Strings, Arrays, Files and Subprograms

A string is an ordered sequence of characters; a list stores an ordered collection. Python uses zero-based indexing, so a list of length n has normal non-negative indexes 0 through n−1. Traversal processes elements in a useful order. Distinguish an element from its index. A two-dimensional structure can represent rows and columns; apply the indexing convention consistently. Validate an index before using it when input may be invalid.

A function packages a named task with parameters and, where appropriate, a return value. Parameters receive the supplied arguments for that call. Local variables are scoped to the function, reducing accidental interference. Returning a value is different from printing it: the caller can use a returned value in another calculation. Decomposition into subprograms improves readability and testing when each has a clear responsibility.

Opening a file in write mode can replace existing contents, while append mode adds to them. Reading retrieves stored text that may need parsing or conversion. Close files reliably; Python’s with statement helps manage that resource. Validate format and handle expected errors appropriately instead of assuming every line is valid. Keep file I/O separate from pure calculations when useful so data-processing logic can be tested without repeatedly changing real files.

def count_passes(scores):
    return sum(1 for score in scores if score >= 50)

print(count_passes([49, 50, 80]))

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.