Python Loops Tutorial with Examples
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In Python, statements are executed sequentially from top to bottom. However, when you want to execute a sequence of statements multiple times, you can use loop.
Python provides you with 2 types of loops:
- while loop
- for loop
Statements are associated with the loop
- else
Statements can be used within a loop:
- continue
- break
- pass
| Control Statement | Description |
|---|---|
|
break |
Terminates the loop statement. |
|
continue |
Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating. |
|
pass |
The pass statement in the loop is simply a mark, prompting you to add some code in the future. It is a null statement (Do nothing). |
The syntax of a while loop:
while (condition) :
# Do something here
# ....
Example:
whileLoopExample.py
print("While loop example");
# Declare a variable, and assign value of 2.
x = 2;
# Condition is x < 10
# If x < 10 is true then run block
while (x < 10) :
print("Value of x = ", x);
x = x + 3;
# This statment is outside of while block.
print("Finish");
Running the example:
While loop example
Value of x = 2
Value of x = 5
Value of x = 8
Finish
The simplest example of a for loop in Python is to use 'for' with 'range'. For example, the variable 'x' has a value that runs within the scope (3, 7) (x = 3, 4, 5, 6).
forLoopExample.py
print("For loop example");
# for x = 3, 4, 5, 6
for x in range (3, 7) :
print("Value of x = ", x);
print(" x^2 = ", x * x);
# This statment is outside of for block.
print("End of example");
Running the example:
For loop example
Value of x = 3
x^2 = 9
Value of x = 4
x^2 = 16
Value of x = 5
x^2 = 25
Value of x = 6
x^2 = 36
End of example
Using the for loop can help you traverse on the elements of the array.
forLoopExample3.py
print("For loop example");
# Declare an array.
names =["Tom","Jerry", "Donald"];
for name in names:
print("Name = ", name);
print("End of example");
Output:
For loop example
Name = Tom
Name = Jerry
Name = Donald
End of example
Traverse elements of the array through index:
forLoopExample3b.py
print("For loop example");
# Declare an array.
names =["Tom","Jerry", "Donald"];
# len() function return length of array.
# index = 0,1,.. len-1
for index in range(len(names)):
print("Name = ", names[index] );
print("End of example");
break is a statement that may be located in a loop. This statement ends the loop unconditionally.
loopBreakExample.py
print("Break example");
# Declare a variable and assign value of 2.
x = 2;
while (x < 15) :
print("----------------------\n");
print("x = ", x);
# If x = 5 then exit the loop.
if (x == 5) :
break;
# Increase value of x by 1
x = x + 1;
print("x after + 1 = ", x);
print("End of example");
Output:
Beak example
----------------------
x = 2
x after + 1 = 3
----------------------
x = 3
x after + 1 = 4
----------------------
x = 4
x after + 1 = 5
----------------------
x = 5
End of example
continue is a statement which may be located in a loop. When caught the continue statement, the program will ignore the command lines in block, below of continue and start of a new loop.
loopContinueExample.py
print("Continue example");
# Declare a variable and assign value of 2
x = 2
while (x < 7) :
print("----------------------\n")
print("x = ", x)
# % is used for calculating remainder
# If x is even, then ignore the command line below of continue
# and start new iteration.
if (x % 2 == 0) :
# Increase x by 1.
x = x + 1
continue
else :
# Increase x by 1.
x = x + 1
print("x after + 1 =", x)
print("End of example");
Output:
Continue example
----------------------
x = 2
----------------------
x = 3
x after + 1 = 4
----------------------
x = 4
----------------------
x = 5
x after + 1 = 6
----------------------
x = 6
End of example
In Python programming, pass is a null statement. The difference between a comment and pass statement in Python is that, while the interpreter ignores a comment entirely, pass is not ignored.
However, nothing happens when pass is executed
However, nothing happens when pass is executed
The pass statement in the loop is simply a mark, prompting you to add some code in the future. It is a null command (Nothing).
loopPassExample.py
number = 0
for number in range(5):
number = number + 1
if number == 3:
print(" do something here " , number)
pass
print(" >> " ,number )
print('Out of loop')
Running the example:
>> 1
>> 2
do somthing here 3
>> 3
>> 4
>> 5
Out of loop
You can remove the pass statement in the example without changing anything.
loopPassExample.py (Remove pass statement)
number = 0
for number in range(5):
number = number + 1
if number == 3:
print(" do something here " , number)
# pass (Remove pass)
print(" >> " ,number )
print('Out of loop')
The else statement can be associated with a loop. The else statement is executed if the loop runs and ends in normal way, not broken by the break statement.
forLoopElseExample.py
print("For loop example");
# for x = 3, 4, 5, 6
for x in range (3, 7) :
print("Value of x = ", x);
print(" x^2 = ", x * x);
else :
print("finish for loop")
# This statment is outside of for block.
print("End of example");
Output:
For loop example
Value of x = 3
x^2 = 9
Value of x = 4
x^2 = 16
Value of x = 5
x^2 = 25
Value of x = 6
x^2 = 36
finish for loop
End of example
If the loop is stopped by the break statement, the else statement is associated with the loop will not be executed.
forLoopElseExample2.py
print("For loop example");
# for x = 3, 4, 5, 6
for x in range (3, 7) :
print("Value of x = ", x);
if x == 5:
print("Break!")
break;
else :
# If the break statement has been called in the loop,
# this statement will not be executed.
print("This command will not be executed!")
# This statment is outside of for block.
print("End of example");
Output:
For loop example
Value of x = 3
Value of x = 4
Value of x = 5
Break!
End of example
