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Welcome back. In this lesson,

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we're going to learn about some operations that we can use to modify dictionaries.

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So here on line 1,

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we create a dictionary.

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In this dictionary, we have four key-value pairs.

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So the key pears is associated with the integer 217.

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The key and the string apples is associated with 430, and so on.

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So, overall we have four key-value pairs and that's all created on line 1.

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Now, on line 2, we do an operation called del.

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So, del is short for Delete,

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and Delete deletes a key-value pair from our dictionary.

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So, when we say "del" and then we say the name of the dictionary,

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and then in square brackets the key,

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whose key-value pair we want to delete,

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that's going to get rid of that key-value pair from our dictionary.

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So, here we're saying del inventory sub pears.

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So, here our key-value pair pears is associated with the value 217.

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Line 3 says get rid of this key-value pair.

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So, when I actually run line 3,

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you'll notice the number of key-value pairs will go from four to three,

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if instead we created the same dictionary.

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So again, we're back to having four key-value pairs.

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Now, in line 3, rather than deleting the value associated with pears,

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then we're going to set it to zero.

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So, if you remember from when we first introduced dictionaries,

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we can set a key-value pair by saying the name of the dictionary, sub,

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and then whatever key we want to set,

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equals whatever value we want it to be associated with.

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But the difference here is that by the time we run line 3,

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we'll already have a key-value pair that associates

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the key pears with the different value 217.

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So, when we say inventory sub pears now equals zero,

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that's going to update the value associated with the key pairs.

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So, in other words, line 3 is going to say pears is not 217 anymore,

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it's now associated with the value zero.

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So here, if we now run line 3,

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then you'll see the value of pears is now zero.

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So, let's reuse that dictionary in another example.

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So on line 1, we create the same dictionary that

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has four key-value pairs that we saw from the previous examples.

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Now, on line 2,

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we're setting that dictionary,

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so inventory sub bananas to inventory sub bananas plus 200.

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Now, when we look at this,

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this expression is a little confusing because here we're

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repeating this expression inventory sub bananas.

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But remember that when we do in assignment,

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then what Python does is it first

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evaluates the value that we're going to be assigning to.

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So, the first thing Python does on line 2 is it asks what's the value of this expression,

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inventory sub bananas plus 200?

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After it computes this value,

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then it's going to assign whatever that value is to inventory sub bananas.

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So, in other words, to figure out what this is going to do,

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we first have to ask what's the value of this expression?

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So, to figure out the value of this overall expression, let's break it down.

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So, we add inventory sub bananas to 200,

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so we ask what's the value of inventory sub bananas?

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To figure that out, we look up this dictionary and see that

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bananas is associated with 312.

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So this value is 312.

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Then, we add 200 to that to get 512.

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Then, Python takes this integer 512 and it

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makes it the new value associated with the key bananas.

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So, this is going to now be 512 when we run line 2.

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So, let's run line 2, and we see bananas is now 512.

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Now, if you remember the "len" function from strings or lists or tuples,

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remember that len gives you the number of items in a collection.

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So, in the case of strings,

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len gives us the number of characters in that string.

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Len also works with dictionaries.

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So, if we pass in the dictionary inventory,

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then when we call len on it,

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the value of this expression gives us the number of key-value pairs in this dictionary.

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So, the value of this expression is going to be one,

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two, three, four, because there are four key-value pairs.

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So, we'll see you that "numItems" in

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our frame right here is going to be assigned to the value, the integer four.

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So, let's run line 4 and we see that numItems is now four.

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So, let's do some more questions.

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In this question we ask what is printed by the following statements?

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So, here we create a dictionary with three key-value pairs.

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Then, we say my dictionary sub mouse equals my dictionary sub cat,

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plus my dictionary sub dog.

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So, the value of this expression,

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my dictionary sub cat, is 12.

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The value of this expression,

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my dictionary sub dog, is six,

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meaning that the value of this overall expression,

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my dictionary sub cat plus my dictionary sub dog is 12 plus 6 or 18.

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So, then we assign the value 18 to being associated with the key mouse.

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So, by the time we print out my dictionary sub mouse,

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we're going to print out 18.

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So our answer is C. So,

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this question asks us to update the value for Phelps in the dictionary swimmers,

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to include his medals from the Rio Olympics by adding five to the current value.

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So, Phelps will now have 28 total medals,

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and it asks us do not rewrite the dictionary.

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So, here on line 2,

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we assigned swimmers to be a dictionary that has one, two, three,

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four, five, six key-value pairs,

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but we want to update the value associated with the key Phelps.

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In the way that we want to update it is by adding five to the current value.

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So, the way that we're going to do that is we're going to say,

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swimmers sub Phelps is now its previous value.

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So swimmers sub Phelps equals swimmers sub Phelps plus five.

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So, whatever it started out with,

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it's now going to be that, plus five.

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That's all for now. Until next time.