Is this the correct way to assign memory to heap and deallocate it in C? Did I do this right?-CodePudding

I've stumbled upon an interesting problem while preparing for my exam, and I'm unsure as to whether I've solved it or not. I will provide the text of the problem, as well as my code:

void charAlloc(){
char *arrPtr;
arrPtr = malloc(sizeof(char)*100);
int i;
for (i=0; i<101; i  ){
    if (*(arrPtr i) % 5 == 0){
        *(arrPtr   i) = 'a';
    }
    if (*(arrPtr i) % 3 && *(arrPtr i) % 7 != 0 && *(arrPtr i) / 2 != 0){
        *(arrPtr i) = 'b';
    }
    else{
        *(arrPtr i) = 'c';
    }
}
free(arrPtr);
}

Since it's a void function there isn't really a way to check if it's doing what it's supposed to in this case, so I thought I'd ask here. I do sometimes struggle with pointers so it's quite possible I maybe messed up something there. There's no errors or warnings in the compiler.

CodePudding user response：

Question has been answered in the comments. Firstly, instead of going through the loop for 101 times, it should iterate 100 times: for(i=0; i<100; i ).

The next mistake was in the if branches within the loop. What *(arrPtr i) does is it selects the value stored at that address. This is not the value of the index, but rather a garbage value appointed to the heap.

What I should've done instead is checked for the values of i, and assign the values given in the problem to *(arrPtr i).

There was another mistake pointed out within the if branches. In the second if conditional, for assigning value 'b' to certain indexes, the testing statements are incorrect. They stated:

if (*(arrPtr i) % 3 && *(arrPtr i) % 7 != 0 && *(arrPtr i) / 2 != 0)

Which is incorrect in several parts. The first condition is not even finished, it should say if (*(arrPtr i) % 3 == 0, and the last condition does not check properly if the number is divisible by two. *(arrPtr i) % 2 != 0 would be the proper way to do that.

With all this combined, the final result should be something like this:

void charAlloc(){
     char *arrPtr;
     arrPtr = malloc(sizeof(char)*100);
     int i;
     for (i=0; i<100; i  ){
         if (i % 5 == 0){
            *(arrPtr i) = 'a';}
         else if (i % 3 == 0 && (i % 7 != 0) && (i % 2 !=0)){
             *(arrPtr i) = 'b';}
         else {
             *(arrPtr i) = 'c';}
     }
     free(arrPtr);
     }

I'd like to thank all the commenters for helping me understand this problem and I hope other programmers with issues similar to mine can find this useful.