02 writing first c program

8/14/2019 02 Writing First C Program

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Topic 2

Writing First C Program


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Smallest C Program

A C program consists of one or more blocks of codecalled functions .

Minimum number of functions in a program is one.

The name of one of the functions must be main.

The main function is where program executionbegins.


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Smallest C Program

When a C program runs, it gives back a result toindicate whether program execution is successful.0 (zero) shows successful execution.Non-zero shows there was some problem.


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Data Type int and Constant

The numbers (0,1,2, etc.) are integers i.e. wholenumbers which do not have decimal point.

Integer values are one category of constants i.e.

values that appear in a program.

In C, this type of numbers belong to a group calledint i.e. this type of data or this data type is called inttype.


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The main Function

Smallest C program has these lines:

int main(void){

return 0;}

Function header marksthe beginning of a function.

Indicates that the functionname is main and thefunction gives back aninteger type value (int) andthe function does not needany information (void).


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The main Function


int main(void){

return 0;}

Opening curly brace -indicates the start of the

body of the function.

Closing curly brace -indicates the end of thebody of the function.


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The main Function


int main(void){

return 0;}

The return statementterminates the execution

of the function and returnscontrol to the operatingsystem. It also givesback the functions result.

Zero indicates theprogram executed withouterror.


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Statements

A statement causes an action to be performed by theprogram.

It translates directly into one or more machine

language instructions.


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Statements

int main(void){

return 0;}

Notice the statement endswith a semicolon (;).


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Statements

This program can run but it doesnt do anything.

int main(void){

return 0;}


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Input-Process-Output

The simplest program that does some useful workshould:get some input data,process that data, and

produce some output result.

ProcessInput Output


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Working with Data

A program usually processes data.

But how do we store the data?In memory cells

How does the program refer to these memory cells?By giving names to the memory cells.Example names are x, y, number, n1, n2.


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Declaration Statement

Before we can use a memory cell in the program, weneed to tell the compiler the name and the type ofdata it will store.

We do this using a declaration.Example:

int x;Declaration that tells thecompiler the name of the

memory cell is x and the typeof data it will store is integer.


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Declaration Statement

We can declare more than one name in onedeclaration.Example:

int x, y, z;

Tells compiler theprogram uses 3 memorycells with the names x, yand z to store integervalues.

Notice the names are

separated by commas(,).


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Assignment Statement

How do we tell the program to store a value in thememory cell?By writing an assignment statement.Example:

x = 5;Assignment statement thattells the program to store/assignthe value 5 in the memory cellwith the name x.

= is the assignment operator. It does not mean equal.


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How do we tell the program to store a value in thememory cell?By writing an assignment statement.Example:

x = 5; Think of it like this:x 5which you can read as:

put the value 5 in the memorycell with the name x

or simply:assign 5 to x

or x becomes 5.


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How do we tell the program to copy a value from onememory cell to another?Use assignment statement.Example:

x = 5;y = x;

5x


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How do we tell the program to copy a value from onememory cell to another?Use assignment statementExample:

x = 5;y = x;

5x

y

Copy valueof x into y


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How do we tell the program to copy a value from onememory cell to another?Use assignment statementExample:

x = 5;y = x;

5x

5y

Copy valueof x into y


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Complete Program

int main(void){

int x;int y;

x = 5;y = x;

return 0;}


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Complete Program

int main(void){

int x;int y;

x = 5;y = x;

return 0;}

Function Header

Function Body


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Complete Program

int main(void){

int x;int y;

x = 5;y = x;

return 0;}

Declarations

Statements


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Complete Program

int main(void){

int x;int y;

x = 5;y = x;

return 0;}

Assignment statements

return statement


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Showing Results

The above program will run but it does not show anyresults.Everything happens inside the computers mainmemory only.

How do we make the program display the valuesstored in the memory cells?

By asking a special function to do it i.e. calling or

activating a function to display the values, just like youcall your friend to do something for you.The name of the function is printf.


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Showing Results

How do we tell function printf what to display?By passing it two pieces of information:the value to displaythe format to display the value

Example:

printf( "Y=%d" , y);

The name offunction to call

Parentheses surrounding theinformation to pass to the function


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Showing Results

How do we tell function printf what to display?By passing it two pieces of information:the value to displaythe format to display the value

Example:


Indicates that we wantto display the value of y.


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Showing ResultsHow do we tell function printf what to display?

By passing it two pieces of information.the value to displaythe format to display the value

Example:


This is a string i.e. a sequence of characters. A string is always surrounded by double quotes ().


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By passing it two pieces of information:the value to displaythe format to display the value

Example:


Here the string is called a format string .It tells the printf function how we want todisplay the value.


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By passing it two pieces of information:the value to displaythe format to display the value

Example:


We are saying we want the output to be Y= followed by thevalue of y. The %d is a placeholder that marks the positionto display an integer type value.


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Program Now

int main(void){

int x;int y;

x = 5;y = x;printf( "Y=%d" , y);

return 0;}


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Showing ResultsThe printf function is one of many functions availablein standard libraries .

A library is a collection of functions.

Each library has a standard header file whose nameends with .h which contains information about thefunctions.

The information for the printf function is in a filecalled stdio.h .


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Showing ResultsFor the program to use printf function, thisinformation must be inserted into the program.

To do this , we add this line in our program:

#include

This is called a preprocessor directive .


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Showing Results A preprocessor or precompiler processes the text ofa program before it is compiled.

