Python provides following types of loops to handle looping requirements
- while loop
- for loop
While Loop is used to execute a block of statements repeatedly until a given a condition is satisfied. And when the condition becomes false, the line immediately after the loop in program is executed. Using else statement with while loops, while loop executes the block until a condition is satisfied. The else clause is only executed when while condition becomes false.
#1 while loop count = 0 while (count < 3): count = count + 1 print("Hello") # Output Hello Hello Hello #2 Combining else with while count = 0 while (count < 3): count = count + 1 print("Hello") else: print("Welcome") # Output Hello Hello Hello Welcome #3 Single statement while block count = 0 while (count == 0): print("Hello")
For loops are used for sequential traversal. For example
#1 Iterating over a list l = ["List1", "List2", "List3"] for i in l: print(i) # Output List1 List2 List3 #2 Iterating over a tuple t = ("T1", "T2", "T3") for i in t: print(i) # Output T1 T2 T3 #3 Iterating over a String s = "String" for i in s : print(i) # Output S t r i n g #4 Iterating over dictionary d = dict() d['xyz'] = 123 d['abc'] = 345 for i in d : print("%s %d" %(i, d[i])) # Output xyz 123 abc 345
We can also use the index of elements in the sequence to iterate. The key idea is to first calculate the length of the list and in iterate over the sequence within the range of this length. We can generate a sequence of numbers using range() function. range(10) will generate numbers from 0 to 9 (10 numbers). We can also define the start, stop and step size as range(start,stop,step size). step size defaults to 1 if not provided.
# Iterate over the list using index genre = ['pop', 'rock', 'jazz'] for i in range(len(genre)): print(genre[i])
We can also combine else statement with for loop like in while loop. But as there is no condition in for loop based on which the execution will terminate so the else block will be executed immediately after for block finishes execution.
digits = [0, 1, 5] for i in digits: print(i) else: print("No items left!") # Output 0 1 5 No items left!
Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. Python supports the following control statements.
# 1 Prints all letters except 'e' and 'd' for letter in 'england': if letter == 'e' or letter == 'd': continue print (letter) # 2 Example for letter in 'england': # break the loop as soon it sees 'e' or 'd' if letter == 'e' or letter == 'd': break print (letter)
We use pass statement to write empty loops. Pass is also used for empty control statement, function and classes.
# An empty loop for letter in 'loop': pass