Bitwise Manipulation and Assignment Operator, Programming Constructs

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CS201 Introduction to Programming

Lecture Handout

Introduction to Programming

Lecture No. 22

Reading Material

Deitel & Deitel - C++ How to Program

Review previous lectures

Summary

Bitwise Manipulation and Assignment Operator

Design Recipes

Variables

Data Types

Operators

- Arithmetic operators

- Logical operators

- Bitwise operators

Programming Constructs

Decisions

- if statement

- Nested if statement

Loops

- while loop

- do-while loop

- for loop

switch, break and continue Statements

Functions

- Function Calling

- Top-Down Methodology

Arrays

Pointers

File I/O

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Bitwise Manipulation and Assignment Operator

Last time we discussed bitwise operators, we will continue with the elaboration of bitwise

manipulation and assignment operator.

C/C++ are well constructed languages, at start we used to write:

a = a + 1;

This is used to increment the variable. Then we came to know of doing it in a different

manner:

a += 1;

This is addition and assignment operation using single operator +=.

The same thing applies to bitwise operators; we have compound assignment operators for

& (bitwise AND), | (bitwise OR) and ^ (bitwise exclusive OR). It is written in the same

way as for the above mentioned arithmetic operators . Suppose we want to write:

a = a & b;

It can be written as:

a &= b;

Similarly for | and ^ operations we can write the statement in the following fashion.

a |= b;

and

a ^= b;

Remember, the ~ (NOT) operator is unary as it requires only one operand. Not of a

variable a is written as: ~a. There is no compound assignment operator available for it.

Now we will recap topics covered in the previous lectures one by one.

Design Recipe

Our problems, typically, are of real world nature, e.g., Payroll of a company. These

problems are expressed in words. As a programmer we use those words to understand the

problem and to come up with its possible solution.

To begin with the comprehension and resolution process, we analyze the problem and

express the problem in words in reduced and brief manner. Once we have reduced it into

its essence, we put some examples to formulate it. For example, if the problem is to

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calculate the annual net salary of employees, we can take an example for a particular

employee X. Later we will refine the problem, write program and test it. Finally we

review it if it has met objectives. We have discussed all these steps in Design Recipe.

There was a main heading in the topic, "Pay attention to the detail". Never forget this as

our computers are very dump machines. They perform exactly whatever we tell them.

It is important to keep in mind that we are using C/C++ as a vehicle to understand

programming concepts.

Variables

The computer memory can be thought of as pigeon holes each with an address. To store

numbers or characters in computer memory, we need a mechanism to manipulate it and

data types are required for different types of data. Instead of using hard coded memory

addresses with data types, symbolic names are used. These symbolic names are called

variables because they can contain different values at different times. For example,

int i;

double interest;

i and interest are symbolic names or variables with types of int and double respectively.

Data Types

int type is used to store whole numbers. There are some varieties of data types to store

whole numbers e.g., short and long. unsigned qualifier is used for non-negative

numbers. To represent real numbers we use float data type. For bigger-sized real

numbers double data type is used. char data type is used to store one character.

Generally, the size of the int type on our machines is 4 bytes and char is 1 byte. chars

are enclosed in single quotation mark. ASCII table contains the numeric values for chars.

We further discussed a bit later stage about the aggregations or collections of basic data

types (int, float and char etc) called arrays. Arrays are used to aggregate variables of

same data type.

Operators

We discussed three types of operators:

- Arithmetic Operators

- Logical Operators

- Bitwise Operators

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Arithmetic Operators

+ operator is used to add two numbers, - is used to subtract one number from the other, *

is used to multiply two numbers, / is used to divide numbers. We also have a modulus

operator % , used to get the remainder. For example, in the following statement:

c = 7 % 2;

7 will be divided by 2 and the remainder 1 will be the stored in the variable c. We also

used this operator in our programs where we wanted to determine evenness or oddness of

a number. There are also compound arithmetic operators +=, -=, *=, /= and also %= for

our short hand. It is pertinent to note that there is no space between these compound

operators.

Logical Operators

The result for logical operators is always true or false. && (AND operator) and || (OR

operator). Logical Comparison operators are used to compare two numbers. These

operators are: <, <=, ==, >, >=. Don't confuse the == operator of equality with = operator

of assignment.

It is important for us to remember the difference between these two operators of equality

(==) and assignment (=) . However, C/C++ creates a little problem for us here. When we

write a statement as:

a = b;

The assignment statement itself has a value, which is the same as that of the expression

on the right hand side of the assignment operator. We can recall from our last lecture that

we only wrote a number inside the if statement. We also know that if the resultant inside

the if statement is non-zero then its code block is executed. In case, the result is zero, the

control is transferred to the else part.

If we want to compare two variables a and b inside if statement but wrongly write as:

if ( a = b )

{

// if code block

// do something

}

else

{

// do something else

}

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In this case, if the value of the variable b is non-zero (and hence value of the statement a

= b is non-zero) then if code block will be executed. But this was not required, it is a

logical fault and compiler was unable to detect it. Our objective was to compare two

variables. For that purpose, we should have used assignment operator == for that as:

if ( a == b )

One should be very careful while using comparison operators. You should not miss any

case of it and be sure about what you want to do and what will be the output of a

comparison statement.

