5.2 For loop

Learning objectives

By the end of this section you should be able to

Explain the for loop construct.
Use a for loop to implement repeating tasks.

For loop

In Python, a container can be a range of numbers, a string of characters, or a list of values. To access objects within a container, an iterative loop can be designed to retrieve objects one at a time. A for loop iterates over all elements in a container. Ex: Iterating over a class roster and printing students' names.

Note

For loop over a string container

A string variable can be considered a container of multiple characters, and hence can be iterated on. Given the following code, answer the questions.

str_var = "A string"

count = 0
for c in str_var:
  count += 1

print(count)

`Range` function in for loop

A for loop can be used for iteration and counting. The range function is a common approach for implementing counting in a for loop. A range function generates a sequence of integers between the two numbers given a step size. This integer sequence is inclusive of the start and exclusive of the end of the sequence. The range function can take up to three input values. Examples are provided in the table below.

**Table 5.1** Using the range function.
Range function	Description	Example	Output
`range(end)`	Generates a sequence beginning at `0` until `end`. Step size: `1`	`range(4)`	`0, 1, 2, 3`
`range(start, end)`	Generates a sequence beginning at `start` until `end`. Step size: `1`	`range(0, 3)`	`0, 1, 2`
		`range(2, 6)`	`2, 3, 4, 5`
		`range(-13, -9)`	`-13, -12, -11, -10`
`range(start, end, step)`	Generates a sequence beginning at `start` until `end`. Step size: `step`	`range(0, 4, 1)`	`0, 1, 2, 3`
		`range(1, 7, 2)`	`1, 3, 5`
		`range(3, -2, -1)`	`3, 2, 1, 0, -1`
		`range(10, 0, -4)`	`10, 6, 2`

Example 1 Two programs printing all integer multiples of 5 less than 50 (Notice the compactness of the for construction compared to the while)

# For loop condition using # range function to print # all multiples of 5 less than 50 for i in range(0, 50, 5): print(i) # While loop implementation of printing # multiples of 5 less than 50 # Initialization i = 0 # Limiting the range to be less than 50 while i < 50: print(i) i+=5

Note

Counting spaces

Write a program using a for loop that takes in a string as input and counts the number of spaces in the provided string. The program must print the number of spaces counted. Ex: If the input is "Hi everyone", the program outputs 1.

usr_in = input()

# TODO: counting the number of space characters in usr_in

Note

Sequences

Write a program that reads two integer values, n1 and n2, with n1 < n2, and performs the following tasks:

Prints all even numbers between the two provided numbers (inclusive of both), in ascending order.
Prints all odd numbers between the two provided numbers (exclusive of both), in descending order.


    Input: 2 8


    Prints:
    2 4 6 8
    7 5 3

Note: the program should return an error message if the second number is smaller than the first.

n1 = int(input("Enter an integer: "))
n2 = int(input("Enter another integer larger than the first one: "))

# TODO: printing even numbers in increasing order (inclusive)

# TODO: printing odd numbers in decreasing order (exclusive)

5.2 For loop

Learning objectives

For loop

Range function in for loop

`Range` function in for loop