HowToProgramC : Lesson 3

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Variables and Data Types :

During programming we need to store data. This data is stored in variables. Variables are locations in memory for storing data. The memory is divided into blocks. It can be viewed as pigeon-holes. You can also think of it as PO Boxes. In post offices there are different boxes and each has an address. Similarly in memory, there is a numerical address for each location of memory (block). It is difficult for us to handle these numerical addresses in our programs. So we give a name to these locations. These names are variables. We call them variables because they can contain different values at different times.

The variable names in C may be started with a character or an underscore ( _ ). But avoid starting a name with underscore ( _ ). C has many libraries which contain variables and function names normally starting with underscore ( _ ). So your variable name starting with underscore ( _ ) may conflict with these variables or function names.

In a program every variable has
o Name
o Type
o Size
o Value

The variables having a name, type and size (type and size will be discussed later) are just empty boxes. They are useless until we put some value in them. To put some value in these boxes is known as assigning values to variables. In C language, we use assignment operator for this purpose.
In C language equal-to-sign (=) is used as assignment operator. Do not confuse the algebraic equal-to with the assignment operator. In Algebra X = 2 means the value of X is 2, whereas in C language X = 2 (where X is a variable name) means take the value 2 and put it in the memory location labeled as X, afterwards you can assign some other value to X, for example you can write X = 10, that means now the memory location X contains the value 10 and the previous value 2 is no more there.

Data Types

A variable must have a data type associated with it, for example it can have data types like integer, decimal numbers, characters etc. The variable of type Integer stores integer values and a character type variable stores character value. The primary difference between various data types is their size in memory. Different data types have different sizes in memory depending on the machine and compilers. There are very few data types in C language. These data types are reserved words of C language. The reserve words can not be used as a variable name. Let’s take a look into different data types that the C language provides us to deal with whole numbers, real numbers and character data.

Whole Numbers

The C language provides three data types to handle whole numbers.
o int
o short
o long

The data type int is used to store whole numbers (integers). The integer type has a space of 4 bytes (32 bits for windows operating system) in memory. And it is mentioned as ‘int’ which is a reserved word of C, so we can not use it as a variable name. In programming before using any variable name we have to declare that variable with its data type. If we are using an integer variable named as ‘i’, we have to declare it as

int i ;

The above line is known as declaration statement. The declaration statement int i ; reserves 4 bytes of memory and labels it as ‘i’. This happens at the execution time.

We noted that the integer occupies four bytes in memory. So if we have to store a small integer like 5, 10 or 20, C provides another data type for storing small whole numbers which is called short. The size of short is two bytes and it can store numbers in range of -32768 to 32767. So if we are going to use a variable for which we know that it will not increase from 32767, for example the age of different people, then we use the data type short for age.

short age = 5;

On the other side if we have a very large whole number that cannot be stored in an int then we use the data type long provided by C. So when we are going to deal with very big whole numbers in our program, we use long data type. We use it in program as:

long x = 300500200;

Real Numbers

The C language provides two data types to deal with real numbers (numbers with decimal points e.g. 1.35, 735.251). The real numbers are also known as floating point numbers.
o float
o double

To store real numbers, float data type is used. The float data type uses four bytes to store
a real number. Here is program that uses float data types.

float f = 325.32501

If we need to store a large real number which cannot be store in four bytes, then we use double data type. Normally the size of double is twice the size of float. In program we use it as:
double x = 345624.769123;

char Data Type

So far we have been looking on data types to store numbers, In programming we do need to store characters like a,b,c etc. For storing the character data C language provides char data type. By using char data type we can store characters in variables. While assigning a character value to a char type variable single quotes are used around the character as ‘a’.

/* This program uses short data type to store values */
#include <iostream.h>
char x;
x = ’a’;
cout << “The character value in x = “;
cout << x;

Arithmetic Operators

In C language we have the usual arithmetic operators for addition, subtraction, multiplication and division. C also provides a special arithmetic operator which is called modulus. All these operators are binary operators which means they operate on two operands. So we need two values for addition, subtraction, multiplication, division and modulus.
Addition +
Subtraction -
Multiplication *
Division /
Modulus %

There is one thing to note in division that when we use integer division (i.e. both operands are integers) yields an integer result. This means that if, for example, you are dividing 5 by 2 (5 / 2) it will give integer result as 2 instead of actual result 2.5. Thus in integer division the result is truncated to the whole number, the fractional part (after decimal) is ignored. If we want to get the correct result, then we should use float data type. The modulus operator returns the remainder after division. This operator can only be used with integer operands. The expression x % y returns the remainder after x is divided by y. For example, the result of 5 % 2 will be 1, 23 % 5 will be 3 and 107%10 will be 7.

Precedence of Operators

The arithmetic operators in an expression are evaluated according to their precedence. The precedence means which operator will be evaluated first and which will be evaluated after that and so on. In an expression, the parentheses ( ) are used to force the evaluation order. The operators in the parentheses ( ) are evaluated first. If there are nested parentheses then the inner most is evaluated first. The expressions are always evaluated from left to right. The operators *, / and % have the highest precedence after parentheses. These operators are evaluated before + and –
operators. Thus + and – operators has the lowest precedence. It means that if there are * and + operators in an expression then first the * will be evaluated and then its result will be added to other operand. If there are * and / operators in an expression (both have the same precedence) then the operator which occurs first from left will be evaluated first and then the next, except you force any operator to evaluate by putting parentheses around it.

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