Exercise Solutions: Strings

Remember, the solutions below represent ONE way to solve the exercise. If your output matches what is being asked for, you have correctly solved the exercise.

Part One: Bracket Notation

Bracket Notation Basics

Use bracket notation to:

  1. Print a slice of the first 12 characters from "Strings_are_sequences_of_characters."

  2. Print a slice of the last 12 characters from the same string. You should NOT have to count the index values yourself!

  3. Print a slice of the middle 12 characters from the same string.


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Looping Through a String

Use index values to loop backwards through a string:

  1. First, print one letter per line.

    max_index = len(word)-1
    for index in range(max_index, -1, -1):
  2. Next, instead of one letter per line, use the accumulator pattern to build up and print the reversed string. For example, if given the string 'good', your program prints doog.

    new_word = ""
    for index in range(max_index, -1, -1):
       new_word += word[index]
  3. Finally, use concatenation to print the combination of the original and reversed string. For example, given the string 'tomato', your program prints tomatootamot. (If you want to be fancy, include the | character to make the output look almost like a mirrored image: tomato | otamot).

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Part Two: String Methods and Operations

String Methods and Data Types

The len() function returns the number of characters in a string. However, the function will NOT give us the length of an integer. If num = 1001, then len(num) throws an error instead of returning 4.

  1. Use str() to change num from an int to a string data type.

  2. Print the length (number of digits) in num.

  3. Modify your code to print the number of digits in a float value (e.g. num = 123.45 has 5 digits but a length of 6). The digit count should NOT include the decimal point.

    num = 123.45
    new_num = str(num).replace(".", "")
  4. What if num could be EITHER an integer or a decimal? Add an if/else statement so your code can handle both cases. (Hint: Consider using the find() method or the in operator to check if num contains a decimal point).

    if '.' in str(num):
       print(len(str(num)) - 1)

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Loops, Conditionals, and Strings! Oh my!

Given word = 'bag':

  1. Set up a loop to iterate through the string of lowercase vowels, 'aeiou'.

  2. Inside the loop, create a new string from word, but with a different vowel. Use the replace() string method.

  3. Print the new string.

    word = "bag"
    vowels = "aeiou"
    for vowel in vowels:
       new_word = word.replace("a", vowel)

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Method Chaining Fun with DNA

  1. Use the strip() method to remove the leading and trailing whitespace, and then print the result.

  2. Change all of the letters in the DNA string to UPPERCASE and print the result.

    dna = dna.strip().upper()
  3. Let’s use string methods to do more work on the same DNA strand:

    1. Use replace() to remove the sequence 'GCT', and then print the altered strand. Don’t forget about the extra hyphen!

      dna = dna.replace("-GCT","")
    2. Look for the sequence 'CAT' with find(). If found print, 'CAT found', otherwise print, 'CAT NOT found'.

      if dna.find("CAT") > -1:
         print("CAT gene found")
         print("Cat gene NOT found")
    3. Use count() to find the number of hyphens (-) in the string, then print the number of genes (in this case, a gene is a set of 3 letters) in the DNA strand. Note that the number of genes will be 1 more than the number of hyphens.

      hyphen_count = dna.count("-")
      gene_count = hyphen_count+1
      print(hyphen_count, gene_count)
    4. Finally, use an f-string to print the output "The DNA string is ___ characters long and contains ___ genes." Fill in the blanks with the length of the string and the number of genes.

      print("The DNA string is {0} characters long and contains {1} genes.".format(len(dna), gene_count))

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Part Three: String Formatting

Template Literals

Assign your favorite, school-appropriate number and word to two variables.

  1. Use format() and index values to print the string, "Here is my number: ___, and here is my word: ___, and here is my number again: ___."

    output = "Here is my number: {0}, and here is my word: {1}, and here is my number again: {0}."
    print(output.format(my_num, my_word))
  2. Print the string, "Here is my word 3 times: ___/___/___, and here is my number squared: ___."

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Debugging Practice

No starter code for this one.

The following code sample works, but it can be improved.

  1. Assuming that advice remains a string, when will the code produce the wrong output?
  2. Why will the code do this?
  3. What should the programmer do to fix the code?
#One option is to avoid hard coding the print line and use variables.

advice = "Don't Panic"

output = "The text, '{0}' contains {1} characters."

print(output.format(advice, len(advice)))

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