C Programming (Important Questions Set 1)

Summary

This video script discusses various C programming concepts, including the behavior of printf functions, character data types, and integer arithmetic. It explains how printf can print strings and return the number of characters printed, the impact of using %10s for formatting strings, and the arithmetic modulo operation with character overflow. Additionally, it covers the declaration of integers with modifiers like signed and unsigned, and how the internal representation of integers in computers affects the output of printf statements, emphasizing the importance of careful code analysis for exams like GATE.

Takeaways

• 💻 The script discusses a C program fragment involving nested printf functions and explains their output.
• 🔢 The outer printf function prints the number of characters printed by the inner printf function, which prints 'Hello World!'.
• 📏 The script explains the use of format specifiers like %d for integers and %s for strings in printf functions.
• 📑 It highlights the importance of including the null character when counting the total number of characters in a string.
• 📐 The script demonstrates how printf can be used with padding (%10s) to control the width of the output.
• 🔄 The script explores the concept of data types and arithmetic operations, particularly with character variables.
• 🔒 It explains how character variables can only hold values up to 255 and uses modulo arithmetic to show how values exceeding this are handled.
• 📝 The script discusses the declaration of integer variables in C, including signed and unsigned integers.
• 🔄 It clarifies that omitting the data type for a variable defaults to int, which is implicitly assumed by the compiler.
• 🔢 The script explains the internal representation of negative numbers in computers using 2's complement form.
• 💡 It emphasizes the importance of carefully reading each line of code, especially for understanding questions in competitive exams like GATE.

Q & A

Question 1: What does the first printf function in the program do?

The first printf function prints 'Hello World!' on the screen, followed by the number of characters in the string, which is 12. Hence, the output is 'Hello World!12'.

Question 2: Why does the second printf function use '%10s' instead of '%s'?

The '%10s' format specifier ensures that the string is printed within 10 characters wide. Since 'Hello' has 5 characters, 5 white spaces are printed first, followed by 'Hello'. This is used to align the text in a wider field.

Question 3: How does the character variable behave when assigned a value exceeding its range?

When a character variable is assigned a value exceeding its range (which is 255 for 8 bits), it follows arithmetic modulo 256. For example, assigning 265 results in 265 mod 256, which equals 9. Thus, the value 9 is printed.

Question 4: How does a signed integer differ from an unsigned integer in the context of printf?

A signed integer allows negative values, while an unsigned integer only handles positive values. When using '%u', the printf function interprets the value as unsigned, potentially printing a large number instead of a negative value, as it interprets the binary form differently.

Question 5: Why does the program print '4294967293' when using '%u'?

The value -3 is represented in two's complement form, and when printed as an unsigned integer using '%u', the value is interpreted as 4294967293 due to how unsigned integers handle binary representation on a 4-byte machine.

Question 6: What is the significance of arithmetic modulo 2 raised to the power n in the context of character data types?

For a character variable, which is 8 bits long, the maximum value it can hold is 255 (2^8 - 1). If a value exceeds this range, it wraps around according to modulo arithmetic (value mod 256).

Question 7: Why is 'signed i;' considered a correct definition?

Even though 'int' is not explicitly mentioned, the compiler implicitly assumes the 'int' data type. Therefore, 'signed i;' is equivalent to 'signed int i;', making it a valid declaration.

Question 8: What is the internal representation of negative integers like -3 in computers?

Negative integers are represented using two's complement. To get the two's complement of a number, first find the one's complement (invert the bits) and then add 1 to the result. This allows negative numbers to be handled in binary form.

Question 9: What happens if you mix signed and unsigned integers in arithmetic operations?

When you mix signed and unsigned integers, the result may be unexpected because the unsigned integer takes precedence in most cases, leading to a potential misinterpretation of the result as a large positive number instead of a negative one.

Question 10: Why is it important to carefully read format specifiers like '%d' and '%u' in C programming?

