Structuring if Statements

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Del curso dictado por University of Toronto

Learn to Program: The Fundamentals

2734 calificaciones

University of Toronto

Learn to Program: The Fundamentals

2734 calificaciones

Behind every mouse click and touch-screen tap, there is a computer program that makes things happen. This course introduces the fundamental building blocks of programming and teaches you how to write fun and useful programs using the Python language.

De la lección

Booleans, Import, Namespaces, and if Statements

This module introduces Booleans (logical values True and False), how to convert between types, how to use Boolean expressions in if statements to selectively run code, and the concept of a Python module.

If Statements9:11

No if Required4:18

Structuring if Statements6:08

Conoce a los instructores

Jennifer Campbell
Associate Professor, Teaching Stream
Department of Computer Science
Paul Gries
Associate Professor, Teaching Stream
Department of Computer Science

When working with 'if statements' we are going to have some decisions to make about

how to structure our code. In this lecture, we will examine two

problems, and explore alternative solutions involving 'if' for each of the

problems. This example problem involves two grades;

grade one, and grade two. We'll give grade one the value of seventy,

and grade two gets eighty Now, there's an if' lif statement' on lines four through

seven. There's a Boolean expression associated

with the if, and another Boolean expression associated with the Lif.

These expressions are evaluated from top to bottom.

And when one of the expressions is true, the body associated with it gets executed.

And the if statement exits at that point, without continuing on to evaluate the

subsequent expressions. Because of that, at most one of the two

print statements will get executed in this case.

So, let's look at this code. If the grape one is greater than 50, and

it is, we're enter into the body of the if.

And at this point, becaue one of the expressions associated with the if was

true, the if statement will exit after executing this body.

So we jump down to line nine. In line nine there's another if statement

and on line eleven there is a third if statement.

The condition associated with the if on line nine is the same as the one on line

four and the condition associated with the if on line eleven is the same as that

associated with the Lif on line six. However because this code from lines nine

through twelve is two separate if statements, as opposed to the if Lif that

exists on lines four through seven. It behaves differently.

We first evaluate the expression associated with if online nine, and it's

true. So, we enter into its body and print the

message. And, at this point we will exit that if

statement, moving onto the subsequent code.

And, that code is line eleven, where there is a separate if statement.

This Boolean expression is also true, so in this case its body is executed as well.

So the if Lif is not equivalent to two if statements.

And we need to take that into account when we're running our code, and take care to

use the appropriate structure in each situation.

Let's consider another problem. This problem involves predicting whether

we should bring an umbrella or not. So we'll take into consideration whether

precipitation is expected using a true or false value.

And what the temperature is for today. The temperature, in this case is +eight

degrees Celsius. This if statement for it checks the

condition to see if precipitation is expected and that is true.

So we enter into the body of the if and in the body there is actually another if

statement. One if statement is nested inside the

other. We can then check that boolean condition

and find that it is true, so we print the message bring your umbrella.

I want to modify this code so that we write a second version without the

nesting. Notice that when precipitation is true,

the condition associated with the first if, and the condition associated with the

second if is also true. That's when we print bring your umbrella.

So they both need to be true. We'll use the Boolean operator "and" to

state that, and that was we can structure this as a single If statement.

Let's visualize the execution. In the first case, the variables have

values that, causes to print Bring Your Umbrella.

And, in the second case, we evaluate this Boolean condition, precipitation refers to

the value TRUE, temperature refers to the value +eight, so this expression is also

true, and the message Bring Your Umbrella is displayed.

Now, let us expand on this example. I'm going to add an Else to the if

statement on line five. This else describes the statement to print

when temperature is not greater than zero. I'm going to now edit the code on line ten

to make that if statement equivalent to the one on line four.

Let's look at the conditions under which the Else code executes.

The precipitation condition needs to be true, the temperature greater than zero

condition must be false and then the Else is executed.

So we can express this using an L if. We want to make sure that the first

condition is true. And we don't need to check the second

condition. Because we'll only reach the L if when

that is false. In that case, we'll print the same message

as above. Let's visualize the execution now.

In fact there's one more thing to do. Let's make this a negative three so that

it's below freezing and we print those messages.

So if precipitation is true, we enter into the body of the if.

Checking this condition, because I just changed what temperature refers to, it's

false. And we print the statement from the Else.

In the second If statement, we check the first condition, which is false because

the temperature is not greater than zero. Now we must check the L if clause and this

condition is true, so we enter into the body of that L if and we print the same

message. So these two pieces of code are

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