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52 of 108Python - Collection For Loop
for Loop with List
If we want to execute an action for every element present in a list, we use the for loop.
Example 1 - Print All Elements of a List
main.py
0 1 2 3 4 5 6 7 8 9 10
Explanation
The loop visits every element of the list one by one.
Example 2 - Print Only Even Numbers
main.py
0 2 4 6 8 10
Explanation
The if statement filters only the even numbers.
Example 3 - Display Positive and Negative Index
main.py
A is available at positive index: 0 and at negative index: -3 B is available at positive index: 1 and at negative index: -2 C is available at positive index: 2 and at negative index: -1
Understanding the Program
len(l)returns the total number of elements in the list.range(x)generates index values from 0 to x-1.l[i]accesses the list element using its index.i-xgives the corresponding negative index.
Important Points - List
- The
forloop processes one list element at a time. - Every element is visited exactly once.
- You can use
ifconditions inside the loop. - You can iterate through indexes using
range(len(list)).
List Summary
| Statement | Description |
|---|---|
for x in list: |
Iterates through each list element. |
if inside for |
Filters elements based on a condition. |
len(list) |
Returns the total number of elements. |
range(len(list)) |
Generates valid index positions. |
list[i] |
Accesses an element using its index. |
Important Notes
- Lists are iterable objects.
- The
forloop automatically visits every element. range(len(list))is useful when index values are required.- Both positive and negative indexes can be displayed using a loop.
for Loop with Tuple
A tuple is also a sequence, so we can use the for loop to access each element one by one.
Example 1 - Print Tuple Elements
main.py
10 20 30 40 50
Example 2 - Find Sum of Tuple Elements
main.py
Sum = 100
Important Points - Tuple
- Tuple elements are processed one by one.
- Tuple is immutable, so elements cannot be modified.
- The
forloop can only read tuple elements.
for Loop with Set
A set is also iterable. We can access every element using the for loop.
Since a set is unordered, the output order may change every time the program runs.
Example 1 - Print Set Elements
main.py
Possible Output 40 10 30 20
Explanation
The order of elements in a set is not fixed.
Therefore, the output order may be different on different executions.
Example 2 - Print Squares of Set Elements
main.py
Possible Output 4 16 36 64
Important Points - Set
- Set elements are unordered.
- Duplicate values are not allowed.
- The output order may vary.
for Loop with Dictionary
A dictionary stores data as key : value pairs.
By default, the for loop iterates through dictionary keys.
Example 1 - Print Dictionary Keys
main.py
100 200 300
Explanation
When a dictionary is directly used inside a for loop, only the keys are returned.
Example 2 - Print Dictionary Values
main.py
Sunny Bunny Chinny
Example 3 - Print Keys and Values
main.py
100 ----> Sunny 200 ----> Bunny 300 ----> Chinny
Explanation
The items() method returns both keys and values.
This is the most common way to iterate through a dictionary.
Important Points - Dictionary
- Dictionary keys are unique.
- Dictionary values can be duplicated.
- The
forloop iterates through dictionary keys.