The format specifier controls how values are printed. '%d' prints signed integers, while '%u' prints unsigned integers. Misusing these specifiers can lead to incorrect output, such as interpreting a negative value as a large positive number, which can lead to confusion.

Outlines

00:00

💻 C Program Output Analysis

The paragraph discusses the behavior of C program fragments involving the printf function. It starts with an introduction to the lesson, emphasizing that the content is a review of previous topics. The instructor invites viewers to pause the video and try to understand the code written on the screen before the explanation. The code in question involves a printf function within another printf function, which is unusual. The inner printf uses the format specifier %s to print the string 'Hello World!'. The instructor explains that printf returns the number of characters printed, including the space, totaling 12 characters. The expected output is 'Hello World!12'. The next part of the lesson involves another printf function with the format specifier %10s, which prints the string 'Hello' followed by spaces to fill a field of 10 characters wide. The final part of the paragraph discusses what happens when a character variable is assigned a value beyond its capacity, such as 255, and then incremented. It explains that character variables can hold values up to 255 due to their 8-bit size, and exceeding this results in a modulo operation with 2^8 (256), leading to a remainder of 9, which is what gets printed.

05:02

🔢 Understanding Data Types in C

This section delves into the correct definitions of integers in C programming. It starts with a multiple-choice question about which statements correctly define an integer. The instructor explains that the first statement, defining a signed integer, is correct. It clarifies that omitting the 'int' keyword and just using 'signed' is also valid due to compiler assumptions. The paragraph also covers the definitions of 'unsigned', 'long', and 'long long' integers, confirming that all the given options are correct. The discussion then shifts to a program that prints the result of adding an unsigned integer to a negative integer. The instructor explains that the output is not -3 as one might expect, but rather depends on the machine due to the use of the %u format specifier, which interprets the result as an unsigned integer. The maximum value for an unsigned integer is 4294967295 for a 4-byte integer, and the actual output is one less than this maximum value due to the initial value of -4 being added to 1.

10:03

📚 Recap and Importance of Careful Reading

The final paragraph serves as a recap of the lesson and emphasizes the importance of careful reading and understanding of each line of code, especially for exams like GATE. It highlights that such questions test the ability to comprehend every detail within a code snippet. The instructor reminds viewers to pay close attention to every line and concludes the lecture with a farewell, indicating the next lecture will continue the discussion.

Keywords

💡printf function

The printf function is a standard library function in C programming used to send formatted output to the standard output stream. It is a versatile function that can output various data types, including integers, characters, and strings. In the video, the instructor uses printf to demonstrate how to print strings and integers, as well as to return the number of characters printed. For example, the script shows a printf function calling another printf within its arguments, which is used to print 'Hello World!' followed by the number of characters printed.

💡format specifier

A format specifier in C programming is a code that tells the printf function the expected data type of the argument to be printed. The script mentions '%d' for integers and '%s' for strings. The instructor explains that '%d' is used to print an integer, while '%s' is for printing a string of characters, such as 'Hello World!'.

💡character

In the context of the video, a character refers to a data type in C that can hold a single letter or digit. The instructor discusses how a character variable can be assigned an integer value and how it behaves when the value exceeds the maximum limit for a character (255). The example of incrementing a character variable from 255 to 265 and printing it demonstrates the concept of overflow and modulo arithmetic.

💡arithmetic modulo

Arithmetic modulo is a mathematical operation that finds the remainder after division of one number by another. In the video, the instructor explains how character data types, being 8 bits long, use modulo 2^8 arithmetic to handle values that exceed their storage capacity. The example of assigning 265 to a character variable and printing it results in 9, demonstrating modulo arithmetic in practice.

💡integer

An integer is a whole number that can be positive, negative, or zero. In C programming, integers are a fundamental data type used to store whole numbers. The script discusses various ways to declare integer variables, such as 'int', 'signed int', and 'unsigned int', and the implications of using unsigned integers when performing arithmetic operations.

