Section 13.9 Fun With Loops!

Remember: the best way to learn to code is to write code! So let’s practice our loops. Pick one of the following prompts. And most importantly: have fun! If you get done early, pick another one!

Subsection 13.9.1 Option 1

Activity 13.8.

Write a program that prompts the user to enter a positive integer n and then computes the sum of the numbers from 1 through n. For example, if the user enters 5, then your program’s output should be:

"The sum of the numbers from 1 through 5 equals 15."

A quick way to check that your program works is to ensure that the sum of the numbers from 1 to n equals \(\frac{n(n+1)}{2}\text{.}\)

Write two versions of this program: One using a while-loop, and another one using a for-loop.

Compile and run the code below in your own C environment.

#include <stdio.h> 
int main (void){
    int n, sum;
    
    printf ("Enter a positive integer: ");
    scanf ("%d", &n);
    
    /* add your code here. Be sure to declare any additional variables
       you need. */
    
    printf("The sum of the numbers from 1 through %d equals %d.", n, sum);
    return(0);
}

Subsection 13.9.2 Option 2

Activity 13.9.

Write a program that continuously asks the user to enter a nonzero number and adds up all of the numbers entered. The program should terminate when the user enters a zero and then print out the total sum.

Here is a sample run of the program:

Please enter a number: 5
Please enter a number: 11
Please enter a number: -3
Please enter a number: 0
The numbers you entered add up to 13.

Compile and run the code below in your own C environment.

#include <stdio.h>

int main(void){
    
    int n, total;
    
    /* Please add your code here. Be sure to declare
       any additional variables you need. */h
    
    printf("\nThe numbers you entered add up to %d.", total);
    
    return(0);
}

Subsection 13.9.3 Option 3

Activity 13.10.

Write a program that prompts the user for two numbers p and q, then calculates the greatest common divisor of p and q. You may do so using the "brute force" method: If q \(\leq\) p, for example, you simply check each of the divisors q, q-1, q-2, ..., 1, until you find one that divides both p and q.

Challenge: implement the Euclidean algorithm!

Compile and run the code below in your own C environment.

#include <stdio.h>

int main(void){
    
    int p, q, divisor;
    
    printf("Please enter two numbers to find the GCD: ");
    scanf("%d%d", &p, &q);
    
    /* Please add your code here. Be sure to declare any additional
       variables you need. */
    
    printf("The greatest common divisor of %d and %d is %d.", p, q, divisor);
    return 0;
}

Subsection 13.9.4 Option 4

Activity 13.11.

Write a program that determines whether a number entered by the user is prime (again, you may use the “brute force” method, going through all possible divisors 2, 3, 4, …, n).

(You only really need to check factors up to \(\sqrt{n}\text{.}\))

Compile and run the code below in your own C environment.

#include <stdio.h>
int main(void){
    
    int number, notPrime;
    
    printf("Please enter a number: ");
    scanf("%d", &number);
    
    /* Please add your code here. Be sure to declare any
       additional variables you need. */
    
    if (notPrime) {
        printf("The number %d you entered is not prime.", number);
    } else {
        printf("The number %d you entered is prime.", number);
    }
}

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