Testing and searching text

What you will be able to do

After this lesson you can

• ask a yes or no question about a piece of text,
• count how often a piece occurs and find where it is,
• tell what kind of characters a string has, and
• use these tools to check a word, a code, or a number that a user typed.

Text that answers questions

So far we have made new text. Now we ask questions about text. Each of these gives True or False.

These answers are bool values, the fourth type of value. In the course on if, you will use them to make decisions: "if the text is a number, then calculate". For now, print them and read them.

Counting and finding

You can use a position that find() gives in a slice. text[text.find(" ") + 1:] is everything after the first space.

With no second number, find() starts at position 0, so it gives the first match of the whole text.

A small use: the first word

The first word of a sentence ends at the first space. The position of the first space is sentence.find(" "). So the first word is sentence[:sentence.find(" ")]. For "Python is easy", find(" ") is 6, and sentence[:6] is Python. You will use this idea in the project.

Common mistakes

• Using in without a string on the left. 5 in "2026" is an error. Write "5" in "2026".
• Forgetting that case matters. "Python" in "python is fun" is False. Use .lower() on both sides first.
• Expecting isdigit() to accept a minus sign or a decimal point. "-5".isdigit() and "3.5".isdigit() are False.
• Forgetting that find() gives -1 when the piece is missing. A position of -1 is not the last position here.
• Using index() for something that may be missing. It stops with an error. Use find().

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Text detective. Answer five questions about two sentences. The checker confirms each one.
2. Build. Open the problem Counting inside a word, and the problem Does it read the same backwards?
3. Stretch. Open the problem Count and find. Predict each answer before you run the program.

Remember these

• in, startswith(), endswith(), isdigit(), isalpha() and isupper() answer with True or False.
• count() counts a piece. find() gives the position of the first match, or -1.
• index() is like find() but stops with an error if the piece is not there.
• Case matters. Change both sides to small letters first, when it does not.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What are "cat" in "concatenate" and "Dog" in "dog"?

2. What do "banana".count("a") and "banana".find("n") give?

3. How do you check that the text s is a whole number?

4. What does "hello".find("z") give?