💡unsigned integer

An unsigned integer is a data type that can only represent non-negative integers. The video explains that when an arithmetic operation results in a negative value with unsigned integers, the result wraps around to a large positive value. This is demonstrated when adding -4 to an unsigned integer 1, which results in a large number depending on the machine's integer size.

💡2's complement

2's complement is a method for representing signed integers in binary form used in computer systems. The video script explains how -3 is represented in 2's complement, involving taking the 1's complement of the binary representation of 3 and then adding 1. This concept is crucial for understanding how negative numbers are stored and manipulated in computers.

💡ASCII table

The ASCII (American Standard Code for Information Interchange) table is a character encoding standard for electronic communication. The video mentions the ASCII table in the context of character variables and how they can represent characters according to their ASCII values. This is relevant when discussing the output of printing character variables.

💡GATE examination

The GATE (Graduate Aptitude Test in Engineering) examination is a standardized test in India for postgraduate admissions in engineering and the sciences. The script mentions that understanding each line of code is critical for GATE exams, where questions often test the candidate's ability to comprehend and predict the behavior of code snippets.

💡data type

A data type in programming defines the type of value a variable can hold. The video script covers various data types in C, such as int, signed int, unsigned int, long, and long long int. The instructor explains that the compiler implicitly assumes certain data types if not explicitly stated, which is important for understanding variable declarations and their defaults.

Highlights

The lesson reviews previous topics and does not introduce new material.

Students are encouraged to pause the video and try to understand the written C program fragment.

A printf function within another printf function is explained, which is unusual.

The inner printf function returns the number of characters printed.

The number of characters in a string includes spaces and should be counted accurately.

The output of the nested printf functions is expected to be 'Hello World !12'.

The concept of format specifiers in printf is explained with %10s.

The %10s format specifier prints a string with leading spaces to fill up to 10 characters wide.

The output of the modified printf with %10s is 'Hello' followed by spaces.

A character variable can hold integer values, and exceeding the range results in modulo arithmetic.

The value 265 assigned to a character variable results in a modulo operation with 256.

The result of 265 mod 256 is 9, which is printed on the screen.

The definition of integers is discussed, including signed and unsigned modifiers.

Compiler assumptions about data types for variables like 'signed i;' are explained.

All the given statements about integer definitions are correct.

The output of a program with unsigned and signed integers is machine-dependent.

The internal representation of -3 in computer using 2's complement form is explained.

The difference between printing with %d and %u in printf changes the output significantly.

Careful reading of each line is emphasized for understanding and answering questions correctly.

The importance of such questions in GATE examinations is highlighted.

Transcripts

Today we are going to consider some

questions on the topics

that we had already studied

in our previous lessons.

These questions are related to

the previous topics that we had

already studied.

Therefore there is nothing new,

if something is new, I will talk about that

in this lesson.

1. What is the output of the following

C program fragment:

Before I will explain the program,

I would recommend to you to please

pause the video for a while,

and try to understand whatever

I have written over here.

Then come back again

and see what I explained

in this particular session.

OK,

Here you see,

I have written a printf function

and inside this printf function

I have written two arguments.

One is %d,

as we all know, this is a format specifier

used to print an integer

and another argument is

printf function.

Now this is something weird.

In this argument we usually write

a variable or some integer value

but here, instead of that

I have written printf function.

Now what does this printf function do

Inside this printf function,

I have provided the argument as

%s.

Now this is something new to you.

%s means, we need to print

string of characters on to the screen.

%s is used to print

"string of characters."

Here I have provided,

a string of characters as

"Hello World !"

If you want to print this "Hello World !"

on the screen,

then we have to use %s.

OK, what this printf function will do

is that it prints "Hello World !" on screen.

This is OK.

But what does this printf function do?

What does it print?

printf not only prints

the content on the screen.

It also returns the

number of characters that it

successfully prints on the screen.

