CSPP 50101 - Lab 4

  /* returns the square of n */
  int square( int x )
  {
    return x*x;
  }

  int main()
  {
    int x=2;
    int y=square(x);
    ...
  }

In contrast, under call by reference argument evaluation, a function receives a memory address (or pointer) of the variable passed as an argument.

The function swap() in swap.c is an example of a call-by-reference function:

   void swap (int * px, int * py)
   {
      int tmp = *px;
      *px = *py;
      *py = tmp;
   }

Old value of x: 4
Old value of y: 8
Calling swap...
New value of x: 8
New value of y: 4

int main()
{
   int x=4, y=8;
   printf("Old value of x: %d\n",x);
   printf("Old value of y: %d\n",y);

   printf("Calling swap...\n");
   swap(&x,&y);

   printf("New value of x: %d\n",x);
   printf("New value of y: %d\n",y);

   return 0;
}

In the argument list of swap(), * ("star") tells us that the arguments are "pointers to integers", or memory addresses of integers.

   void swap (int * px, int * py)  /* argument list */

In the body of swap(), the operator * is used to dereference pointers px and py, or to get the integer values that px and py point to. For example, here px is dereferenced, and the result is assigned to tmp:

   int tmp = *px; /* assignment statement */

Note that these are two different uses of *. In the argument list, * indicates a pointer data type. In the assignment statement, * is an operator that dereferences a pointer variable.

What is the data type of expression *px in the above assignment statement? (Hint: What is the data type of tmp, on the left sign of operator =?)

You can see that the & operator is used in main() to pass the addresses of variables x and y to swap():

   swap(&x,&y);

Add a new function to this file called reorder(), which is similar to swap() but performs a swap only if the integer value at px is greater than the value at py. Otherwise, reorder() does not perform a swap. So after reorder() returns, the second argument always points to the larger of the two integer values.

You are given this function prototype:

   void reorder(int * px, int * py);

main() includes two tests for reorder(). You will have to modify main() to call reorder() for each test case:

   printf("Calling reorder ...\n");

   /* add call to reorder here */

   printf("New values: x=%d, y=%d\n",x,y);

Your output should look like this (note a swap occurs in the first test but not the second):

Test 1: x=8, y=4
Calling reorder ...
New values: x=4, y=8

Test 2: a=5, b=9
Calling reorder ...
New values: a=5, b=9

Exercise 2

norm_time() will take two arguments:

• A pointer to an integer that represents a number of hours
• A pointer to an integer that represents a number of minutes

• Add a function prototype for norm_time()
• Implement norm_time()
• Modify main() to include calls norm_time:

     print_time(hours,mins);
  
     /* add call to norm_time() */
  
     printf(" Printing normalized time...\n");
  

Test 1:
 1 hours, 135 minutes
 Printing normalized time...
 3 hours, 15 minutes

Test 2:
 5 hours, 59 minutes
 Printing normalized time...
 5 hours, 59 minutes

Arrays and array arguments

Compile and run this program. You will see this output:

Calling print_array() ...
0: 10
1: 11
2: 12
3: 13
4: 14
5: 15
6: 16
7: 17

Calling set_array() ...

Calling print_array() ...
0: 42
1: 42
2: 42
3: 42
4: 42
5: 42
6: 42
7: 42

   int a[LEN] = {10,11,12,13,14,15,16,17};

void print_array(int * arr, int len)
{
  int i=0;
  while(i<len)
  {
    printf("%d: %d\n",i,arr[i]);
    ++i;
  }
}

Next look at function set_array():

void set_array(int * arr, int len, int val)
{
  int i=0;
  while(i<len)
    arr[i++]=val;
  printf("\n");
}

Update main() so that it calls set_odd() instead of set_array(). Your output should look like this:

Calling print_array() ...
0: 10
1: 11
2: 12
3: 13
4: 14
5: 15
6: 16
7: 17

Calling set_odd() ...

Calling print_array() ...
0: 0
1: 42
2: 0
3: 42
4: 0
5: 42
6: 0
7: 42

Strings and string arguments

The file str_demo.c contains a simple function that updates the first character in a string:

  void set_first(char str[],char c)
  {
    str[0] = c;
  }

Enter a word: UChicago
Calling set_first() ...
New string: ZChicago

  void set_first(char * str, char c)
  {
    *str = c;
  }

Exercise 4

This function should make changes in-place, meaning that it should change the string that is passed as input. Your function flip_case() should not call printf().

You can use str_demo.c as a starting point. Your output should look something like this:

Enter a word: U-Chicago
Calling set_first() ...
New string: u-cHICAGO

Submitting your assignment

Submit your *.c files for Lab 4: ex1.c, ex2.c, ex3.c, ex4.c

First, be sure that every source file you submit includes this information in comments at the top of the file:

• Your first and last name
• Your username
• Any other comments you want to add about your program (optional)

Also include a simple README file in your lab4 directory. This file should include your first and last name, your username, the list of files that you are submitting, and any other comments or questions for the Lab TA.

Do not submit any compiled binary files

You will submit this lab with the hwsubmit program.

$ hwsubmit cspp50101lab <path to your lab4 directory>

prepared by Sonjia Waxmonsky