Programming, Data Structures and Algorithms in Python

So far we have seen some basic Types of values in Python.
You began with the numeric types, which divided into two categories int and float. So, int represented whole numbers or integers, and float represented values which have a decimal point. And for these, we had arithmetic operations such as plus, minus, times, divide and also other functions which we can import using the math library, which is built into Python. Then we introduced a new type of value, which may not be so familiar for logical values true and false which are of type bool.
We can operate on these values using functions such as not, which negates the value makes it the opposite 'and' and 'or'. And when we do comparisons between numeric values, for instance, the outcome of such a comparison is typically a bool value and we can combine these comparisons using 'not' and 'and' to make complex conditions.

In the previous lecture, we looked at strings. So, strings are used to represent text. A string is of type str. It is a sequence of characters. And since it is a sequence we can talk about positions in the sequence. The position starts numbering at 0 and goes up to n minus one where n is the length of the string. If we say s square bracket i for a string value s then we get the ith position using this numbering convention. And a slice gives us a sub sequence a string from position i to position j minus one written s square bracket i colon j. The basic operation we can do with strings is to glue them together using the plus operation. Plus means concatenation for strings and not addition in the arithmetic sense; we can extract the length of a string using the len function and we said that we will look at more complex string functions later on.

Today we move on to lists. A list is also a sequence of values, but a list need not have a uniform type. So, we could have a list called factors, which has numbers 1, 2, 5, 10. We could have a list called names, which are Anand, Charles, and Muqsit. But we could also have a list, which we have called mixed that contains a number, a Boolean, and a string. Now it is not usual to have lists which have different types of values at different positions, but Python certainly allows it. While we will normally have lists which are all integers or all strings or all Boolean values, it could be the case that different parts of a list have different types.

A list is a sequence in the same way as a string is and it has positions 0 to n minus 1 where n is the length of the list. So, we can now extract values at a given position or we can extract slices. In this example, if we take the list factors and we look at the third position remember that the positions are labeled 0, 1, 2, 3, then factors at position 3 is 10. Similarly, if you take the list mixed and we take the slice from 0 to 2 minus 1, then we get the sub list consisting of 2 and 1. As with a string, the length of the list is given by the function len. So, len of names is 3 because there are 1, 2, 3 values in names. Remember that length is just a normal length, whereas the positions are numbered from 0 to n minus 1.

There is one difference between lists and strings and what we have seen so far. We said that there was no concept of a single character. In a string, if we take the value at a single position or we take a string a substring of length 1, we get the same thing. So, if we have the string h, which has position 1, 2, 3, 4, 5 sorry 1, 2, 3, 4 said as length 5. And if we ask for the 0th position then this gives us the letter h. But the letter h in python is indistinguishable from the string h similarly if we ask for the sub sequence from 0 to 1, but not including 1 then again we get the string h. So, in one case it's as though we constructed a substring of length one in one case we got a single character, but Python does not distinguish. So, h at position 0 is actually equal to the substring h[0:1].

Now, nested lists can contain other lists, so this is called nesting. For example, we can have a nested list. This contains a single value at the beginning which is another list. This is position 1. This is position, sorry position 0. This is position 1 and this is position 2. Position 1 is a single simple value, an integer 0, an integer 4. Position 0 is a list, which in turn has itself two positions 0 and 1. And the value position 1 is itself another list. So, it is a third level of nested list which has a single value 37. Similarly, the value at position 2 is itself a string and therefore, this has sequence, and it has its own position.

One fundamental difference between list and string or indeed any of the values we have seen before is that a list can be updated in place. So, if we have a list nested as we had before and we say nested of 1 is 7. Remember when we try to change a position in a string we got an error. We cannot change the second l in hello to p just by saying that we want position three to be replaced by p, but for a list this is allowed. If we want to replace 4 with 7, we can just say nested one equal to seven and this will give us the list [2, 37, 7, 'hello'], we can do this inside as well. We can say that we want to go into this list which is nested 0 then we want to go into this list which is nested 0, 1 then we want to go into this value and change this value. We want to change the value at the position 0 of the nested list at position 1 of this initial value.

It is important to understand the distinction between mutable and immutable values, because this plays an important role in assignment. Let us look at what happens when we assign names. Suppose we go through the following sequence of assignments. We initially assign the value 5 to the name x, then we assign the name the value and the name x to the value y, and then we reassign x to seven. We started with x being 5; then we said y is also 5, because y is a value of x. And now we changed x to 7. So, the question we would like to ask is has the value of y changed.