Aditech » C

Saving the output of a C program in a text file(Linux/Unix)

adithya — Sun, 30 Mar 2008 11:46:17 +0000

Consider the following c program

#include
main()
{
int i,n;
printf("Enter limit\n");
scanf("%d",&n);
printf("I will print %d times\n",n);
for(i=0;i
the output will be


Enter limit

4

I will print 4 times

I luv C

I luv C

I luv C

I luv C


This is a simple program which will print "I luv C" n times.Now, wouldn't be nice if we saved the output in a text file

then in such case you have to use directed symbol ">>"



Usage:



./a.out >> filename.txt


let the filename for the above c program be adi.c.In order to direct the output of this program to a text file.Issue commands


bash-2.05b$ cc adi.c

bash-2.05b$ ./a.out >> adi.txt


where adi.txt is the text file in which you want to save the output
When you type ./a.out >> adi.txt and press enter ,nothing will be displayed.

Now type 5 and press enter.

Open adi.txt file by issuing command vi adi.txt


bash-2.05b$vi adi.txt


the following will be it's contents


Enter limit

I will print 5 times

I luv C

I luv C

I luv C

I luv C

I luv C


when you direct the output to a text file,2 things are important
 1.Only scanning takes place in the terminal.Means no output will be there.You have to remember the sequence in which you should enter values.

2.Your input is not saved in the text file as you can see ,the file contains "Enter limit" but doesn't contain what i inputted.
1.If the text file already exists,then the output will be appended(added) to the existing contents

2.The text file is saved under the same directory.If you want to store it some other place then give the path

ex:



bash-2.05b$ ./a.out >> \home\adithya\output\adi.txt



will save the output to a text file named adi.txt saved in \home\adithya\output\ directory[/B]
Tip:
if you want the contents of the textfile to be of natural output ,then after every scanf() statement issue appropriate printf() statement of the value you scanned.

Ex:if you want to make the directed output file of the above program match exatly with real output then make this change


printf("Enter limit\n");

scanf("%d",&n);

printf("%d\n",n);


now the contents of the text file will be like this


Enter limit

5

I will print 5 times

I luv C

I luv C

I luv C

I luv C

I luv C



     
You may also like
Dynamic Output in C
enum data type
Wordpress 2.6 -A complete review and Guide
floor and ceil
Advantages of running c in Linux/Unix-1
Edit Set

Important instructions for programs using math.h library function

adithya — Sun, 23 Mar 2008 15:14:25 +0000

if your program uses math.h header file then sometimes the compilation will lead to errors.In such cases you have to suffix -lm to the compilation file command

Usage
cc filename.c -lm

lm means link mathematical header file.Note that this step is not always required use it only if your program is correct and compilation is leading to errors.
and all other procedures are same.

ex:
consider the following program

#include
#include
 main()
{
float num,sq;
printf("Enter the number\n");
scanf("%f",&num);
sq=sqrt(num);
printf("Sqrt=%f",sq);
}

suppose the name of this c file is adi.c
then sometimes compilation of this file will lead to an error like this

bash-2.05b$ cc adi.c /tmp/cck2Ps76.o(.text+0x42): In function `main': : undefined reference to `sqrt' collect2: ld returned 1 exit status

In such case compile like this

bash-2.05b$ cc adi.c -lm

Advantages of running c in Linux/Unix-1

adithya — Sat, 22 Mar 2008 13:59:28 +0000

C is strongly linked with linux/unix .The kernel of Unix/Linux is written in C.Running C in linux/unit has several advantages .The most important among it is full word length utilization.
Consider the following C program

#include
int main()
{
int a;
float b;
printf("Size of int datatype =%d bits\n",(sizeof(a)*8));
printf("Size of float datatype =%d bits\n",(sizeof(b)*8));
return 1;
}

sizeof() returns the amount of bytes that the variable is awarded

We want the answer in bits,hence we have to multiplied it by 8 (1 byte=8bits)
now run this c program in turbo c in windows
the output will be

Size of int datatype=16 bits Size of float datatype =32 bits

Even though your computer is 32 or 64 bit computer, Turbo C is utilizing only 16 bits to int which should have been 32 bits(Int is allocated with a space equal to wordlength of the computer)

now run the above c program in linux
the output will be

Size of int datatype=32 bits Size of float datatype =64 bits

^^if your processor is 32 bit

Size of int datatype=64 bits Size of float datatype =128 bits
^^if your processor is 64 bit

Ok…running C program in linux/unix utilizes full word length.what’s it’s use for me??

