Question

Suppose that str1, str2, and str3 are string variables, and str1 = "English", str2 = "Computer Science", and str3 = "Programming". Evaluate the following expressions: a. str1 >= str2 b. str1 != "english" c. str3 < str2 d. str2 >= "Chemistry

Short answer

a. True, b. True, c. False, d. False.

Step-by-step solution

Understand String Comparison

In Python, strings are compared lexicographically using the underlying ASCII values of their characters. Therefore, capital letters are considered smaller than lowercase letters because their ASCII values are lower.

Evaluating Expression a

Evaluate `str1 >= str2`. Compare the string "English" with "Computer Science" lexicographically. Since 'E' in "English" has a higher ASCII value than 'C' in "Computer Science", "English" is greater than "Computer Science". Thus, `str1 >= str2` is true.

Evaluating Expression b

Evaluate `str1 != "english"`. The strings "English" and "english" are not the same because they have different letter cases ('E' vs 'e'). Since the strings are different, the expression evaluates to true.

Evaluating Expression c

Evaluate `str3 < str2`. Compare the string "Programming" with "Computer Science" lexicographically. Since 'P' has a higher ASCII value than 'C', "Programming" is greater than "Computer Science". Thus, `str3 < str2` is false.

Evaluating Expression d

Evaluate `str2 >= "Chemistry"`. Compare "Computer Science" with "Chemistry" lexicographically. Since 'C' in "Chemistry" and "Computer Science" is the same, the second character 'h' in "Chemistry" (which has a higher ASCII value) makes "Chemistry" greater. Therefore, the expression is false because `str2` is not greater than or equal to "Chemistry".

Key concepts

Lexicographical Order

String comparison relies on a method called lexicographical order. Think of it as a dictionary where words are arranged based on their sequence of characters. When comparing two strings, we look at the first character in each word. If they are identical, we then compare the second character, and so on. This continues until we find a difference or confirm that the strings are identical. For instance, when comparing "apple" to "orange", the first letters 'a' and 'o' are directly compared. Since 'a' comes before 'o' in the alphabet, "apple" is considered less than "orange" in lexicographical order. If one string is a prefix of another, the shorter string is considered smaller. This method is crucial when sorting strings or evaluating conditions in programming.

ASCII Values

ASCII (American Standard Code for Information Interchange) assigns a unique numerical value to every character in the character set, including letters, numbers, and symbols. For example, 'A' has an ASCII value of 65, while 'a' is 97. These values help computers perform string comparison by converting text into a numerical format.
This is why 'B' is smaller than 'c' because 66 is less than 99. When programming languages like Python compare strings, they compare the ASCII values of each character starting from the leftmost position. This system allows consistent character comparison across different platforms and systems, ensuring that data is interpreted accurately.

String Variables

In programming, string variables hold sequences of characters, such as words or sentences. These strings are manipulable, allowing for operations like comparison, concatenation, and slicing. For example, in Python, the operation `str1 + str2` combines two strings, while comparison operations like `str1 == str2` check for equality.
Defining a string variable typically involves assigning it to a piece of text. For example, `str1 = "Hello"`. Once defined, these variables can be used in various expressions or functions, making it easy to work with text data efficiently. Understanding how to use string variables is fundamental in programming, as they allow you to manipulate text and symbols effectively.

Case Sensitivity

Programming languages often treat uppercase and lowercase characters differently, a principle known as case sensitivity. This means that "Hello" and "hello" are not considered equal, as 'H' and 'h' have different ASCII values. Operations sensitive to case will return false when comparing "English" to "english", highlighting the importance of the character's case in evaluations.
This behavior is especially crucial in string comparisons, database queries, and while working with text processing. In cases where the case should not affect the result, functions like `str.lower()` or `str.upper()` are used to convert strings to a standard format before comparison. This ensures that comparisons are consistent, regardless of the original case of the characters.