Working with Loops and Conditionals

Why Search Selectively

A text file may contain hundreds or thousands of lines, while your program may need only a small subset. A log file might contain many entries, but your task could be to find only errors. A contact list might contain many names, but you may be looking for one specific name. Processing every line in the same way would do unnecessary work. A targeted file search instead reads each line, checks whether it matches a condition, and processes only the lines that pass.

The Three-Step Search Pattern

The loop visits the file line by line. For each line, the filter applies a search condition. String operations such as the in operator, startswith, and find can test whether the line contains particular text, begins with a particular prefix, or includes a substring. If the condition is true, the line moves to the processing step. If the condition is false, the line is skipped and the loop continues with the next line.

Tracing One Line

with open("system.log") as file: for line in file: if "ERROR" in line: print(line.strip())

Progressing Through the File

The same logic is applied repeatedly as the loop advances from the first line to the last. Each line receives one opportunity to pass the filter. A matching line is processed immediately; a nonmatching line is skipped immediately. The search therefore makes one pass through the file instead of first collecting or processing all lines.

Why Single-Pass Processing Helps

The pattern is useful when the file is large and only a small portion is relevant. Each line is read once, tested immediately, and either processed or skipped. This avoids unnecessary processing of nonmatching lines. It also avoids loading the entire file into memory at once, making the approach memory-efficient for large text files.

Choosing the Search Test

• Use the in operator when the line should contain particular text anywhere.
• Use startswith when the line should begin with a particular prefix.
• Use find when you need to test for a substring's presence or position.
• Place the string test inside the if statement so it is evaluated once for each line.

with open("contacts.txt") as file:
    for line in file:
        if line.startswith("Ada"):
            print(line.strip())

This example changes the filter rather than the overall pattern. The loop still reads a line, the condition still decides whether it matches, and the processing step still runs only for a matching line. Only the search requirement has changed: the line must begin with Ada.

Mistakes to Avoid

• Forgetting to strip whitespace

  A line read from a text file may include surrounding whitespace, including the line ending.

  Fix: Use line.strip() when the processing step needs the surrounding whitespace removed.

• Using case-sensitive comparisons incorrectly

  The condition may not match a line containing ERROR when the searched text uses different capitalization.

  Fix: Choose the capitalization used by the file and the search condition carefully.

• Processing before filtering

  The line is already being processed by print before the condition decides whether it belongs in the result.

  Fix: Put the relevant processing statement inside the conditional block.

• Not closing the file after reading

  A file should be closed when reading is finished.

  Fix: Close the file explicitly or use a file-handling structure that closes it after the reading block.

Practice the Pattern

Key Takeaways

1. A targeted file search follows the sequence read, filter, process.
2. The loop reads one line at a time and the conditional decides whether that line continues.
3. String methods and operations such as in, startswith, and find can build the filter condition.
4. Single-pass filtering avoids unnecessary processing and avoids loading the entire file into memory at once.
5. Strip whitespace, handle capitalization carefully, and close the file after reading.