The maximum number of singed int datatype(normal int) that can be accomodated if you run c in windows is ((2^15)-1)=32767
that means if ‘a’ is made to store number greater than 32767 it will cause data overflow and you will get garbage values.(Read How C Compiler decides the Garbage values in case of data overflow for more information)

Now ,lets take the scenario in linux/unix

The maximum number of singed int datatype(normal int) that can be accomodated if you run c in linux is ((2^31)-1)=2147483647 (assuming your comp is 32 bit)

so the int variable ‘a’ can store a maximum value of 2147483647.Compare with this mere 32767..

This is for int datatype consider other datatypes such as float,long double,double,long int etc..
Imagine how many big values it can store

So you can play with larger values if you run a C program in linux/unix.
Apart from this the speed of execution will be more than double since it utilizes full available word length.

C in linux/unix-First step

adithya — Sat, 22 Mar 2008 13:26:50 +0000

First let’s learn how to run a c program in linux/unix.Nearly every distro support’s running c programs.

First thing is to type the c program.you can use any editor you want but here i use vi editor which comes with allmost all distro

vi editor

To create a new c file or to open an existing c file,type vi followed by filename.c
ex:
[student@localhost student]$vi adi.c

will create a new file adi.c or will open an exisiting file adi.c if it’s present

now press insert key to edit or modify or add new content
after you complete writing the program,press Esc key and type :w (colen w) and press enter to save the file then press :q (colen q) and press enter to exit the vi editor.

Next stop:Compilation

Compilation

To compile a c file enter cc followed by the filename
ex:
student@localhost student]$cc adi.c

if the compilation is successful ,then no message will be displayed.Otherwise a message along with line number where the error is present is displayed.
Next stop:Running the compiled file

Running the compiled file

To run the compiled file,enter ./a.out
ex:
[student@localhost student]$./a.out

the c program will be running.Just work the same way as you work in windows.
note that irrespective of the file name ,you have to always enter “./a.out” to run the c program which you just compiled.

let’s see an example program to print hi linux
[student@localhost student]$vi first.c

press insert and type the following program

#include main() { printf("\nHi linux\n"); }
note that i have given just main() not void main()

now press Esc key and type :w to save the file and :q to exit the vi editor

[student@localhost student]$cc first.c [student@localhost student]$./a.out

if you do these steps correctly, then you should get the output like this

Hi linux [student@localhost student]$

Noteif you get a error like this

[student@localhost student]$ cc adi.c bash: cc: command not found [student@localhost student]$

means there is no gcc package in your distro and you are unable to run c programs in that

ASCII Chart

adithya — Wed, 20 Feb 2008 12:29:10 +0000

After seeing this chart if you want to print say square root symbol all you have to do is specify the ASCII value for %c in printf
i.e to print square root..

printf(“%c Hi! %c”,251,251);

and to print heart symbol

printf(“%c Hi! %c”,3,3);

atoi,atol and atof functions

adithya — Wed, 20 Feb 2008 12:18:42 +0000

These functions are available under stdlib.h .These are very unknown functions.Lets see what these do..

atol()
atol returns the value in terms of long datatype of the string containing numbers.the syntax is

atol(stringvariable)

ex: consider the following program

#include
#include
void main()

char a[10],b[10];
puts("Enter the value of a");
gets(a);
puts("Enter the value of b");
gets(b);
printf("%s+%s=%ld and %s-%s=%ld",a,b,(atol(a)+atol(b)),a,b,(atol(a)-atol(b)));
getch();
}

and similarly there is atof() which returns the float value of the string containing numbers and atoi() which returns integer value.

now to see the difference between the 3 look at this program

#include
#include
main()
{
char a[]={"2545.965"};
printf("atol=%ld\t atof=%f\t atoi=%d\t\n",atol(a),atof(a),atoi(a));
}

the output will be

atol=2545 atof=2545.965000 atoi=2545

The output of atoi() and atol() functions will be same but atof() function may give different output depending upon the compiler.That is,in some compiler the output may be like this atof=2545.96498 or something.The above output is extarcted from red hat

let’s look what happens when the string does not contain numeric value.change this statement in the above program

char a[]={“adithya”};

now when you run the program the following will be the output

atol=0 atof=0 atoi=0

that means the string should contain numeric value
now modify this program as

char a[]={“007adi”};

the output in this case(tested in Red hat) will be

atol=7 atof=7.000000 atoi=7

so the functions has taken 007 only not the remaining part
However note that this depends upon compiler.Some will take ,while some will give output as 0.

