Lab 4

Sequences

Last week’s lecture contained material on sequences, in particular on the following:

Indexing: Recall that you can access individual elements in a list by providing their index: if L = ["cat", "dog", "potato"], then L[0] == "cat", L[1] == "dog" and L[2] == "potato"]. You can also use negative indices to start from the back: L[-1] == "potato", L[-2] == "dog" and L[-3] == "cat".
Slicing: Additionally, you can use slicing to access a range of elements from a list by providing multiple indices: L[0:2] will give a list of the elements of L, from indices 0 to (but not including!) 2. For example, L[0:2] == ["cat", "dog"] and L[2:3] == ["potato"]. You can also use negatives indices here: L[0:-1] == ["cat", "dog"]. If you don’t specify a starting index, it will default to the first one; similarly, if you don’t specify a final index, it will default to the last one: L[:2] == ["cat", "dog"], L[1:] == ["dog", "potato"] and L[:] == ["cat", "dog", "potato"]. Slicing also works on strings: "potato"[:3] == "pot".
You can use len to get the length of a list or string, e.g., len(L) == 3.
You can concatenate sequences with + and repeat them a number of times with *: ["cat", "dog"] + ["potato"] = ["cat", "dog", "potato"] and ["cat", "dog"] * 3 = ["cat", "dog", "cat", "dog", "cat", "dog"].

Do the following exercises on CodingBat:

Last lecture covered loops, in particular the basics of while- and for-loops. Both types allow you to repeat something multiple times:

While-loops repeat something as long a given condition is true. For example, the following loop will keep doubling a given number until it is at least 1000. Note that, if the number is 0, this will never stop.
```
while number < 1000:
  number *= 2
```
For-loops repeat something a set number of times, once for every element in a given sequence. For example, the following loop will print the squares of the integers 0 to 9:
```
for i in range(10):
  print(i * i)
```
The following loop will print the first two characters of every string in a given list:
```
for word in ["cat", "dog", "potato"]:
  print(word[0:2])
```

Do the following exercises on CodingBat:

As always, if you have any problems or additional questions, make sure to post them on Brightspace.