Pointers and Arrays as function arguments

• Pointers as function arguments.

  Remember, when a function is called, the values of arguments are copied to the local variables of the function. In other words, a function can not modify the original value.
  

  Let's see what happens if you pass a pointer as the function argument.

  #include <stdio.h>
  void func(int *p);
  
  int main (void) {
    int i = 10, j = 20;
    int *iPtr = &i;
    
    func(iPtr);
    func(&j);
    
    printf("In main: i is %d  j is %d\n", i, j);
  }
  
  void func(int *p) {
    printf("In func: %d\n", *p);
    *p += 1;
    return;
  }
  

  The definition of func() tells that it receives a pointer to integer as an argument.

  You can copy the source from /home/progClass/src/drift/pntrFunc.c.

  Do you notice that the function can modify the value of local variables in main()? This is because the value of the pointer (address) is copied to the local pointer variable p. The address in the memory is unique, so the function can directly modify the local variable residing in another function through dereferencing the pointer.

• Array as a function argument

  Similarly, the following example shows a function which will create an array with random values. After the function call, the array a contains 10 random numbers.

  • Similarly you can pass the name of an array to a function:

    int main(void) {
      double a[10], sum;
      
      sum = SumArray(a, 10);
    }
    
    double SumArray(double arr[], int size) {
      int i;
      double result = 0;
      for (i = 0; i < size; i++) {
        result += arr[i];
      }
      return(result);
    }
    

  • Note double arr[] in the function definition. This tells that the argument is an array. Alternatively, you can use double *arr. These two ways of defining the type of arguments is equivalent.

  • When you are calling the function, you are passing the address of the 1st element of the array (a is used without following squrare brackets [].

  • You can pass a pointer as the argument to SumArray().

• A function returning multiple values.

  Previously, we could returns only 1 value from a function. You can create a function which ``returns'' several calculated values by passing the address.

  int main(void) {
    int a[10], sum, product;
    sumProdArray(a, 10, &sum, & product);
  }
  
  void SumProdArray(int *arr, int arrSize, int *sumAnswer, int *productAnswer) {
    int i, sumResult=0, prodResult=0;
    for (i = 0; i < arrSize; i++) {
      sumResult += arr[i];
      prodResult *= arr[i];
    }
    *sumAnswer = sumResult;
    *productAnswer = prodResult;
    return;
  }
  

  Explanation: You prepare 2 variables (sum and product) which you want to receive the answers with.

  Then the address of these two variables are passed to the function, and the function put the answers into the two variables.

• A function returning an array.

  int main(void) {
    double a[10];
    srand48(time(NULL));
    MakeRandArray(a, 10);
  }
  
  void MakeRandArray(double *a, int arrSize) {
    int i;
    for (i = 0; i < arrSize; i++) {
      a[i] = drand48(); 
    }
  }
  

• Exercises:
  1. Write a function which takes an array as an input, and returns the minimum and maximum values.
  2. Write a function SumArrays(), which takes two arrays (of same size) as input arguments. The function add each element in the arrays together and place the values in a third array.
  3. Make a function int MemberQ(int *array, int arrSize, int x), which takes an array of size arrSize, and check if one of the element has the same value as x (i.e., if x is a member of array. This function returns 1 if x is a member, and returns 0 if x doesn't exist in array.
  4. Write a function which creates an array with n random integers between 0 and max. Similar to MakeRandArray() above.
  5. Modify the above function, so that it will create n unique random integers.