The #include directive notifies the

preprocessor that some names used in the program(such as printf) are defined in the file stdio.h.

std standardio input/output

h header file


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Complete Program

#include int main(void){

int x;int y;

x = 5;y = x;printf( "Y=%d" , y);

return 0;

}

5xMemory Cells

5y

Y=5

Computer Screen


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Showing ResultsHow do we make the program display the following?

Y is 5

5 is the value in y.

printf( "Y is %d ", y);

printf( "%d is the value in y. ", y);


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What is the output of this program?


int a, b;

a = 475;b = a;printf( "a=%d", a);printf( " b=%d", b);

return 0;

}


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Showing Results

How can we get the two values displayed on twoseparate lines?

Use a special character sequence ( \n) which represents anewline escape sequence to display the output on a

new line.This makes the cursor (position) on the screen movedown to a new line.

Computer Screen

a=475 b=475


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Showing ResultsExample:

printf( "a=%d\n ", a);printf( "b=%d\n ", b);

a=475b=475-

The cursor is nowon the third line

Computer Screen


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Showing ResultsWhat is the output of the following?

printf( "a=%d", a);printf( "\nb=%d\n ", b);


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Showing ResultsWhat is the output of the following? Same as above

Can we do this?

printf( "a=%d\nb=%d\n ", a, b);



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Showing ResultsWhat is the output of the following? Same as above

Can we do this? Yes

printf( "a=%d\nb=%d\n ", a, b);

Placeholderfor a

Placeholderfor b



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VariablesPrograms store data in memory cells.

A memory cell has a name, a data type, a value,and an address.

In programming, these memory cells are calledvariables because the value in the memory cell canchange.


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Variable Declaration and Assignment

int n;Variable declaration tellscompiler the name and datatype of the variable (i.e.memory cell).

n = 72; Assigns value 72 to variable n(i.e. memory cell with name n).


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Variable Declaration and Assignment

int n;

n = 72;

Only variables are allowed on theleft-hand side of an assignmentstatement.


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Getting Input DataHow do we make a program input the data from theprogram user in order to do computation?

By asking a special function to do it.The name of the function is scanf.

After we get the data, where do we store it?In a variable


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Getting Input DataHow do we tell function scanf where to store thedata?

By passing it two pieces of information:the address of variable we want to store the data

the type of the input dataExample:

scanf( "%d", &num);

Indicates we want to storethe data in variable num.


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scanf( "%d", &num);

& (ampersand character) is the address-of operator that gives the address of the variable


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scanf( "%d", &num);

The format string indicates thatthe data type is integer.


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Getting Input DataJust like for the function printf, the stdio.h fileprovides additional information about the functionscanf .

We need to add the preprocessor directive in ourprogram:#include


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Complete Program


int num;

scanf( "%d", &num);

return 0;}


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Getting Input Data

Better to let the user know that the program isexpecting some data.How do we do this?

We display a prompt using printf function

_Cursor appears andprogram waits for theuser to enter a number.

Computer Screen


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Complete Program


int num;

printf( "Enter a number: ");scanf( "%d", &num);

return 0;}

Only formatstring inparentheses

Displaysa prompt.


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Getting Input Data

Enter a number: _

Prompt displayedby printf function

Computer Screen Cursor appears andprogram waits for theuser to enter a number


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Getting Input Data

Enter a number: 91Computer Screen User types a number

and presses Enterkey.

The input data isstored as an integer inthe variable num.

91num


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Getting Input Data

Enter a number: 91Computer Screen User types a number

and presses Enterkey.

The input data isstored as an integer inthe variable num.

91num

Note: the underline is usedto show that this is data

entered by the user.


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What does this program do?


int num, numPower2;

printf( "Enter n: ");scanf( "%d", &num);numPower2 = num * num;printf( "n x n = %d\n ", numPower2);

return 0;}


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Enter n: 5n x n = 25

Computer Screen5 num

25 numPower2


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What does this program do?#include int main(void){

int a, b, sum;printf( "Enter a: ");scanf( "%d", &a);printf( "Enter b: ");scanf( "%d", &b);

sum = a + b;printf( "a + b = %d\n ", sum);return 0;

}


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Enter a: 531Enter b: 24

a + b = 555

Computer Screen 531 a

555 sum

24 b


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int a, b, sum;

printf( "Enter a and b: ");scanf( "%d%d", &a, &b);

sum = a + b;printf( "%d + %d = %d\n ", a, b, sum);

return 0;}


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Enter a and b: 531 24

531a ?sum24b


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Enter a and b: 531 24531 + 24 = 555

531a 555sum24b


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int a, b;

a = 3;b = 5;printf( "a= %d, b=%d\n ", a, b);a = b;

b = a;printf( "a= %d, b=%d\n ", a, b);return 0;

}


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3a ?b

a = b;b = a;

a = 3;b = 5;


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3a 5b

a = b;b = a;

a = 3;b = 5;


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5a 5b

a = b;b = a;

a = 3;b = 5;


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5a 5b

a = b;b = a;

a = 3;b = 5;


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int a, b, temp;

a = 3;b = 5;printf( "a= %d, b=%d\n ", a, b);


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temp = a;a = b;b = temp;

printf( "a= %d, b=%d\n ", a, b);

return 0;}


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3a ?b ?tempa = 3;b = 5;



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3a 5b ?tempa = 3;b = 5;



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3a 5b 3tempa = 3;b = 5;



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5a 5b 3tempa = 3;b = 5;



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5a 3b 3tempa = 3;b = 5;


02 writing first c program

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