Loops in Python explained with examples

1. For Loop

Like R and C programming language, you can use for loop in Python. It is one of the most commonly used loop method to automate the repetitive tasks.

How for loop works?

Suppose you are asked to print sequence of numbers from 1 to 9, increment by 2.

for i in range(1,10,2):
  print(i)

Output

1
3
5
7
9

range(1,10,2) means starts from 1 and ends with 9 (excluding 10), increment by 2.

Iteration over list
This section covers how to run for in loop on a list.

mylist = [30,21,33,42,53,64,71,86,97,10]
for i in mylist:
    print(i)

Output

30
21
33
42
53
64
71
86
97
10

Suppose you need to select every 3rd value of list.

for i in mylist[::3]:
    print(i)

Output

30
42
71
10

mylist[::3] is equivalent to mylist[0::3] which follows this syntax style list[start:stop:step]

Python Loop Explained with Examples

Example 1 : Create a new list with only items from list that is between 0 and 10

l1 = [100, 1, 10, 2, 3, 5, 8, 13, 21, 34, 55, 98]

new = [] #Blank list
for i in l1:
    if i > 0 and i <= 10:
        new.append(i)

new

Output: [1, 10, 2, 3, 5, 8]

It can also be done via numpy package by creating list as numpy array. See the code below.

import numpy as np
k=np.array(l1)
new=k[np.where(k<=10)]

Example 2 : Check which alphabet (a-z) is mentioned in string

Suppose you have a string named k and you want to check which alphabet exists in the string k.

k = "deepanshu"

import string
for n in string.ascii_lowercase:
    if n in k:
        print(n + ' exists in ' + k)
    else:
        print(n + ' does not exist in ' + k)

string.ascii_lowercase returns 'abcdefghijklmnopqrstuvwxyz'.

Practical Examples : for in loop in Python

Create sample pandas data frame for illustrative purpose.

import pandas as pd
np.random.seed(234)
df = pd.DataFrame({"x1" : np.random.randint(low=1, high=100, size=10),
"Month1" : np.random.normal(size=10),
"Month2" : np.random.normal(size=10),
"Month3" : np.random.normal(size=10),
"price"  : range(10)
                     })

df

1. Multiple each month column by 1.2

for i in range(1,4):
    print(df["Month"+str(i)]*1.2)

range(1,4) returns 1, 2 and 3. str( ) function is used to covert to string."Month" + str(1) means Month1.

2. Store computed columns in new data frame

import pandas as pd
newDF = pd.DataFrame()
for i in range(1,4):
    data = pd.DataFrame(df["Month"+str(i)]*1.2)
    newDF=pd.concat([newDF,data], axis=1)

pd.DataFrame( ) is used to create blank data frame. The concat() function from pandas package is used to concatenate two data frames.

3. Check if value of x1 >= 50, multiply each month cost by price. Otherwise same as month.

import pandas as pd
import numpy as np
for i in range(1,4):
    df['newcol'+str(i)] = np.where(df['x1'] >= 50,
                                   df['Month'+str(i)] * df['price'],
                                   df['Month'+str(i)])

In this example, we are adding new columns named newcol1, newcol2 and newcol3.np.where(condition, value_if condition meets, value_if condition does not meet) is used to construct IF ELSE statement.

4. Filter data frame by each unique value of a column and store it in a separate data frame

mydata = pd.DataFrame({"X1" : ["A","A","B","B","C"]})

for name in mydata.X1.unique():
    temp = pd.DataFrame(mydata[mydata.X1 == name])
    exec('{} = temp'.format(name))

The unique( ) function is used to calculate distinct values of a variable. The exec( ) function is used for dynamic execution of Python program. See the usage of string format( ) function below -

s= "Your Input"
"i am {}".format(s)

Output: 'i am Your Input'

Loop Control Statements

Loop control statements change execution from its normal iteration. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.

Python supports the following control statements.

1. Continue statement
2. Break statement

Continue Statement
When continue statement is executed, it skips the further code in the loop and continue iteration.
In the code below, we are avoiding letters a and d to be printed.

for n in "abcdef":
    if n =="a" or n =="d":
       continue
    print("letter :", n)

letter : b
letter : c
letter : e
letter : f

Break Statement
When break statement runs, it breaks or stops the loop.
In this program, when n is either c or d, loop stops executing.

for n in "abcdef":
    if n =="c" or n =="d":
       break
    print("letter :", n)

letter : a
letter : b

for loop with else clause

Using else clause with for loop is not common among python developers community.

The else clause executes after the loop completes. It means that the loop did not encounter a break statement.

The program below calculates factors for numbers between 2 to 10. Else clause returns numbers which have no factors and are therefore prime numbers:

for k in range(2, 10):
    for y in range(2, k):
        if k % y == 0:
            print( k, '=', y, '*', round(k/y))
            break
    else:
        print(k, 'is a prime number')

2 is a prime number
3 is a prime number
4 = 2 * 2
5 is a prime number
6 = 2 * 3
7 is a prime number
8 = 2 * 4
9 = 3 * 3

While Loop

While loop is used to execute code repeatedly until a condition is met. And when the condition becomes false, the line immediately after the loop in program is executed.

i = 1
while i < 10:
    print(i)
    i += 2 #means i = i + 2
    print("new i :", i)

Output:
1
new i : 3
3
new i : 5
5
new i : 7
7
new i : 9
9
new i : 11

While Loop with If-Else Statement

If-Else statement can be used along with While loop. See the program below -

counter = 1 
while (counter <= 5): 
    if counter < 2:
        print("Less than 2")
    elif counter > 4:
        print("Greater than 4")
    else: 
        print(">= 2 and <=4")    
    counter += 1