writing a prime factorization program C - c

Question

I am trying to write a program that shows the results of prime factorization as below:
prompt: Input a positive integer
result examples:
100 = 2^2*5^2
It is a composite integer !
13 = 13
It is a prime number !
I tried to write it only with the basic loops, not using sophisticated techniques because it was for beginner's class. The problem is that when I enter 100 as an input, the result is just
100 =
being printed and the program stops there. Other than that, it gives me results I intended. Can anyone help me find what part of this code is the problem?
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int inputNumber (int* input);
int primeFactors (int input);
int main (void) {
int input;
while ( 1 ) {
inputNumber (&input);
primeFactors (input);
}
return 0;
}
int inputNumber (int* input){
printf("\nInput a positive integer : ");
scanf("%d", input);
if(*input == 0){
printf("\n End of program");
exit(0);
}
return;
}
int primeFactors (int input){
int cnt = 0, i = 3, cnt_sum;
printf("%d = ", input);
if(input > 0){
while ( input % 2 == 0){
cnt++;}
if(cnt == 1){
printf("%d",2);}
else if( cnt > 0 && cnt != 1){
printf("%d^%d",2,cnt);
}
cnt_sum += cnt;
for ( ; i <= sqrt(input); i = i+2){
cnt = 0;
while(input % i == 0){
cnt++;
input /= i;
}
if(cnt == 1){
printf("%d",i);
}
else if( cnt > 0 && cnt != 1){
printf("%d^%d",i,cnt);
}
cnt_sum += cnt;
}
if(cnt_sum > 1){
printf("\nIt is a composite number !\n");
}
else{
printf("%d\nIt is a prime number !\n",input);
}
}
else{
printf("\nIt is an invalid number !\n");
}
}

Answer 1

There were few bugs.
while ( input % 2 == 0){cnt++;} loops infinitely if input is even( #Marian )
if(input > 1){ instead of if(input > 0){. since 1 is neither prime nor composite.
Corrected function:
int primeFactors (int input){
int cnt = 0, i = 3, cnt_sum;
printf("%d = ", input);
if(input > 1){
while ( input % 2 == 0){
input/=2;
cnt++;
}
if(cnt == 1){
printf("%d",2);}
else if( cnt > 0 && cnt != 1){
printf("%d^%d",2,cnt);
}
cnt_sum += cnt;
for ( ; i <= sqrt(input); i = i+2){
cnt = 0;
while(input % i == 0){
cnt++;
input /= i;
}
if(cnt == 1){
printf("%d",i);
}
else if( cnt > 0 && cnt != 1){
printf("%d^%d",i,cnt);
}
cnt_sum += cnt;
}
if(cnt_sum > 1){
printf("\nIt is a composite number !\n");
}
else{
printf("%d\nIt is a prime number !\n",input);
}
}
else{
printf("\nIt is an invalid number !\n");
}
}
ideone

Answer 2

You seem to be missing some steps in your algorithm, as well as checking redundant conditions in your primeFactors function.
int cnt = 0, i = 3, cnt_sum;
Because these won't ever be negative, you can change the type to unsigned.
while ( input % 2 == 0){
cnt++;
}
This while statement will loop infinitely if input is odd, because nothing is done to change input. You can add input = input / 2; to stop this.
if(cnt == 1){
printf("%d",2);}
else if( cnt > 0 && cnt != 1){
printf("%d^%d",2,cnt);
}
The else if expression can be reduced to an else statement, because unsigned variables are always greater than 0 and we already know cnt != 1, otherwise the first statement would be triggered.
while(input % i == 0){
cnt++;
input /= i;
}
The while statement is all good here, how peculiar. :P
if(cnt == 1){
printf("%d",i);
}
else if( cnt > 0 && cnt != 1){
printf("%d^%d",i,cnt);
}
Same deal here as with the if / else if statements before: change the else if to else.
With these corrections applied (and some style inconsistencies fixed), the function now looks like this:
int primeFactors (int input) {
unsigned cnt = 0, i = 3, cnt_sum;
printf("%d = ", input);
if (input > 0) {
while (input % 2 == 0) {
cnt++;
input /= 2;
}
if (cnt == 1) {
printf("%d", 2);
} else {
printf("%d^%d", 2, cnt);
}
cnt_sum += cnt;
for ( ; i <= sqrt(input); i += 2){
cnt = 0;
while (input % i == 0) {
cnt++;
input /= i;
}
if (cnt == 1) {
printf("%d", i);
} else {
printf("%d^%d", i, cnt);
}
cnt_sum += cnt;
}
if (cnt_sum > 1) {
printf("%d\nIt is a composite number !\n");
} else {
printf("%d\nIt is a prime number !\n",input);
}
}
else{
printf("\nIt is an invalid number !\n");
}
}