I don't understand this crash - c

Question

This is my code for solving the problem "Students Marks Sum" in Hackerrank:
(Link: https://www.hackerrank.com/challenges/students-marks-sum/problem?)
#include <string.h>
#include <math.h>
#include <stdlib.h>
//Complete the following function.
int marks_summation(int* marks, int number_of_students, char gender) {
int result = 0;
switch (gender){
case 'b':
while(marks != NULL){
result += (*marks);
marks += 2;
}
case 'g':
++marks;
while(marks != NULL){
result += (*marks);
marks += 2;
}
break;
}
return result;
}
int main() {
int number_of_students;
char gender;
int sum;
scanf("%d", &number_of_students);
int *marks = (int *) malloc(number_of_students * sizeof (int));
for (int student = 0; student < number_of_students; student++) {
scanf("%d", (marks + student));
}
scanf(" %c", &gender);
sum = marks_summation(marks, number_of_students, gender);
printf("%d", sum);
free(marks);
return 0;
}
When I move this code into an IDE (in my situation, it's DEV C/C++), it was crashed when my debug ran into my function marks_summation.
Error in line:
result += (*marks);

Answer 1

The problem is in your assumption that if you move out of the range of dynamically allocated memory the pointer value will change to NULL:
while(marks != NULL){
result += (*marks);
marks += 2;
}
There are a number of approaches you can pick to iterate over such array.
You can calculate the memory address that will no longer be valid to read and check at each iteration if your pointer is smaller than it:
int marks_summation(int* marks, int number_of_students, char gender) {
int result = 0;
int* marksEnd = marks + number_of_students;
switch (gender){
case 'b':
while(marks < marksEnd){
result += (*marks);
marks += 2;
}
case 'g':
++marks;
while(marks < marksEnd){
result += (*marks);
marks += 2;
}
break;
}
return result;
}