Week 2 Tutorial — C Refresher

1. (Loops)

   1. The factorial of a positive number n, written n!, is defined as follows:

      • 1! = 1
      • n! = n * (n-1)!  for n > 1

      For example, 4! = 4*3*2*1 = 24.

      Write a for-loop to compute the factorial of a given natural number n.

   2. Write a function

      int gcd(int a, int b) { ... }

      that uses a while-loop to implement Euclid's algorithm for computing the greatest common divisor ("gcd") of two natural numbers:

      • Replace the larger of the two numbers by their difference.
      • Stop when the two numbers are equal — this is the gcd.

      Example:

      gcd(651,378) = 21

      a	b
      651	378
      273	378
      273	105
      168	105
      63	105
      63	42
      21	42
      21	21

2. (Arrays, Strings)

   1. What is the output of the following program?

      1 #include <stdio.h> 2 3 int main(void) { 4 int arr[4] = {10,10,10,10}; 5 char str[] = "Text"; 6 int i; 7 8 for (i = 1; i < 4; i++) { 9 arr[i] = arr[i-1] + arr[i] + 1; 10 str[i] = str[i+1]; 11 } 12 printf("Val[3] = %d\n", arr[3]); 13 printf("String = \"%s\"\n", str); 14 return 0; 15 }

   2. What is the purpose of the program statement in line 14?

3. (Input/Output) Write program statements, including variable declarations, to

   1. ask the user for an integer, read the integer, and output it;
   2. ask the user for a floating point number, read the number, and output it;
   3. ask the user for a string, read the string, and output it.

4. (Files) Write program statements, including variable declarations, to

   • open a file named "infile.txt" for reading;
   • read the input file line-by-line and output each character;
   • close the input stream.

5. (Pointers) What is the output of the following program?

   1 #include <stdio.h> 2 3 int main(void) { 4 int *ptr1, *ptr2; 5 int i = 10, j = 20; 6 7 ptr1 = &i; 8 ptr2 = &j; 9 10 *ptr1 = *ptr1 + *ptr2; 11 ptr2 = ptr1; 12 *ptr2 = 2 * (*ptr2); 13 printf("Val = %d\n", *ptr1 + *ptr2); 14 return 0; 15 }