You should keep in mind straight line of Calculus for the sake of completeness, you

should always divide your domain into two regions. If we take >= as one region then the

other region is <. Similarly if we say < as a region, the other region is >=. Depending on

the problem requirements, these regions should be very clear.

Bitwise Operators

& is bitwise AND operator, | is bitwise OR operator, ^ is bitwise Exclusive OR operator

and ~ is bitwise inversion or NOT operator. ~ (NOT operator) is unary operator as it

requires one operator and the remaining operators &, | and ^ are binary operators because

they require two operands.

Programming Constructs

For us, it is not necessary to know who is the one to devise or decide about these

constructs to be part of the program logic. The important thing is the concept of

programming constructs, required to write a program. We have earlier discussed three

constructs.

1. The sequential execution of statements of a program. Execution of statements begins

from very first statement and goes on to the last statement.

2. Secondly we need decisions that if something is true then we need to do something

otherwise we will do something else. We use if statement for this.

3. The third construct is loops. Loops are employed for repetitive structures.

Decisions

Normally, if statement is used where decisions are required.

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If statement

The syntax of if statement is fairly simple i.e.

if (condition)

{

// if code block

}

else

{

// else code block

}

The result of the condition can be either true or false. If the condition is true, if code

block is executed. Braces of the if code block are mandatory but if there is only one

statement in the if code block then the braces can be omitted or are optional. Now if the

condition is false, the if code block is skipped and the control is transferred to the else

part and else code block is executed. Else part is optional to associate with the if part.

So without else the statement looks like the following:

if (condition)

{

// if code block

// Do something here

}

Use of braces is again mandatory. Again, however, if there is only statement inside the

else part then brace is optional.

As a programming practice, use of braces all the time is recommended. It makes your

program more readable and logically sound.

What happens when the condition is complex?

Nested if statement

For complex conditions, we use logical connectives like &&, ||. For example:

if ( a > b && a < c)

If there are nested decisions structure that we want to do something based on some

condition and further we want to do something more based on an additional condition.

Then we use nested if-statements as under:

if ( a > b && a < c )

{

// Do something

if ( a == 100 )

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{

// Do something more

}

else

{

// Do something else more

}

else

{

// Do something else

}

From stylistic and readability perspective, we properly indent the statements inside if-

statements as shown above.

We discussed pictorial representation of if-statement. By using flowchart of if statement

that was a bit different than normally we see inside books, we introduced structured

flowcharting.

In structured flowcharting, we never go to the left of the straight line that joins Start

and Stop buttons. There is a logical reason for it as while writing code, we can't move to

the left outside the left margin. Left margin is the boundary of the screen and indentation

is made towards the right side. So we follow the construct that is equivalent to the

program being written. The major advantage of this approach is achieved when we draw

a flowchart of solution of a complex problem. The flowchart is the logical depiction of

the solution to the problem. One can write code easily with the help of the flowchart.

There will be one to one correspondence between the segments of the flowcharts and the

code.

Loops

Going on from the decision structures we discussed about loops. In our program if we

have to do something repeatedly then we can think of applying loop structure there.

There are few variants of loops in C language. However, other languages might have

lesser number of loop variants but a programming language always has loops constructs.

While Loop

The syntax of the while loop is as follows:

while ( condition )

{

// while code block

// Do something

}

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The condition is a logical expression like a == b that returns true or false. Braces are

mandatory to for while loop when there are multiple lines of code inside the while code

block. If there is only single line inside the while code block, the braces become optional.

It is good practice to use braces. The statements inside the while code block are never

executed, if the while condition results in false for very first time it is entered. In other

words, statements inside the while code block executes 0 to n times.

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The flowchart for the while loop is as follows:

while

False

condition

Exit

True

Process

Do-While Loop

Next loop variant is Do-while. It syntax is as under

{

// do-while code block

// Do something

}

while ( condition )

The important difference of this loop from the rest ones is that it is executed once before

the condition is evaluated. That means the statements of do-while code block execute at

least once.

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The flowchart for do-while loop is given below:

do while

Process

False

condition

Exit

True

For Loop

The for loop becomes bread and butter for us as it gathers three things together. The

syntax for the for loop is as follows:

for ( initialization statements; condition; incremental statements)

{

//for code block

// Do something

}

E.g.,

for ( int i = 0; i < 10; i ++)

{

}

The for loop is executed until the condition returns true otherwise it is terminated.

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The braces are not mandatory if there is single statement in the for code block. But for

sake of good programming practice, the single statement is also enclosed in braces. Some

people write the for loop in the following manner:

for ( initialization statements; condition; incremental statements){

//for code block

// Do something

}

Both the methods for writing of for loop are perfectly correct. You can use anyone of

these. If you indent your code properly, the process will become easier.

