This is a simple calculator written in C language and works under DOS prompt. The calculator performs all the basic arithematic operations including addition, subtraction, trignometric calculations, and many other functions. It can also calculate logrithms, powers, and roots of integer numbers.

```/*******************************************************
*     MYCPLUS Sample Code - http://www.mycplus.com     *
*                                                     *
*   This code is made available as a service to our   *
*      visitors and is provided strictly for the      *
*               purpose of illustration.              *
*                                                     *
* Please direct all inquiries to saqib at mycplus.com *
*******************************************************/

#include
#include
#include
#include

void main() {
clrscr();
char choice, choice2;
int addition=0, sum=0, subtraction=0, sub=0, multiplication=1, multi=1, division=1, dvs=1, square=0;
double logx, resultlog, sinx, resultsin, cosx, resultcos, tanx, resulttan, rootx, resultroot, expx, resultexp;
double fact, factorial, resultfact=1;

printf("WELCOME TO COMSATS INSTITUTE OF INforMATION TECHNOLOGY,ABBOTTABAD n");
printf("BIT-II nn");
printf("AMBER SARDAR nn");
printf("MUNAZZA HAMEEDnn");
printf("Press enter to use the calculator");

getche();
clrscr();

printf("Enter choice n");
printf("Press n");
printf("[2] for Subtraction n");
printf("[3] for Multiplication n");
printf("[4] for Division n");
printf("[5] for Scientific Functions n");
printf("[E] for Exit n");
choice=getch();

if((choice=='e')||(choice=='E'))
exit(0);

else if(choice=='1') {
clrscr();
printf("Add numbers or press q to quit n");
do {
printf("Enter number ");
while(getch()!='q');
printf("Result is %d",sum);
getche();
}

else if(choice=='2') {
clrscr();
printf("This program subtract given number from last given number or press q to quit n");
printf("Enter number ");
scanf("%d",&subtraction);
sub=subtraction;
while(getch()!='q') {
printf("Enter number ");
scanf("%d",&subtraction);
sub=sub-subtraction;}
while(getch()!='q');
printf("Result is %d",sub);
getch();
}

else if(choice=='3') {
clrscr();
printf("Multiply numbers or press q to quit ");
do {
printf("Enter number ");
scanf("%d",&multiplication);
multi=multiplication*multi; }
while(getch()!='q');
printf("Result is %d",multi);
getch();
}

else if(choice=='4') {
clrscr();
printf("This program divide given number from last given number or press q to quit n");
printf("Enter number ");
scanf("%d",&division);
dvs=division;
while(getch()!='q') {
printf("Enter number ");
scanf("%d",&division);
dvs=subtraction/dvs;}
printf("Result is %d",dvs);
getch();
}

else if(choice=='5') {
clrscr();
printf(" Scientific Function n");
printf("Press n");
printf("[1] for x square n");
printf("[2] for root of x n");
printf("[3] for log x n");
printf("[4] for exp of x n");
printf("[5] for Factorial of x n");
printf("[6] for Sin x n");
printf("[7] for Cos x n");
printf("[8] for Tan x n");
printf("[E] for Exit n");
choice2=getch();
}

if((choice2=='e')||(choice2=='E'))
exit(0);

else if(choice2=='1') {
clrscr();
printf("This program calculate square of x n");
printf("Enter number ");
scanf("%d",&square);
square=square*square;
printf("Result is %d",square);
getch();
}

else if(choice2=='2') {
clrscr();
printf("This program calculate Square root of x n");
printf("Enter number ");
scanf("%lf",&rootx);
resultroot=sqrt(rootx);
printf("The square root of %lf is %lfn", rootx, resultroot);
getch();
}

else if(choice2=='3') {
clrscr();
printf("This program calculate Natural log of x n");
printf("Enter number ");
scanf("%lf",&logx);
resultlog=log(logx);
printf("The natural log of %lf is %lfn", logx, resultlog);
getche();
}

else if(choice2=='4') {
clrscr();
printf("This program calculate e ^ x n");
printf("Enter number ");
scanf("%lf",&expx);
resultexp = exp(expx);
printf("'e' raised to the power  of %lf (e ^ %lf) = %lfn",
expx, expx, resultexp);
getche();
}

else if(choice2=='5') {
clrscr();
printf("This program calculate Factorial x n");
printf("Enter number ");
scanf("%lf",&fact);
for(int factorial=fact;factorial>=1;factorial--)
resultfact=resultfact*factorial;
printf("The factorial of %lf is %lf ",fact,resultfact);
getch();
}

else if(choice2=='6') {
clrscr();
printf("This program calculate Sin x n");
printf("Enter number ");
scanf("%lf",&sinx);
resultsin=sin(sinx);
printf("The sin of %lf is %lfn", sinx, resultsin);
getche();
}

else if(choice2=='7') {
clrscr();
printf("This program calculate Cos x n");
printf("Enter number ");
scanf("%lf",&cosx);
resultcos=cos(cosx);
printf("The Cos of %lf is %lfn", cosx, resultcos);
}

else if(choice2=='8') {
clrscr();
printf("This program calculate Tan x n");
printf("Enter number ");
scanf("%lf",&tanx);
resulttan=tan(tanx);
printf("The Tan of %lf is %lfn", cosx, resultcos);
}

}
```