Now consider this

char a[]={“adi007″};

the output of the program will be

atol=0 atof=0.000000 atoi=0

I’m not sure of this but the following is the point that i noticed
now consider this

char a[]={“0XA”};

what do you think the output is???
As you can see 0XA is a hexadecimal number(hexadecimal number in C is written by writing 0X(zero X) folowed by hex number).It’s decimal equivalent is 10.The following is the output i got in Red hat

atol=0 atof=10.000000 atoi=0

Now the question is why atol and atoi couldn’t understand this while atof took the number as hex number?????

Saving the output in a text file

adithya — Wed, 20 Feb 2008 08:54:54 +0000

consider the following c program

#include
#include
main()
{
int i,n;
clrscr();
printf(“Enter limit\n”);
scanf(“%d”,&n);
printf(“I will print %d times\n”,n);
for(i=0;i
printf(“I luv C\n”);
}

the output will be

Enter limit
4
I will print 4 times
I luv C
I luv C
I luv C
I luv C

This is a simple program which will print “I luv C” n times
Now i want to save the output to a text file without copy pasting the output.
Well,in that case, first compile the saved C file[{ALT} {F9}]
Now go to file menu and click DOS SHELL or DOS PROMPT in Turbo C

You are in DOS now.Lets assume the filename of the above program be some output.c.Then issue command

C:\TC\BIN\output.exe >> output.txt

In general,the command is

filename.exe >> textfile.txt

As soon as you press enter, you will be presented a black screen and you see no message.Now enter 5 and press enter.You will again see DOS prompt.Now open the text file(text file will be in same director(C:\TC\BIN)),the following will be it’s content

Enter limit
I will print 5 times
I luv C
I luv C
I luv C
I luv C
I luv C

When you direct the output to a text file,2 things are important

1.Only scanning takes place in DOS.Means no output will be there.You have to remember the sequence in which you should enter values.
2.Your input is not saved in the text file as you can see ,the file contains “Enter limit” but doesn’t contain what i inputted.

Note:

1.If the text file already exists,then the output will be appended(added) to the existing contents
2.This is a general procedure assuming that you have saved your c file in C:\TC\BIN\ directory.If you have saved somewhere else say C:\Pro\ then you should first navigate to that directory by using ‘cd’ command i.e, cd C:\Pro\ and then the remaining procedures are same
3.The text file is saved under the same directory.If you want to store it some other place then give the path
ex:

C:\TC\BIN\output.exe >> C:\output.txt

will save the output to a text file named output.txt saved in C:\
4.Do not forget to compile the file before saving it’s output in a text file
5.If you use getch(); at the end ,remember to press a key.
6.In case of above program,blank screen is presented because we have used clrscr();if it’s not used then you will not see a blank screen.But you see a no prompt.This is the indication that the program is running
7.Save the file before compilation.
8.The compiled file is always having the same name as the saved c file but having extension ‘.exe’

Tip:
if you want the contents of the textfile to be of natural output ,then after every scanf() statement issue appropriate printf() statement of the value you scanned.
Ex:if you want to make the directed output file of the above program match exatly with real output then make this change

printf(“Enter limit\n”);
scanf(“%d”,&n);
printf(“%d\n”,n);

now the contents of the text file will be like this

Enter limit
5
I will print 5 times
I luv C
I luv C
I luv C
I luv C
I luv C

How C Compiler decides the Garbage values in case of data overflow

adithya — Wed, 20 Feb 2008 08:43:45 +0000

Garbage value in case of data overflow

I did some intense R&D for garbage values and found these facts.After understanding these concepts u can tell what will be the garbage value to be stored in a variable in case of dataoverflow.