Now this is quite important for us to know

this printf function will not

only print "Hello World !" on the screen,

but it also returns the value

which is equivalent to

the number of characters

it successfully prints on the screen.

Here you can see, the number of characters

in this string is

1 2 3 4 5 6

You have to include that blank character as well.

Don't forget to include the blank character.

OK,

up till now there are total 6 characters.

After this, 7 8 9 10 11 and then 12.

There are total 12 character that we

need to print on the screen.

Right?

So this printf function will also

return 12 to this

particular printf function

as second argument.

So this printf function will print

12 and this printf function prints

"Hello World !" on the screen.

Therefore the output is

"Hello World !12"

That is what is expected.

Let's consider the next question.

What is the output of the following

C program fragment:

Kindly pause the video for a while

and try to understand

what this printf is going to print

on the screen.

OK here,

I have provided the first argument

as %10s instead of %s.

And second argument is

"Hello"

Now what would be the output

provided by this printf function

Let's try to understand

by executing it on Code blocks.

Here I changed the code a little bit

to actually differentiate between

these two statements.

Let's execute the code.

First printf prints Hello

simply on the screen.

while

the second printf prints Hello

but after some white spaces.

This is because of this 10.

This 10 means print the characters

up to 10 characters wide.

This means,

here in our example

we just have 5 characters to print.

Now because of the shortage of characters,

it will print first white spaces

and then the rest of the 5 characters.

5 white spaces printed

and after that 5 characters

gets printed.

That is the reason why

%10s is used.

If you want to print

string up to 10 characters wide,

then we use %10s.

If we want 15 characters wide,

then we write 15 instead of 10.

That is the difference between

this one and this one.

Let's consider the next question.

What is the output of the

following C program fragment:

Here is this program,

let's try to understand what does

it really do.

Here I have provided a character

variable c and assigned it a value

255. You can assign integer values

to a character variable.

As I already told you,

in the lesson when I taught you

about the character variable.

Here you can see, I have provided an integer

value to this character variable

and after that

I increment that value to 10

and store it again in this variable.

Here after the increment

this will be 265.

and I simply print it on to the screen.

Can you guess what would be the output

of this printf statement?

Is it 265

Is it some character according to ASCII table

Is it 7

or is it 9.

As we know this is 255,

character is capable of holding

the information up to 1 byte only

which is equal to 8 bits.

8 bits means the maximum value

that it can hold is 255.

And cannot be more than that.

If we try to exceed the range,

like in this case

we are exceeding the range

because we are assigning to this c variable

265.

If we try to exceed the range,

then what would happen is that,

as we know this thing

that all the data types like

character, integer, float,

long, double

they all follow

or you can say, they obey the laws of

arithmetic modulo 2 raised to the power n

where n is equal to

8 in this case.

As character data type is of 8 bits long,

therefore n is equal to 8.

Here in this case, the value is 265

therefore we will calculate

265 mod 2 raised to the power 8.

2 raised to power 8 is equal to 256.

Therefore it is 265 mod 256

which is equal to 9.

When we divide 265 by 256

we get the remainder as 9,

that is why, 9 is printed on the screen.

And therefore the answer is option d.

Let's consider this question.

Which of the following statement/ statements

is/ are correct corresponding to the

definition of the integer:

As we know how to define or declare

an integer.

Here are some statements written.

We have to determine that

which of the following statements or statement

is correct corresponding to the

definition of integer.

Is it only I and V ?

Is it just I ?

Are all correct?

Or just IV V and VI are correct?

Take you time and try to interpret this

one by one.

OK. Let's understand them one by one.

The first interpretation is correct.

This is the actual definition of

a signed integer.

You can also write int i;

But instead of int i

you can also write signed int i;.

Both are one and the same thing.

Here writing signed int i;

int is a data type

and signed is our modifier

for integer.

Therefore this is a correct definition.

