Assignment #2

Introductory C Programming

UW Experimental College

Assignment #2

Handouts:

Assignment #2
Assignment #1 Answers
Class Notes, Chapter 3

Reading Assignment:

Class Notes, Chapter 2
Class Notes, Chapter 3, Secs. 3.1-3.2
Class Notes, Chapter 3, Secs. 3.3-3.6 (optional)

Review Questions:

What are the two different kinds of division that the / operator can do? Under what circumstances does it perform each?
What are the definitions of the ``Boolean'' values true and false in C?
Name three uses for the semicolon in C.
What would the equivalent code, using a while loop, be for the example
```
	for(i = 0; i < 10; i = i + 1)
		printf("i is %d\n", i);
```
?
What is the numeric value of the expression 3 < 4 ?

Under what conditions will this code print ``water''?

	if(T < 32)
		printf("ice\n");
	else if(T < 212)
		printf("water\n");
	else	printf("steam\n");

What would this code print?

	int x = 3;
	if(x)
		printf("yes\n");
	else	printf("no\n");

(trick question) What would this code print?

	int i;

	for(i = 0; i < 3; i = i + 1)
		printf("a\n");
		printf("b\n");

	printf("c\n");

Tutorial Section

(This section presents a few small but complete programs and asks you to work with them. If you're comfortable doing at least some of the exercises later in this assignment, there's no reason for you to work through this ``tutorial section.'' But if you're not ready to do the exercises yet, this section should give you some practice and get you started.)

Type in and run this program, and compare its output to that of the original ``Hello, world!'' program (exercise 1 in assignment 1).
```
#include <stdio.h>

int main()
{
	printf("Hello, ");
	printf("world!\n");
	return 0;
}
```
You should notice that the output is identical to that of the original ``Hello, world!'' program. This shows that you can build up output using multiple calls to printf, if you like. You mark the end of a line of output (that is, you arrange that further output will begin on a new line) by printing the ``newline'' character, \n.
Type in and run this program:
```
#include <stdio.h>

int main()
{
	int i;

	printf("statement 1\n");
	printf("statement 2\n");
	for(i = 0; i < 10; i = i + 1)
		{
		printf("statement 3\n");
		printf("statement 4\n");
		}
	printf("statement 5\n");

	return 0;
}
```
This program doesn't do anything useful; it's just supposed to show you how control flow works--how statements are executed one after the other, except when a construction such as the for loop alters the flow by arranging that certain statements get executed over and over. In this program, each simple statement is just a call to the printf function.

Now delete the braces {} around statements 3 and 4, and re-run the program. How does the output change? (See also question 8 above.)
Type in and run this program:
```
#include <stdio.h>

int main()
{
	int i, j;

	printf("start of program\n");

	for(i = 0; i < 3; i = i + 1)
		{
		printf("i is %d\n", i);
		for(j = 0; j < 5; j = j + 1)
			printf("i is %d, j is %d\n", i, j);
		printf("end of i = %d loop\n", i);
		}

	printf("end of program\n");

	return 0;
}
```
This program doesn't do much useful, either; it's just supposed to show you how loops work, and how loops can be nested. The outer loop runs i through the values 0, 1, and 2, and for each of these three values of i (that is, during each of the three trips through the outer loop) the inner loop runs, stepping the variable j through 5 values, 0 to 4. Experiment with changing the limits (initially 3 and 5) on the two loops. Experiment with interchanging the two loops, that is, by having the outer loop manipulate the variable j and the inner loop manipulate the variable i. Finally, compare this program to the triangle-printing program of Assignment 1 (exercise 4) and its answer as handed out this week.

Type in and run this program:

#include <stdio.h>

int main()
{
	int day, i;

	for(day = 1; day <= 3; day = day + 1)
		{
		printf("On the %d day of Christmas, ", day);
		printf("my true love gave to me\n");
		for(i = day; i > 0; i = i - 1)
			{
			if(i == 1)
				{
				if(day == 1) printf("A ");
				else	printf("And a ");
				printf("partridge in a pear tree.\n");
				}
			else if(i == 2)
				printf("Two turtledoves,\n");
			else if(i == 3)
				printf("Three French hens,\n");
			}
		printf("\n");
		}

	return 0;
}

The result (as you might guess) should be an approximation of the first three verses of a popular (if occasionally tiresome) song.

a. Add a few more verses (calling birds, golden rings, geese a-laying, etc.).

b. Here is a scrap of code to print words for the days instead of digits:

	if(day == 1)
		printf("first");
	else if(day == 2)
		printf("second");
	else if(day == 3)
		printf("third");

Incorporate this code into the program.

c. Here is another way of writing the inner part of the program:

		printf("On the %d day of Christmas, ", day);
		printf("my true love gave to me\n");
		if(day >= 3)
			printf("Three French hens,\n");
		if(day >= 2)
			printf("Two turtledoves,\n");
		if(day >= 1)
			{
			if(day == 1) printf("A ");
			else	printf("And a ");
			printf("partridge in a pear tree.\n");
			}

Study this alternate method and figure out how it works. Notice that there are no else's between the if's.

Exercises:
(As before, these range from easy to harder. Do as many as you like, but at least two or three.)

Write a program to find out (the hard way, by counting them) how many of the numbers from 1 to 10 are greater than 3. (The answer, of course, should be 7.) Your program should have a loop which steps a variable (probably named i) over the 10 numbers. Inside the loop, if i is greater than 3, add one to a second variable which keeps track of the count. At the end of the program, after the loop has finished, print out the count.
Write a program to compute the average of the ten numbers 1, 4, 9, ..., 81, 100, that is, the average of the squares of the numbers from 1 to 10. (This will be a simple modification of Exercise 3 from last week: instead of printing each square as it is computed, add it in to a variable sum which keeps track of the sum of all the squares, and then at the end, divide the sum variable by the number of numbers summed.)

If you keep track of the sum in a variable of type int, and divide by the integer 10, you'll get an integer-only approximation of the average (the answer should be 38). If you keep track of the sum in a variable of type float or double, on the other hand, you'll get the answer as a floating-point number, which you should print out using %f in the printf format string, not %d. (In a printf format string, %d prints only integers, and %f is one way to print floating-point numbers. In this case, the answer should be 38.5.)
Write a program to print the numbers between 1 and 10, along with an indication of whether each is even or odd, like this:
```
	1 is odd
	2 is even
	3 is odd
	...
```
(Hint: use the % operator.)
On page 3 of chapter 3 of the notes is a scrap of code to print the words ``Northeast'', ``Southwest'', etc. corresponding to the x and y components of a direction of travel. Modify the code to also print ``North'', ``East'', etc. if x or y is 0.
Write a program to print the first 7 positive integers and their factorials. (The factorial of 1 is 1, the factorial of 2 is 1 * 2 = 2, the factorial of 3 is 1 * 2 * 3 = 6, the factorial of 4 is 1 * 2 * 3 * 4 = 24, etc.) [Extra credit: why did I only ask for the first 7?]
Write a program to print the first 10 Fibonacci numbers. Each Fibonacci number is the sum of the two preceding ones. The sequence starts out 1, 1, 2, 3, 5, 8, ...
Make some improvements to the prime number printing program in section 3.6 of the notes. Other than 2, no even numbers are prime, so have the program test only the odd numbers between 3 and 100. Rather than recomputing sqrt(i) each time through the inner loop, compute it just once for each new value of i (that is, compute it at the beginning of the body of the outer loop, just before the beginning of the inner loop) and assign its value to another variable.