The lines are indented by the same amount from the
- There is always exactly one line in the loop body.
- The loop body ends with an empty line.
- The loop body ends at the next
How many lines does the following code print?
for number in range(10): print("I have", 12 - number, "cookies. I'm going to eat one!")
For the code above, what is the value of
number the third time Python
executes the loop?
For the same code, what is the LAST line printed by the program?
I have 2 cookies. I'm going to eat one!
I have 3 cookies. I'm going to eat one!
I have 10 cookies. I'm going to eat one!
I have 12 cookies. I'm going to eat one!