8.3: Searching/Testing Strings

Learning Objectives

By the end of this section you should be able to

• Use the in operator to identify whether a given string contains a substring.
• Call the count() method to count the number of substrings in a given string.
• Search a string to find a substring using the find() method.
• Use the index() method to find the index of the first occurrence of a substring in a given string.
• Write a for loop on strings using in operator.

in operator

The in Boolean operator can be used to check if a string contains another string. in returns True if the first string exists in the second string, False otherwise.

Checkpoint: What is in the phrase?

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Concepts in Practice: Using in operator to find substrings

1.

What is the output of ("a" in "an umbrella")?

1. 2

False

True

1

2.

What is the output of ("ab" in "arbitrary")?

1. True
2. False

3.

What is the output of ("" in "string")?

1. True
2. False

For loop using in operator

The in operator can be used to iterate over characters in a string using a for loop. In each for loop iteration, one character is read and will be the loop variable for that iteration.

Checkpoint: for loop using in operator

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Concepts in Practice: Using in operator within for loop

4.

What is the output of the following code?

for c in "string":
  print(c, end = "")

1. string

s
t
r
i
n
g

s t r i n g

5.

What is the output of the following code?

count = 0
for c in "abca":
  if c == "a":
    count += 1
print(count)

1. 0
2. 1
3. 2

6.

What is the output of the following code?

word = "cab"
for i in word:
  if i == "a":
    print("A", end = "")
  if i == "b":
    print("B", end = "")
  if i == "c":
    print("C", end = "") 

1. cab
2. abc
3. CAB
4. ABC

count()

The count() method counts the number of occurrences of a substring in a given string. If the given substring does not exist in the given string, the value 0 is returned.

Checkpoint: Counting the number of occurrences of a substring

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Concepts in Practice: Using count() to count the number of substrings

7.

What is the output of (aaa".count("a"))?

1. True

1

3

8.

What is the output of ("weather".count("b"))?

1. 0
2. -1
3. False

9.

What is the output of ("aaa".count("aa"))?

1. 1
2. 2
3. 3

find()

The find() method returns the index of the first occurrence of a substring in a given string. If the substring does not exist in the given string, the value of -1 is returned.

Checkpoint: Finding the first index of a substring

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Concepts in Practice: Using find() to locate a substring

10.

What is the output of "banana".find("a")?

1. 1

3

5

11.

What is the output of "banana".find("c")?

1. 0
2. -1
3. ValueError

12.

What is the output of "b".find("banana")?

1. -1
2. 0
3. ValueError

index()

The index() method performs similarly to the find() method in which the method returns the index of the first occurrence of a substring in a given string. The index() method assumes that the substring exists in the given string; otherwise, throws a ValueError.

Example 8.4

Getting the time's minute portion

Consider a time value is given as part of a string using the format of "hh:mm" with "hh" representing the hour and "mm" representing the minutes. To retrieve only the string's minute portion, the following code can be used:

    time_string = "The time is 12:50"
    index = time_string.index(":")
    print(time_string[index+1:index+3])

The above code's output is:

    50

Concepts in Practice: Using index() to locate a substring

13.

What is the output of "school".index("o")?

1. 3

4

-3

14.

What is the output of "school".index("ooo")?

1. 3
2. 4
3. ValueError

15.

What is the output of the following code?

sentence = "This is a sentence"
index = sentence.index(" ")
print(sentence[:index])

1. "This"
2. "This "
3. "sentence"

Try It: Finding all spaces

Write a program that, given a string, counts the number of space characters in the string. Also, print the given string with all spaces removed.

    Input: "This is great"

    Prints:
    2
    Thisisgreat

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