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Section 15.3 Strings are Immutable

Even though you can manipulate a string to create a new string the original string is immutable which means that they never change. Notice that after you execute the code below the string stored in the variable sentence hasn’t changed.
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While the strings themselves can’t be changed you can change the value of a variable. This throws away the original string and sets the variable’s value to the new string.
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We can reassign a variable to hold the result of calling a function on a string. On line 3 of the example below, we make the new value of sentence be the result of changing the old value of sentence after it was changed to lowercase.
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Note 15.3.1.

We always do all the work on the right side of = and then store the result in the variable listed on the left side.
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Checkpoint 15.3.2.

Given the following code segment, what is the value of the string s1 after these are executed?
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s1 = "xy"
s2 = s1
s1 = s1 + s2 + "z"
  • xyz
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  • s1 will equal "xy" plus another "xy" and then "z" at the end.
  • xyxyz
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  • s1 contains the original value, plus itself, plus "z"
  • xy xy z
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  • No spaces are added during concatenation.
  • xy z
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  • No spaces are added during concatenation, and an additional "xy" should be included at the beginning.
  • s1 was set to "xy" initially, so the final answer will be "xyxyz"
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Checkpoint 15.3.3.

What is the value of s1 after the following code executes?
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s1 = "HEY"
s2 = s1.lower()
s3 = s2.capitalize()
  • Hey
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  • This would be correct if we had asked what the value of s3 was. What is the value of s1?
  • hey
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  • This would be true if we asked what the value of s2 was after the code executes. What is the value of s1?
  • HEY
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  • Strings are immutable, meaning they don’t change. Any function that changes a string returns a new string. So s1 never changes unless you set it to a different string.
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