Here are some of the basic facts required to proceed

In case of windows(TC),

The space allocated for a variable of int datatype is 16 bits.

That means the maximum negative value that can be stored by the variable of int datatype is 2^15=-32768.

You may ask why 2^15 not 2^16.This is because one bit is reserved for sign(+ or -)

simlarly the maximum +ve value that a variable of int datatype can store is 2^15-1=32767.Hang on!!

Why -1.Because 0 is also counted in case of +ve values.

Now lets come to long datatype.
The space allocated for a variable of long datatype is 32 bits.
Maximum negative value= 2^31=-2147483648
Maximum +ve value= 2^31-1=2147483647

In case of linux(32 bit),

The space allocated for a variable of int datatype is 32 bits.
Maximum negative value= 2^31=-2147483648
Maximum +ve value= 2^31-1=2147483647

Now lets come to long datatype.
The space allocated for a variable of long datatype is 64 bits.
Maximum negative value= 2^63=-9.22337e+18
Maximum +ve value= (2^63)-1=9.22337e+18

Since long in linux can store a huge amount of value.let’s not consider it.
Let’s take int and long in case of windows and only long in case of linux

That’s basic stuffs
Now, consider this program

#include
#include
void main()
{
int a;
long int b;
clrscr();
a=45000;
b=4000000000;
printf(“%d\t%ld\n”,a,b);
getch();
}

Since both variables a and b are assigned a value greater than their limits,when you run this program some garbage value will be the output.But, the question is how c decides the garbage value??Can we predict the garbage value without running the program.

Sure,We can.C compiler calculates the garbage value by using the formula

garbage value=overflow value-reference value.

Overflow value is nothing but the value stored in the variable.Now what’s is the reference value??
reference value is the value such that it is greater than the overflow value(neglecting the sign)obtained by the multiplication of the basic reference value by 2 until the value is greater than the overflow value(neglecting the sign) The sign of the reference value is same as that of overflow value Basic reference value is 2^n where n represents the number of bits allocated.

That means Basic reference value in case of int is 2^15=32768 and in case of long,it is 2^31=2147483648
The garbage value thus calculated is checked whether it can fit the variable or not.If not the steps are repeated again.

Ok thats sounds confusing lets take an example

ex1:

a=45000;

overflow value=45000
Basic reference value=32768
since the reference value is less than the overflow value ,we have to multiply it by 2
i.e,32768*2=65536
65536 is greater than the 45000.That means reference value(RV)=65536.SInce RV must be of same sign,RV=+65536
now apply formula
garbage value=overflow value-reference value.
garbage value=45000-(+65536)
garbage value=-20536
-20536 comes within the limits of int datatype.

ex2:

b=4000000000;

overflow value(OV)=4000000000
SInce ‘b’ is of long type Basic reference value=2147483648
since the reference value is less than the overflow value ,we have to multiply it by 2
i.e,2147483648*2=4294967296
4294967296 is greater than the 4000000000.That means reference value(RV)=4294967296.SInce RV must be of same sign,RV=+4294967296
now apply formula
garbage value=overflow value-reference value.
garbage value=4000000000-(+4294967296)
garbage value=-294967296
-294967296 comes within the limits of long datatype.

So the output of the program will be
-20536 -294967296

Now, run the program and see whether the output is same as calculated or not.

Modify the above program

a=-40000;
b=-4500000000;

now , do not run the program.

ex3:

a=-40000;

OV=-40000
discard the sign take 40000.Basic reference value=32768
since the reference value is less than the overflow value ,we have to multiply it by 2
i.e,32768*2=65536
65536 is greater than the 40000.That means reference value(RV)=65536.SInce RV must be of same sign as OV,RV=-65536
Now apply the formula
garbage value=overflow value-reference value.
garbage value=-40000-(-65536)
garbage value=-40000+65536
garbage value=25536
25536 comes within the limits of int datatype.

ex4:

Code:

b=-45000000000;

overflow value(OV)=-4500000000
SInce ‘b’ is of long type Basic reference value=2147483648
discard the sign take 4500000000.
since the reference value is less than the overflow value ,we have to multiply it by 2
i.e,2147483648*2=4294967296 is less than 4500000000.
therefore 4294967296*2=8589934592
8589934592 is greater than the 4500000000.That means reference value(RV)=8589934592.SInce RV must be of same sign,RV=-8589934592
now apply formula
garbage value=overflow value-reference value.
garbage value=-4500000000-(-8589934592)
garbage value=-4500000000+8589934592)
garbage value=4089934592
4089934592 does not come within the limits of long datatype.