Apart from that,

in this second statement,

I have written signed i;

instead of signed int i;

Now this is something weird.

I have not written anything

like int data type

or some character data type

or any other data type here in this case.

Therefore it seems like a incorrect

definition but I would like to tell you

that this is not an incorrect definition.

It is correct.

Because compiler implicitly assume

integer data type.

If you have not written any data type

over here, then compiler

will assume this data type

is integer over here.

Therefore signed i;

is equivalent to

signed int i;

Similarly unsigned i;

is also correct.

Because compiler implicitly assumes

as integer therefore it automatically puts

int in front of this variable.

And unsigned i; is also a

correct definition.

On the other hand long i;

is also a correct definition,

that is equivalent to long int i;

and long long i; is

equivalent to long long int i;

Therefore we can say,

option c is the correct answer.

Because all of them are

actually correct. Therefore C is the correct answer.

Now let us see what this program prints.

Does it print garbage value?

Does it print -3?

Or does it print integer value that

depends from machine to machine?

Or is it none of the above?

Ok,

in the first statement itself,

I have written unsigned i equal to 1;

that is equivalent to unsigned int i;

Automatically compiler puts

int in front of this variable.

Therefore there is no problem.

unsigned int i is equal to 1

means that this integer variable

is going to have a value equal to 1.

in the next statement,

I have declared a variable j

and assigned it a value -4.

Inside this printf function

I have written i+j.

This is an arithmetic expression,

and the result is getting printed.

using this printf function.

i+j is equal to 1-4

which is equal to -3,

and that is getting printed.

You might think the answer is -3

but it is not.

Let's try to understand what is the reason.

But before that,

we must have to know

what would be the internal representation

of -3 in computer.

Internally -3 is represented

in 2's complement form.

Here I have written certain steps,

to know how we can represent

-3 internally in computer.

You first have to take 1's complement of 3.

That is,

This is the binary representation of 3

as this value is set to 1

and this is also set to 1

which is (2 raised to power 1) + (2 raised to power 0)

that is equal to 3.

And then we perform 1's complement of 3.

which is nothing but changing the

0's to 1's and 1's to 0's.

Here in this case I change

all the 0's to 1's and 1's to 0's.

And this gives me the answer

which is 1's complement of 3.

Then in order to produce the

2's complement of 3,

we need to add 1 to the

result of this 1's complement,

which gives us this answer.

Now, this would be the binary

representation of -3.

You can say internally that is

represented in our modern computers.

OK, now if I have written

%d instead of %u

this will print the answer -3.

Now because I have written %u

this will be interpreted as

an unsigned integer.

And we know this thing that

unsigned integers' maximum value

is 4294967295

in the case when integer is of

4 bytes long.

We know this thing.

Here you can say not all the bits

are equal to 1.

This bit is not equal to 1.

If this is 1, then this would be

4294967295.

Because this is not set

and its place value is

2 raised to the power 1 which is equal to 2.

Therefore we need to subtract,

2 from the maximum value

4294967295.

This gives us the answer as

4294967293.

This is on my computer.

Basically in my computer,

because the size of integer is

of 4 bytes,

Therefore the maximum value is

4294967295 and subtracting 2 from it

will give me the answer 4294967293.

May be this is possible in your computer

size of integer is of 2 bytes,

then maximum value would be different

and subtracting 2 from it would give

something else.

Note down this thing.

Because of %u,

the output printed is,

integer value that depends from machine to machine.

And therefore the answer is c.

But if it is %d,

the output would be -3.

And that is the major difference between these things

From this example we have

to know this thing,

and we have to understand this thing,

that we need to actually read

each and every line very carefully.

when we are answering the questions like this

In GATE examination,

such things are very critical

and very frequent.

They are checking your ability

of understanding each and every line.

Therefore such kinds of questions

are frequent in GATE examinations.

OK friends, this is it for now.

See you in the next lecture. Bye.