Execution of If/Else

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来自 Duke University 的课程

Programming Fundamentals

229 个评分

Duke University

Programming Fundamentals

229 个评分

课程 1（共 4 门，Specialization Introduction to Programming in C）

Programming is an increasingly important skill, whether you aspire to a career in software development, or in other fields. This course is the first in the specialization Introduction to Programming in C, but its lessons extend to any language you might want to learn. This is because programming is fundamentally about figuring out how to solve a class of problems and writing the algorithm, a clear set of steps to solve any problem in its class. This course will introduce you to a powerful problem-solving process—the Seven Steps—which you can use to solve any programming problem. In this course, you will learn how to develop an algorithm, then progress to reading code and understanding how programming concepts relate to algorithms.

从本节课中

Reading Code

In this module, you will learn to read code—this means you will be able to execute a piece of code by hand, and clearly illustrate what each statement does and what the state of the program is. Understanding how to read code is the only way to be sure you can write correct code. By the end of this module, you will be able to read and understand code with functions, conditional statements, iteration, and other fundamental techniques.

Execution of If/Else3:17

Execution of Switch/Case3:05

与讲师见面

Andrew D. Hilton
Assistant Professor of the Practice
Electrical and Computer Engineering
Genevieve M. Lipp
Adjunct Professor
Electrical and Computer Engineering/Mechanical Engineering
Anne Bracy
Senior Lecturer
Computer Science, Cornell University

Now, let's see how to execute code with conditional statements in it.

Here, we have a function f which has some if and if else statements.

As always, our program starts its execution at the beginning of main.

The first statement declares a variable a,

so we make a box for it.

Even though this statement initializes a,

it's going to take us several steps to compute that value,

so we're going to leave a's value as a question mark

until we compute the value of f of 3, 4.

To evaluate the call to f,

we make a frame passing the values of the parameters.

We know where to return to and move the execution arrow to the first line of f.

The next line of code is an if statement whose conditional expression is x less than y.

We evaluate that expression and find that three less than four is true,

so we move the execution arrow into

the then clause of this if statement and continue executing.

The next statement is a call to print f,

which you are familiar with.

We write x less than y in our output.

Next, we return y plus x.

This follows the same rules you have already learned for return statements.

We evaluate the expression to get the return value which is seven.

This is what the function call will evaluate to.

And now, we return back to the caller,

destroying the frame for f. Now,

we are ready to finish initializing a since we know that

the call of f evaluated to seven.

So a is now seven.

We are now ready for the next line of code in main.

We make a box for b and call f,

passing in seven and five.

We once again want to evaluate the condition,

x less than y.

However, now, x is seven and y is five,

so x less than five is false.

We find the close curly brace for

this if statement and see that the if statement has an else clause.

So we move the execution arrow into the start of

the else clause and continue executing from there.

First, we execute the print statement,

and then we reach another if statement.

This if statement is nested inside of the else,

but that does not affect the rules of how we evaluate it.

We see that the conditional expression is false,

so we look for the close curly brace of the if statement.

There is no else clause,

so we move the execution arrow immediately past

the close curly brace and continue execution.

There are no more statements inside the else clause,

so we move the execution arrow outside of the else clause and keep going.

Now, we have a return statement so we evaluate the expression, x minus two,

to find that the return value is five,

which will get returned to the place that we have noted.

We return back to the main and destroy the frame for f,

then we finish the assignment statement.

Now, we are at the return statement from main,

we execute that, and exit the program.

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