The process is repeated again
overflow value(OV)=4089934592
RV=+4294967296
garbage value=4089934592-(+4294967296)=-205032704
-205032704 comes within the limits of long datatype.
That means final garbage value is -205032704

So the output will be
25536 -205032704

Now run the program.Is your output matching with mine??

Some special cases:

There is a special overflow value which will return the same overflow value but opposite in sign. The values are 32768 in case of int and 2147483648 in case of long datatype.The garbage value will be -32768 and -2147483648 respectively.
Lets understand what happens.

As i already explained ,the maximum +ve value that can be stored by int datatype variable is 32767 not 32768
That means.OV=32768 Basic RV=32768. RV must be greater than OV.Therfore RV=32768*2=65536
Now, garbage value(GV)=OV-RV=32768-65536=-32768

Similarly apply this to 2147483648 you will get
GV=2147483648-4294967296
GV=-2147483648
Quite fascinating.Isn’t it??

Dynamic Output in C

adithya — Wed, 20 Feb 2008 08:22:54 +0000

Dynamic output

consider the following program

#include
int main()
{
long double a=3.14159265;
printf(“Value of pi=%.4Lf”,a);
return(1);
}

the output of the program will be

Value of pi=3.1416

wouldn’t it be nice if we allow the user to choose how the precession of the answer would be??
that is the output should be like this

Enter precision
6
The value of pi precised to 6 decimal number is 3.141593

Well in that case the program will be like this

#include
int main()
{
long double a=3.14159265;
int pre;
printf(“Enter precision\n”);
scanf(“%d”,&pre);
printf(“The value of pi precised to %d decimal number is %.*Lf”,pre,pre,a);
return(1);
}

see the difference,it is upto the user to decide how the output will be.The value of pi will be rounded off to fit the precision .Such an output is called dynamic output.

This is the part of the program which is important

printf(“The value of pi precised to %d decimal number is %.*Lf”,pre,pre,a);

you can create a dynamic output by just specifing * before control string and specifying the variable having precesion in the arguments list.As you can see i have specified pre twice because one to print the The value of pi precesiced to__ and the other to suggest the compiler to fill the * with the value of pre.

This fact can not only be used to specify the precision of the decimal number but also for various other things.Some of them are

1.Printing first n characters of the given string

consider

printf(“The Output is %.*s”,pre,str);

suppose the string variable str has a string of “adithya” and the value of int datatype variable pre is 3 then the output will

The Output is adi

2.Justification
consider

printf(“The Output is %*.*f.Bye!”,just,pre,num);

suppose num(float)=1983.78
pre(int datatype variable)=2
just(int datatype variable)=25

then the output will be in the form of right justification i.e,

The Output is 1983.78.Bye!

suppose you want the output to be left justified then either you can specify ‘-*.*f’ or give a negative value to the variable ‘just’
suppose for the above example just=-25 then the output will be

Code:

The Output is 1983.78 .Bye!

floor and ceil

adithya — Wed, 20 Feb 2008 08:09:38 +0000

floor and ceil

floor and ceil functions comes under math.h header file.
floor is used to round down the value of the decimal number and ceil is used to round up the value of the decimal number.

consider the following c program

#include
#include
#include
{
float a;
clrscr();
printf(“Enter any decimal number\n”);
scanf(“%f”,&a);
printf(“Using floor the value is %d and using ceil it is %d\n”,floor(a),ceil(a));
getch();
}

Output:
Enter any decimal number
5656.98
Using floor the value is 5656 and using ceil it is 5657

In otherwords floor will neglect the decimal part while ceil will add 1 to the mantissa part.Hence we can say that floor is similar to casting.That means floor(a) is equivalent to (int)a or (long)a

These 2 functions do not depend upon the value after decimal point that means if the program is run with the value 5656.01 then the output will be same.

Similarly there is floorl and also ceill which returns long type integers