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Python: Conditions Inside the Body of the Loop

You can execute instructions in the bodies of loops like you can in functions. This means you can use everything you learned before inside loops, such as conditional constructions.

Imagine a function that counts how many times a letter occurs in a sentence. Example of how it works:

count_chars('Fear cuts deeper than swords.', 'e')  # 4
# If nothing is found, the result is 0 matches
count_chars('Sansa', 'y')  # 0

Before you look at the contents of a function, think about this:

  • Is this an aggregation operation?
  • How will it check if a character occurs in a sentence?
def count_chars(string, char):
    index = 0
    count = 0
    while index < len(string):
        if string[index] == char:
            # We only count the matching characters
            count = count + 1
        # The counter is incremented anyway
        index = index + 1
    return count

This is an aggregation task. Although it doesn't count all the characters to calculate the amount, you still have to analyze each character. The key difference between this loop and the ones we looked at before is that there's a condition inside the body.

The count variable is incremented only when the character in question is the same as the expected one. Otherwise, it's a typical aggregate function that returns the number of characters you want.


The function from the theory is case-sensitive. I.e., A' anda' are different characters from its point of view. Write a version of this function that isn't case-sensitive:

count_chars('HexlEt', 'e')  # 2
count_chars('HexlEt', 'E')  # 2
The exercise doesn't pass checking. What to do? 😶

If you've reached a deadlock it's time to ask your question in the «Discussions». How ask a question correctly:

  • Be sure to attach the test output, without it it's almost impossible to figure out what went wrong, even if you show your code. It's complicated for developers to execute code in their heads, but having a mistake before their eyes most probably will be helpful.
In my environment the code works, but not here 🤨

Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.

My code is different from the teacher's one 🤔

It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.

In some rare cases, the solution may be adjusted to the tests, but this can be seen immediately.

I've read the lessons but nothing is clear 🙄

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