# 7.11. Exercises: Strings¶

Note

If your teacher added you to a Trinket course, complete the exercises there.

## 7.11.1. Part One: Bracket Notation¶

1. Identify the result for each of the following statements:

1. `'Characters'[8]`

2. `"Strings are sequences of characters."[5]`

3. `len("Wonderful")`

4. `len("Do spaces count?")`

There’s no starter code for this one, just try it on your own with old-fashioned pencil and paper!

2. 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.

3. Use index values to loop backwards through a string:

1. First, print one letter per line.

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`.

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`).

Note

Code part 1, steps 2 and 3 at repl.it or Trinket.

## 7.11.2. Part Two: String Methods and Operations¶

1. 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.

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).

2. 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.

4. Properly done, your output should look something like this:

```bag
beg
big
bog
bug
```
5. Try other words besides `'bag'`!

3. Consider a string that represents a strand of DNA: `dna = " TCG-TAC-gaC-TAC-CGT-CAG-ACT-TAa-CcA-GTC-cAt-AGA-GCT    "`. There are some typos in the string that you need to fix:

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.

3. Note that you need to reassign the changes back to the `dna` variable in order to see them printed. Apply these fixes to your code so that `print(dna)` prints the DNA strand in UPPERCASE with no whitespace.

4. 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!

2. Look for the sequence `'CAT'` with `find()`. If found print, ```'CAT found'```, otherwise print, `'CAT 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.

Note

Code part 2, steps 3 & 4 at repl.it or at Trinket.

## 7.11.3. Part Three: String Formatting¶

1. 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: ___."```

2. Print the string, ```"Here is my word 3 times: ___/___/___, and here is my number squared: ___."```

Code it at repl.it or Trinket

2. For part 2, exercise 4, 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.

3. 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?

 ```1 2 3 4 5``` ```advice = "Don't Panic" output = "The text, '{0}' contains {1} characters." print(output.format("Don't Panic", 11)) ```

Console Output

```The text, 'Don't Panic' contains 11 characters.
```