Concepts / Working with Loops and Conditionals

Working with Loops and Conditionals

The loop visits the file line by line.

  • Programming

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.

Key point

The central pattern is read, filter, process: read one line, filter it with a condition, and process it only when the condition is true.

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.

Read lineone lineSearch conditionstring methodProcess linematching lineNext linenonmatching lineinspecttruefalse
How does each line move from being read, through a search condition, to being processed only when it matches?

Tracing One Line

Code walkthrough
with open("system.log") as file:
    for line in file:
        if "ERROR" in line:
            print(line.strip())
  1. 1
    The file is opened for reading and represented by file.
  2. 2
    The loop obtains one line at a time from the file.
  3. 3
    The in condition checks whether the line contains the text ERROR.
  4. 4
    When the condition is true, line.strip() removes surrounding whitespace before the line is printed.
  5. 5
    When the condition is false, the print statement is skipped and the loop continues.
Current linetext from fileERROR in linetrue or falsePrint linematching resultNext linecontinue loopchecktruefalse
What happens next for a line that matches the condition, and what happens next for a line that does not?

What do you think happens?

Suppose the current line is "INFO: backup complete\n". What happens when the condition is "ERROR" in line?

  • The line is printed

  • The line is skipped

  • The loop stops

Reveal answer
Answer:

The line is skipped.

The condition is false because the line does not contain ERROR. The processing statement is not reached, so the loop moves to the next line.

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.

First lineread and testNext lineread and testLater lineread and testLast linefinish searchcontinuecontinuecontinue
How does the program progress from the first line to the last while maintaining the same search logic?

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.

ApproachWhat happens to each lineWhy it matters
Process every lineEvery line moves to processingWork is performed even for irrelevant data
Filter before processingOnly matching lines move to processingUnnecessary processing is avoided

The difference between processing all file data and using a targeted search condition.

Every lineprocessedMatching lineprocessedNonmatching lineskippedprocessprocessskip
What is the difference between processing every line and processing only lines that satisfy the search condition?

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.
python

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

  • Common mistake

    Forgetting to strip whitespace

    What it looks like
    print(line)
    Why it fails

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

    Do this instead

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

  • Common mistake

    Using case-sensitive comparisons incorrectly

    What it looks like
    if "error" in line:
    Why it fails

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

    Do this instead

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

  • Common mistake

    Processing before filtering

    What it looks like
    for line in file:
    print(line)
    if "ERROR" in line:
        process(line)
    Why it fails

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

    Do this instead

    Put the relevant processing statement inside the conditional block.

  • Common mistake

    Not closing the file after reading

    Why it fails

    A file should be closed when reading is finished.

    Do this instead

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

Practice the Pattern

MEDIUM

Write a Python file search that reads lines from settings.txt and prints only the lines that contain the text "timeout". Make sure the processing statement is controlled by the conditional and that surrounding whitespace is removed before printing.

Hints
  • Open the file and loop through it one line at a time.
  • Use the in operator inside an if statement.
  • Call strip() in the processing statement.
Worked example

Tracing a Timeout Search

A file contains three lines: "host=server", "timeout=30", and "mode=safe". The search condition is "timeout" in line.

  1. 1
    Read the first line

    The line host=server does not contain timeout, so the condition is false and the line is skipped.

  2. 2
    Read the second line

    The line timeout=30 contains timeout, so the condition is true and the line moves to processing.

  3. 3
    Read the third line

    The line mode=safe does not contain timeout, so the condition is false and the line is skipped.

Result

Only timeout=30 is processed.

Key Takeaways

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

At a glance

  • Search files line by line using the read, filter, process pattern.
  • Use string tests such as in, startswith, and find inside a conditional.
  • Process only matching lines so irrelevant data is skipped.
  • A single-pass search is useful for large files because it avoids unnecessary processing and memory use.
  • Remember to remove unwanted whitespace, handle case carefully, and close files.

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