Powers of base 2 in C
by Hanly on Feb.21, 2009, under Computer Science
This program takes a user input from 1 through 512 (inclusive), and calculates all powers of two that are less than or equal to the value. The program prints out all of these values.
NOTE: Due to my usage of the math library’s log function, it is necessary to compile using -lm under Unix systems for the linker to properly define the log function. Information referenced from: http://c-faq.com/fp/libm.html
/* Assignment1: Powers of 2
* by Hanly De Los Santos
* Copyright ©2009, Hanly De Los Santos. All Rights Reserved.
* http://www.hdelossantos.com
*
* This program takes a user input from 1
* through 512 (inclusive), and calculates all powers of
* two that are less than or equal to the value. The
* program prints out all of these values.
*
* NOTE: Due to my usage of the math library's log function,
* it is necessary to compile using -lm under Unix systems
* for the linker to properly define the log function.
* Information referenced from: http://c-faq.com/fp/libm.html
*/
/* Load standard I/O library */
#include <stdio.h>
#include <math.h>
/* Define macro */
#define ARRAYSIZE 10
/* Declare function prototypes to define the output of the
* following functions.
*/
void printPowers(int ar[], int firstindex, int lastindex);
void setPowers(int userinteger, int ar[], int lastindex);
main(int argc, char *argv[]){
int powers[ARRAYSIZE];
/* This determines whether there are 2 arguments in the command
* line input. If there are less than 2 arguments, then a message
* is printed prompting the user to enter a positive integer value.
*/
if( argc < 2)
printf("Invalid command line. Positive integer value required.\n");
/* If there are 2 variables, then the second variable is tested
* for the required integer values.
*/
else{
int usrval = atoi(argv[1]);
/* If the user input value is within the accepted range
* 1-512, then the operation is done with the user value.
*/
if( usrval >= 1 && usrval <= 512 ){
int maxexp = ((log(usrval))/log(2));
setPowers( usrval, powers, maxexp);
printf("Powers of 2 that are less than or equal to %d are:\n", usrval);
printPowers(powers, 0, maxexp);
}
/* If the value is greater than 512, the operation is done with the
* maximum value of 512
*/
else if ( usrval > 512 ){
printf("Powers of 2 that are less than or equal to 512 are:\n", usrval);
setPowers( 512, powers, 9);
printPowers(powers, 0, 9);
printf("argc is equal to: %d", argc);
}
/* If a character/negative number is entered instead of an integer, the user is
* prompted that a positive integer is required to complete the
* operation.
*/
else if ( !isdigit(usrval) || usrval <= 0 ){
printf("Invalid command line. Positive integer value required.\n");
printf("argc is equal to: %d", argc);
}
}
return 0;
}
/* This function sets the array's values; only the array elements
* whose values are less than or equal to the userinteger are set.
* It calls function power() for each value.
*/
void setPowers(int userinteger, int ar[], int lastindex){
int i;
for( i = 0; i <= lastindex; i++)
ar[i] = power( 2, i );
}
/*This function calculates and returns baseexponent utilizing the
* math library function 'pow'.
*/
int power(int base, int exponent){
int e;
e = pow(base, exponent);
return e;
}
/* This function prints the program's output. For each array
* element in the range of integers defined by firstindex and
* lastindex, the function prints one line of the output.
*/
void printPowers(int ar[], int firstindex, int lastindex){
int p;
for( p = 0; p <= lastindex; p++)
printf("2^%d = %d\n", p, ar[p] );
}
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