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C Exercises: Find the sum of all the multiples of 3 or 7 below 100

C Programming Practice: Exercise-17 with Solution

The natural numbers below 10 that are multiples of 3 or 7 are 3, 7, 6 and 9. The sum of these multiples is 25.
Write a C programming to find the sum of all the multiples of 3 or 7 below 100.

C Code:

#include <stdio.h>
int main(void)
{
  int sum3 = 0, sum7 = 0, sum21 = 0;
  int i;
  for (i = 0; i < 100; i++) {
    if (i % 3 == 0) {
      sum3 += i;
    }
    if (i % 7 == 0) {
      sum7 += i;
    }
    if (i % 21 == 0) {
      sum21 += i;
    }
  }
  printf("%d\n", sum3 + sum7 - sum21);
  return 0;
}

Sample Output:

2208

Flowchart:

C Programming Flowchart: Find the sum of all the multiples of 3 or 7 below 100.

C Programming Code Editor:

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Previous: Write a C programming to find the length of the longest valid (correct-formed) parentheses substring of a given string.
Next: Write a C programming to find the sum of the even-valued terms from the terms in the Fibonacci sequence whose values do not exceed one million.

What is the difficulty level of this exercise?

C Programming: Tips of the Day

C Programming - Why do all the C files written by my lecturer start with a single # on the first line?

In the very early days of pre-standardised C, if you wanted to invoke the preprocessor, then you had to write a # as the first thing in the first line of a source file. Writing only a # at the top of the file affords flexibility in the placement of the other preprocessor directives.

From an original C draft by the great Dennis Ritchie himself:

12. Compiler control lines

[...] In order to cause [the] preprocessor to be invoked, it is necessary that the very first line of the program begin with #. Since null lines are ignored by the preprocessor, this line need contain no other information.

That document makes for great reading (and allowed me to jump on this question like a mad cat).

I suspect it's the lecturer simply being sentimental - it hasn't been required certainly since ANSI C.

Ref : https://bit.ly/2Mb8OVZ