Aprende Searching and Measuring

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Once you can read pieces of a string, the next step is to ask questions about its contents: "does it contain this?", "where is it?", "how many times?", "does it start/end with…?".

Membership

Use in to check if a substring appears anywhere in a string. It returns a Boolean.


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# Checking if a user's bio mentions specific skills
user_bio = "Experienced in Python programming and data analysis."

print("Python" in user_bio)      # True → the bio mentions Python
print("Java" not in user_bio)    # True → Java is not listed as a skill

Finding Positions

find(sub) returns the starting index of the first match, or -1 if not found;
rfind(sub) searches from the right and returns the last match index (or -1);
index(sub) is like find, but raises ValueError if the substring is missing.


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# Searching for keywords inside a product description
description = "This brand new bracelet is made from recycled materials."

print(description.find("bra"))     # 5  → first occurrence of "bra"
print(description.rfind("bra"))    # 15  → only one "bra" found
print(description.find("gold"))    # -1 → not found, returns -1
print(description.index("bra"))    # 5  → same as find(), but raises an error if not found
print(description.index("gold"))   # ValueError → "gold" not in the text

Counting Occurrences

count(sub) returns how many non-overlapping times the substring appears.


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# Counting occurrences of words or letters in a customer review
review = "Amazing banana smoothie with banana slices on top!"

print(review.count("banana"))  # 2 → the word appears twice
print(review.count("a"))       # 8 → letter 'a' appears multiple times

Starts and Ends

To check a string's start or end, use startswith or endswith, these are clearer and safer than slicing when you only need a yes/no answer.


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# Checking if the uploaded file has the correct name and format
uploaded_file = "report_final.pdf"

print(uploaded_file.startswith("report"))  # True → file name starts correctly
print(uploaded_file.endswith(".pdf"))      # True → valid file format for upload

Case-Insensitive Checks

String methods are case-sensitive. For case-insensitive search, normalize both sides with .lower() (or .upper()).


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# Checking a user's message for a polite greeting
user_message = "Hello, team! Let's start the meeting."

print("hello" in user_message.lower())          # True → greeting detected
print(user_message.lower().startswith("hello")) # True → message begins with "hello"

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Sección 1. Capítulo 15

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Sección 1. Capítulo 15

Searching and Measuring

Membership

Finding Positions

Counting Occurrences

Starts and Ends

Case-Insensitive Checks

1. What will this code output?

2. Which statement does not raise an error when the substring is missing?

3. Given `s = "Banana"`, which expression returns `True` case-insensitively for checking the prefix `"ba"`?

Searching and Measuring

Membership

Finding Positions

Counting Occurrences

Starts and Ends

Case-Insensitive Checks

1. What will this code output?

2. Which statement does not raise an error when the substring is missing?

3. Given s = "Banana", which expression returns True case-insensitively for checking the prefix "ba"?

3. Given `s = "Banana"`, which expression returns `True` case-insensitively for checking the prefix `"ba"`?