9 Odd
- Remainder
9 % 2 = 1
Check Odd Number in C
What you’ll learn
- What “odd” means when you divide by two (leftover of one for nonnegative integers).
- A tiny
isOddhelper usingn % 2 != 0, plus printing odds between 1 and 10. - A live preview and why zero is not odd.
Overview
Odd numbers are the other half of the number line alongside evens: if splitting a pile into two equal whole rows leaves exactly one left over, the count is odd. In C you detect that with the remainder of division by 2.
Two programs
One value (15) and a fixed range 1–10.
Live preview
Type an integer; see odd vs not odd with the same rule as the C samples.
Plain facts
Zero is even, so it fails the “odd” test—by design.
Prerequisites
The modulo operator %, if, and for loops.
#include <stdio.h>,int main(void),printf.- Knowing that division can leave a remainder (that is what
%gives you).
What is an odd number?
For counting numbers 1, 2, 3, …, an integer is odd when it is not divisible by 2 without remainder. Practically: divide by two; if there is a leftover unit, the number is odd (1, 3, 5, 7, …).
In code you rarely prove it with words; you ask C for the remainder using %.
Odd n % 2 != 0
Even n % 2 == 0
Zero even
Remainder mod 2
Odd integers are those congruent to 1 modulo 2 (in the usual nonnegative story). Even integers are congruent to 0. So “odd” and “even” chop the integers into two teams.
1515 = 2 · 7 + 1, remainder 1 when divided by 2—odd.
Quick examples
8 Even
- Remainder
8 % 2 = 0
Takeaway: same symbol %, opposite branch from the even-number lesson.
Live preview
Uses JavaScript safe integers. The rule matches the C idea: odd when n % 2 !== 0.
Algorithm
Goal: return true iff n is not divisible by 2 (equivalently, remainder nonzero).
Compute n % 2
If the remainder is not 0, treat n as odd (for typical beginner nonnegative inputs this is simply remainder 1).
Range scan
Loop from start to end; print values that pass the odd test.
📜 Pseudocode
Pseudocode
function isOdd(n):
return (n mod 2) != 0
function printOddsInRange(start, end):
for i from start to end:
if isOdd(i):
output i1
n % 2 != 0
Helper returns 1 or 0 in C; matches the classic interview style with 15.
c
#include <stdio.h>
int isOdd(int number) {
return number % 2 != 0;
}
int main(void) {
int number = 15;
if (isOdd(number)) {
printf("%d is an odd number.\n", number);
} else {
printf("%d is not an odd number.\n", number);
}
return 0;
}Explanation
isOdd is true when there is any nonzero remainder mod 2. Change number to experiment (14 prints the else branch).
2Odds in
Odds in [1, 10]
Same idea as the reference: walk the range and print 1 3 5 7 9.
c
#include <stdio.h>
void printOddNumbersFrom1To10(void) {
printf("Odd numbers in the range 1 to 10:\n");
for (int i = 1; i <= 10; ++i) {
if (i % 2 != 0) {
printf("%d ", i);
}
}
printf("\n");
}
int main(void) {
printOddNumbersFrom1To10();
return 0;
}Explanation
You could also loop for (i = 1; i <= 10; i += 2) for odds only—fewer iterations, same printed line when start is odd.
Notes
Step by two. To list only odds in [start, end], advance i by 2 after aligning start to the next odd if needed.
Bit trick. (n & 1) != 0 detects odd integers on two’s complement; prefer % until you are comfortable.
Interview: mention that 0 is even, and that isOdd and isEven partition the integers.
❓ FAQ
It is a whole number that cannot be split into two equal whole piles without a leftover—dividing it by 2 leaves remainder 1 (for nonnegative integers). Examples: 1, 3, 15, 101.
No. Zero counts as even (you can write 0 = 2 · 0). So “odd” tests such as n % 2 != 0 correctly classify 0 as not odd.
The remainder when dividing by 2 is 0 for evens and nonzero for odds (with typical nonnegative ints). For negatives in C, the remainder sign follows C99 rules, but n % 2 != 0 still matches “not divisible by 2” for odd integers.
For integers, yes: exactly one of the two is true. In code, isOdd can be (n % 2 != 0) and isEven can be (n % 2 == 0).
On two’s complement, the least significant bit is 1 for odd integers. It is a common micro-optimization; n % 2 != 0 is usually clearer for beginners.
A single test is O(1) time and O(1) space. Scanning a range [a, b] is O(b - a + 1) with O(1) extra space.
🔄 Input / output examples
Swap number in Example 1 or extend Example 2 with parameters start and end if you want a flexible range.
| n | Odd? |
|---|---|
0 | No (even) |
15 | Yes |
22 | No |
-5 | Yes (not divisible by 2) |
Edge cases
The phrase “not odd” includes evens and zero—do not confuse “not odd” with “even and positive.”
Zero
n = 0
0 % 2 == 0, so isOdd(0) is false.
Negatives
Signed integers
Odd negatives still satisfy n % 2 != 0 in C; the printed remainder may be -1 for some negative odds under C99 rules—the inequality test still works.
Inclusive range
i <= end
Keep both endpoints when the problem says “from 1 to 10” inclusively.
⏱️ Time and space complexity
| Operation | Time | Extra space |
|---|---|---|
isOdd(n) | O(1) | O(1) |
Range [a, b] | O(b - a + 1) | O(1) |
| Step-by-two odd loop | half as many iterations | O(1) |
No heap allocation is required for these snippets.
Summary
- Test:
n % 2 != 0(or(n & 1) != 0once you are ready). - Zero is even, so it is not odd.
- Ranges: reuse the same test inside a
forloop.
Did you know?
Every whole number is either even or odd—never both, never neither. Zero is even, so the test n % 2 != 0 correctly says zero is not odd.
About the author
8 people found this page helpful