The while' loop can be used to solve any item search problem, but it is notable for being verbose. Forwhile` it is necessary to set a stopping condition and enter a counter. When there are few loops, this is fine, but in real code loops occur at every step. It is therefore tedious to manage the conditions manually, especially when the stopping condition is obvious.

For example, if we want to go through the characters in a string, the computer can figure out by itself when the string ends. For such situations, Python introduced the for loop. It knows itself when to stop, because it only works with collections - sets of elements that need to be searched.

A string is a collection because it consists of a set of characters. The other types of collections are studied in detail in another course.

Example:

text = 'code'
for symbol in text:
    print(symbol)

# => c
# => o
# => d
# => e

In the code above, for goes through each character in the string, writes it in a variable symbol and calls the internal code block where that variable is used. The name of this variable can be anything. The general structure of the for' loop looks like this:for in '.

Let's see how to implement the string flip function through the for loop:

def reverse_string(text):
    # Initial value
    result = ''
    # char - a variable in which the current character is written
    for char in text:
        # Connect in reverse order
        result = char + result
    # The cycle ends when the whole line is passed
    return result


reverse_string('go!')  # => '!og'

Now let's calculate the number of mentions of a character in the string, not case-sensitive:

# text - any text
# char - a symbol to consider
def chars_count(text, char):
    # Since we are looking for the sum, the initial value is 0
    result = 0
    for current_char in text:
        # We convert everything to lower case,
        # so as not to depend on the current register
        if current_char.lower() == char.lower():
            result += 1
    return result


chars_count('hexlet!', 'e')  # 2
chars_count('hExlet!', 'e')  # 2
chars_count('hExlet!', 'E')  # 2

chars_count('hexlet!', 'a')  # 0

Implement the function normalize_filename(), which prepares a file name for saving. The function takes a string and returns a new string where every space is replaced with _.

An example of a call:

normalize_filename('my photo.png')        # 'my_photo.png'
normalize_filename('final report.pdf')    # 'final_report.pdf'
normalize_filename('already_ready.txt')   # 'already_ready.txt'