How do I change index of array values with indexes that are null

Question

I was able to do this with an array of 2 indexes, but when I have an array of 3 it only changes the index of one value, and when I have 9 indexes my validation goes alert.

I am trying something like this when inputting 11 in an array of 3 to receive 011, but I get 101.

I have been looking at my second function where I am looping through the indexes to switch values.

int validateInt(int digitAmount);
char* validateCharAmount(int charAmount);

int main(void) {
    do{

        printf("Please enter Student's ID': ");
        studentID=validateInt(3);
    }while(studentID!=0);

    return 0;
}

int validateInt(int digitAmount)
{
    int value;
    char *entrySegments;    
    int index=0;
    char temp;
    entrySegments=validateCharAmount(digitAmount);
    while(index<digitAmount){
        while((entrySegments[index]-48)<0 || (entrySegments[index]-48)>9){
            printf("Invalid Value! Please input an integer: ");
            entrySegments=validateCharAmount(digitAmount);
            index=0;
        }
        value=10*value+(entrySegments[index]-48);
        index++;
    }
    printf("%d",value);
    return value;   
}

char* validateCharAmount(int charAmount){

    char entrySegments[charAmount];
    int index=0;
    entrySegments[0]= NULL;
    entrySegments[charAmount]= NULL;
    scanf("%s",entrySegments);
    while(entrySegments[charAmount]!=NULL){
        printf("You entered too many characters! Please input %d: ", charAmount);
        scanf("%s",entrySegments);      
    }
    while(index<charAmount){
        while((entrySegments[index])==NULL){        
            entrySegments[index]=entrySegments[index-1];
            entrySegments[index-1]=48;
        }
        index++;
    }   
    return entrySegments;
}

Answer 1

entrySegments is a local scoped variable inside validateCharAmount function.

You cannot return it to the caller because its life ends when the function ends.

You can use malloc & co function to do so.

Moreover entrySegments[charAmount] addresses the array out of bounds. Last accessible item is entrySegments[charAmount-1].