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cpointersgetchar

Using pointers to count _ and ! in a string outside of the main function in C

I am working on an assignment for school and am having trouble getting the correct output. I am unsure if there is a problem with my loop or a problem with the method of saving values with pointers. When I run the code I end up with something like this:

Output: There are 369224989 underscores and 0 exclamation points in the sentence.

The assignment specifies to use the prototype and the getchar() function to read the input. I feel like since the first value is so high it is an issue with my loop but I have been working on this for two days and haven't seen anything wrong with it (though I could be staring through it at this point).
In addition I get these warnings when I try and compile the program:

characters.c:28: warning: value computed is not used
characters.c:31: warning: value computed is not used

This makes me think that maybe it isn't communicating properly with the main function.

#include<stdio.h>
 //this function prototype was required for the assignment

void count(int* num_, int* num_exclamation);

// intended to count the number of _ and ! in a string using pointers

int main()
{
        int num_, num_exclamation;

        count(&num_, &num_exclamation);

        return 0;
}


void count(int* p_num_, int* p_num_exclamation)
{
        char ch;

        *p_num_ = *p_num_exclamation = 0;

        //attempts to scan a string get the first character
        printf("Enter a sentence: ");
        ch = getchar();

        //attempts to loop while incrementing if it is a ! or _

        while(ch != '\n')
                {
                if(ch == '_')
                        *++p_num_;

                if(ch == '!')
                        *++p_num_exclamation;

                ch = getchar();

                }
        //prints result

        printf("Output: There are %d underscores and %d exclamation points 
in the sentence.\n", *p_num_, *p_num_exclamation);

}

This is the second time I have really interacted with pointers with the first being the other half of this assignment which is working properly. I am not particularly comfortable with them yet nor aware of all of their nuances. Any advice to get me looking in the right place would be greatly appreciated.

Solution

  • You have Undefined behavior in your code. *++p_num_; increments the pointer first and then it dereferences it. The value of it is not used. And this way, the pointer points to some memory which are not the variables you supposed it to be. Then you dereference it - that location contains indeterminate value and you print it. Accessing some memory which you don't have permisssion to is - UB.

    (*p_num_)++

    is the one you wanted. This holds for the other variable also - namely p_num_exclamation. Also return value of getchar is int not char - you should use int to hold the value returned by getchar.