The flowchart for for loop is as under:

for

Initialization

Statements

for

False

condition

True

Process

for

Incre/Decre

Statements

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switch, break and continue Statements

For multi-way decisions, we can use nested if-statements or separate if-statements or

switch statement. There are few limitations of switch statement but it is necessary to use

break statements in every case inside the switch statement. If a case results in true when

there is no break statement inside it, all the statements below this case statement are

executed. break statement causes to jump out of the switch statement. We use break at

the end of every case statement. By using break, the jumping out from switch statement

is in a way bit different from the rules of structured programming. But break statement is

so elegant and useful that you can use it inside switch statement and inside loops. If we

use break inside a loop, it causes that loop to terminate. Similarly continue statement is

very useful inside loops. continue statement is used, when at a certain stage, you don't

want to execute the remaining statements inside your loop and want to go to the start of

the loop.

Functions

In C/C++, functions are a way of modularizing the code. A bigger problem is broken

down into smaller and more manageable parts. There is no rule of thumb for the length of

each part but normally one function's length is not more than one screen.

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Function Calling

We covered Functions Calling by value and by reference. The default of C language is

call by value. Call by value means that when we call a function and pass some parameter

to it, the calling function gets the copy of the value and the original value remains

unchanged. On the other hand, sometimes, we want to call a function and want to see the

changed value after the function call then call by reference mechanism is employed. We

achieved call by reference by using Pointers. Remember while calling functions, call by

value and call by reference are different techniques and default for ordinary variables is

call by value.

Top-Down Methodology

We discussed top-down design methodology. How do we see a problem at a high level

and identify major portions of it. Then by looking at each portion we identify smaller

parts inside it to write them as functions.

Arrays

After discussing functions and playing little bit with function calling, we had elaborated

the concept of Arrays. As discussed previously in this lecture, arrays are used to

aggregate variables of same data type. We wrote little functions about it and did some

exercises e.g., when we wanted to store age of students of our class. Then instead of

using a separate variable for each student, an array was employed to store the ages of the

students. Then to manipulate or to access individual array elements, a technique array

indexing was used. One important point to remember is that array indexes start from 0.

Let's say our array name is a of 10 ints, its first element will be a[0] while the last one

will be a[9]. Other languages like Fortran carry out 1-based indexing. Due to this 0 based

indexing for arrays in C language, programmers prefer to start loops from 0.

Arrays can also be multi-dimensional. In C language, arrays are stored in row major order

that a row is stored at the end of the previous row. Because of this storage methodology,

if we want to access the first element of the second row then we have to jump as many

numbers as the number of columns in the first row. This fact becomes important when we

are passing arrays to functions. In the receiving function parameters, we have to write all

the dimensions of the array except the extreme-left one. When passing arrays to

functions, it is always call by reference by default, it is not call by value as in the default

behavior of ordinary variables. Therefore, if the called function changes something in the

array, that change is actually made in the original array of the calling function. When we

pass ordinary variables to functions, they are passed by value because of the default

behavior. But when an array is passed to a function, the default behavior changes and

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array is passed by reference. We also did some examples of arrays by using Matrices and

did some exercises by transposing and reversing a squared matrix. Arrays are not just

used in Mathematics or Linear Algebra but are employed in a number of other problems

like when we store ages, names, and grades or want to calculate grade point of average.

This is very useful construct especially when used with loops. Normally it is very rare

that you see an array in a program and loop is not being used to manipulate it.

Like nested if-statements, we have nested loops, used with multi-dimensional arrays. A

while loop can have an inner while loop. Similarly a for loop can have a for loop inside.

It is also not necessary that a while loop should have only a while loop but it can be a for

loop also or any other construct like if-statement.

Pointers

It is very important topic of C/C++ . Pointers are different types of variables that contain

memory address of a variable instead of a value.

The very first example we discussed for pointers was for implementing function calling

by reference. Suppose we want to interchange (swap) two numbers by making a function

call. If we pass two variables to the function, these will be passed as ordinary variables by

value. Therefore, it will be ineffective as swapping of variables inside the function will

only be on the copies and not on the original variables. So instead of passing variables we

pass their addresses. In the called function, these addresses are taken into pointer

variables and pointers start pointing the original variables. Therefore, the swapping

operation done inside the function is actually carried out on the original variables.

We also saw that Pointers and Arrays are inter-linked. The array name itself is a pointer

to the first element. It is a constant pointer that cannot be incremented like normal pointer

variables. In case of two-dimensional arrays, it points to the first row and first column. In

three-dimensional array, you can imagine it pointing to the front corner of the cube.

File I/O

We discussed about Files and File I/O for sequential and random files. We used a mixture

of C/C++ for file handling and how the sequential and random files are accessed. We saw

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several modes of opening files. The important functions were seek and tell functions.

Seek functions (seekg and seekp ) used to move into the file and tell functions (tellg and

tellp) provided us the location inside the file.

You are required to go with very clear head, try to understand concepts and assess how

much you have learned so far to prepare for the mid-term examination.

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