Execution of Function Calls

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From the course by Duke University

Programming Fundamentals

225 ratings

Duke University

Programming Fundamentals

225 ratings

Course 1 of 4 in the 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.

From the lesson

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.

Using Functions for Abstraction4:13

Execution of Function Calls4:37

Meet the Instructors

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

Okay, now that you have the basic ideas of function call semantics,

let us see them in action.

We have three functions here.

My function, f and main.

As you have learned, we start execution of a program inside of main.

We've drawn a frame for main and

are ready to begin executing its first statement which declares the variable a.

So we create its box inside of the frame for main.

Next, we're going to set a equal to the value of the function call,

myFunction of 3, 7.

To evaluate this expression, we need to create a frame for

the function that we are calling, in this case, myFunction.

This will hold the parameters and variables of myFunction.

Next, we pass the parameters to myFunction.

We create a box for

each parameter with the names coming from the functional declaration x and y.

We initialize these by copying in the values of the expressions

that were passed, here, 3 and 7.

Next, we need to note where to return when we finish executing myFunction.

This location in the code is named the call site,

the place where the function was called.

We'll note it with marker 1 in the code, and

put the same marker in the corner of the frame.

Finally, we move the execution arrow into myFunction, and

start executing code there.

Here, we declare and initialize z, evaluating the expression, 2 times x-y.

The values for x and y come from the frame for myFunction, 3 and

7 respectively, so z will be negative 1.

Now, we have reached a return statement.

Return statements tell us to leave the current function

returning to the call site noted in the frame.

They also tell us the value to return to the caller.

The first thing we need to do is evaluate this expression to obtain

the return value.

Here, the expression is z times x, so

we evaluate negative 1 times 3 and get negative 3.

Next, find where we should return.

This is the call site we noted in the frame.

Then we copy the return value back to the call site.

The function call evaluates to this return value.

We move the execution arrow back to the call site, and destroy the frame for

the function we just returned from.

Now, we are back in main.

The call to myFunction evaluated to negative 3, so

this line behaves like a gets negative 3.

We finished that assignment putting negative 3 in a's box.

Our next line again has a variable declaration and a function call.

We make a box for b, and go through the same process to call f.

We make a frame for f, and pass parameters.

This time, there's one parameter in whose value is a times a which is 9.

We note where to return and begin executing code inside of f.

Our next statement is a return statement, but

the expression involves a function call.

So we have to evaluate that call before anything else.

We start with a frame and past parameters.

x gets the value of n which is 9, and y gets the value of n + 1 which is 10.

We note the call site.

We use two this time since we are already using one somewhere else, and

move the execution arrow to the start of myFunction, and

start executing code there.

We declare Z and initialize it to 8.

Now, we are ready to return from myFunction.

We evaluate z times x which is 72.

Then we find that the call site noted in the frame and copy the return value there.

Finally, we move the execution back

to the call site destroying the frame from myFunction.

Now, we pick up were we left off an f using 72 for

the value of the call to myFunction.

We evaluate 3 + 72 to get 75.

Since we are evaluating the return statement, this is the return value of f,

so we find the call site and copy the return value there.

Then we return to that call site destroying the frame for f.

Now, we can finish the initialization of b, b gets 75.

Lastly, we reached the return statement from main.

When we return from main, the program exits so we are done.

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