Loops: Iterating Through Files

A Search Result with Two Meanings

When a loop reads a file one line at a time, each line can be tested for a substring. The find() method gives the test a precise result: it returns the starting index of the substring when the substring is present, and it returns -1 when the substring is absent. That single integer lets a program decide whether a line should be kept or skipped.

Mapping a Match to an Index

String positions are zero-indexed, so the first character is at position 0. When find() locates a substring, it reports the position of that substring's first character. For example, in the generated string "alice@example.com", the at sign begins at position 5. Calling find("@") therefore returns 5, not the position after the at sign and not the number of characters in the string.

Preparing Each File Line

A line read from a file includes the newline character at its end. Before searching, call rstrip() on the line. rstrip() removes trailing whitespace, including the newline, so the search operates on the actual line content. The source material recommends calling rstrip() before searching because leaving trailing whitespace in place can make a search fail unexpectedly or behave strangely.

line = line.rstrip() position = line.find("@")

Filtering Lines with a Conditional

The useful distinction is between the two possible kinds of result. A return value of -1 means the substring was not found, so the line can be skipped. Any other returned integer means the substring was found at that index, so the line can be printed or collected. A conditional makes this decision for every line in the file.

for line in file:
    line = line.rstrip()
    if line.find("@") == -1:
        continue
    print(line)

This pattern first removes trailing whitespace. It then searches the current line for the at sign. If find() returns -1, continue skips the rest of the loop body and moves to the next line. If find() returns an index, the condition is false and the line is printed.

Walking Through the File

The loop applies the same three-stage process to every line: prepare the line with rstrip(), search for the chosen substring, and either skip or keep the line. The loop does not need to know the result from the previous line. Each current line receives its own find() result.

file = open("mbox-short.txt")
matching_lines = []

for line in file:
    line = line.rstrip()
    if line.find("@") == -1:
        continue
    matching_lines.append(line)

In this generated variation, a list named matching_lines receives each line whose find() result is not -1. The same filtering rule can instead print the line immediately. The important mechanism is unchanged: each line is cleaned, searched, and either skipped or retained.

Tracing an Incorrect Search

Tracing the integer returned by find() can reveal what went wrong. In the generated line "From alice@example.com", the at sign starts at position 10, so find("@") returns 10. If the search is changed to find("#"), the return value becomes -1 because that substring is not present. Checking the searched substring and the returned index helps distinguish a missing match from an incorrect expectation about its position.

Mistakes in File Searches

• Treating -1 as if it were a valid matching position.

  -1 means the substring was not found.

  Fix: Skip when the result equals -1, and keep the line when the result is not -1.

• Forgetting that positions are zero-indexed.

  The first character is at position 0.

  Fix: Count from 0 when tracing the returned index.

• Searching the raw line before removing trailing whitespace.

  A file line includes its ending newline, and trailing whitespace can make a search fail unexpectedly or behave strangely.

  Fix: Call rstrip() before searching each line.

• Assuming every nonzero result means no match.

  Any returned index identifies where the substring starts.

  Fix: Only -1 means that the substring was not found.

Practice the Filter

Key Takeaways

1. find() returns the starting index of a found substring.
2. The first character has index 0, and -1 means that the substring was not found.
3. Call rstrip() before searching a line read from a file.
4. A loop, find(), a conditional, and continue can filter file lines by substring presence.
5. When debugging, inspect both the searched substring and the integer returned by find().