lang/C/the.ansi.c.programming.language/c.programming.notes/homework/PS2a.html

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   9 <title>Assignment #2 Answers</title>
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  12 <H1>Assignment #2 Answers</H1>
  13
  14
  15
  16
  17
  18 <B>Introductory C Programming
  19 <br>
  20 <br>
  21 UW Experimental College
  22 <br>
  23 <br>
  24 Assignment #2 ANSWERS
  25 </B><br>
  26 <br>
  27 <p>Question 1.
  28 <I>What are the two different kinds of division
  29 that the <TT>/</TT> operator can do?
  30 Under what circumstances does it perform each?
  31 </I><p>The two kinds of division are integer division,
  32 which discards any remainder,
  33 and floating-point division,
  34 which yields a floating-point result
  35 (with a fractional part, if necessary).
  36 Floating-point division is performed
  37 if either operand
  38 (or both)
  39 is floating-point
  40 (that is, if either number being operated on is floating-point);
  41 integer division is performed if both operands are integers.
  42 <p>Question 2.
  43 <I>What are the definitions of the ``Boolean'' values
  44 true and false in C?
  45 </I><p>False is always represented by a zero value.
  46 Any nonzero value is considered true.
  47 The built-in operators
  48 <TT>&lt;</TT>, <TT>&lt;=</TT>, <TT>&gt;</TT>, <TT>&gt;=</TT>, <TT>==</TT>, <TT>!=</TT>,
  49 <TT>&amp;&amp;</TT>, <TT>||</TT>, and <TT>!</TT>
  50 always generate 0 for false and 1 for true.
  51 <p>Question 3.
  52 <I>Name three uses for the semicolon in C.
  53 </I><p>Terminating declarations,
  54 terminating statements,
  55 and
  56 separating the three control expressions in a <TT>for</TT> loop.
  57 <p>Question 4.
  58 <I>What would the equivalent code, using a while loop, be for the example
  59 </I><pre>
  60         for(i = 0; i &lt; 10; i = i + 1)
  61                 printf("i is %d\n", i);
  62 </pre>
  63 <I></I>?
  64 <p>Just move the first (initialization) expression to before the loop,
  65 and the third (increment) expression to the body of the loop:
  66 <pre>
  67         i = 0;
  68         while(i &lt; 10)
  69                 {
  70                 printf("i is %d\n", i);
  71                 i = i + 1;
  72                 }
  73 </pre>
  74 <p>Question 5.
  75 <I>What is the numeric value of the expression <TT>3 &lt; 4</TT> ?
  76 </I><p>1 (or ``true''),
  77 because 3 is in fact less than 4.
  78 <p>Question 6.
  79 <I>Under what conditions will this code print ``water''?
  80 </I><pre>
  81         if(T &lt; 32)
  82                 printf("ice\n");
  83         else if(T &lt; 212)
  84                 printf("water\n");
  85         else    printf("steam\n");
  86 </pre>
  87 <p>If <TT>T</TT> is greater than or equal to 32 <em>and</em> less than 212.
  88 (If you said ``greater than 32 and less than 212'',
  89 you weren't quite right,
  90 and this kind of distinction--paying
  91 attention to the difference between
  92 ``greater than'' and ``greater than or equal''--is
  93 often extremely important in programming.)
  94 <p>Question 7.
  95 <I>What would this code print?
  96 </I><pre>
  97         int x = 3;
  98         if(x)
  99                 printf("yes\n");
 100         else    printf("no\n");
 101 </pre>
 102 <p>It would print ``yes'', since <TT>x</TT> is nonzero.
 103 <p>Question 8.
 104 <I>What would this code print?
 105 </I><pre>
 106         int i;
 107
 108         for(i = 0; i &lt; 3; i = i + 1)
 109                 printf("a\n");
 110                 printf("b\n");
 111         printf("c\n");
 112 </pre>
 113 <p>It would print
 114 <pre>
 115         a
 116         a
 117         a
 118         b
 119         c
 120 </pre>
 121 The indentation of the statement <TT>printf("b\n");</TT> is
 122 (deliberately, for the sake of the question)
 123 misleading.
 124 It <em>looks</em> like it's part of the body of the <TT>for</TT> statement,
 125 but the body of the <TT>for</TT> statement is always a single statement,
 126 or a list of statements enclosed in braces {}.
 127 In this case, the body of the loop is the call
 128 <TT>printf("a\n");</TT>.
 129 The two statements
 130 <TT>printf("b\n");</TT> and <TT>printf("c\n");</TT>
 131 are normal statements following the loop.
 132 <p>The code would be <em>much</em> clearer if the
 133 <TT>printf("b\n");
 134 </TT>line were indented to line up with the
 135 <TT>printf("c\n");
 136 </TT>line.
 137 If the intent was that ``b'' be printed 3 times, along
 138 with ``a'', it would be necessary to put braces
 139 <TT>{}</TT> around the pair of lines
 140 <TT>printf("a\n");
 141 </TT>and
 142 <TT>printf("b\n");
 143 </TT>.
 144 <p>Exercise 1.
 145 <I>Write a program to find out
 146 how many of the numbers from 1 to 10 are greater than 3.
 147 </I><p><pre>
 148 #include &lt;stdio.h&gt;
 149
 150 int main()
 151 {
 152 int i;
 153 int count = 0;
 154
 155 for(i = 1; i &lt;= 10; i = i + 1)
 156         {
 157         if(i &gt; 3)
 158                 count = count + 1;
 159         }
 160
 161 printf("%d numbers were greater than 3\n", count);
 162
 163 return 0;
 164 }
 165 </pre>
 166 <p>Tutorial 4b.
 167 <I>Print ``On the first day'' instead of
 168 ``On the 1 day'' in the days-of-Christmas program.
 169 </I><p>Simply replace the lines
 170 <pre>
 171                 printf("On the %d day of Christmas, ", day);
 172                 printf("my true love gave to me\n");
 173 </pre>
 174 with
 175 <pre>
 176                 printf("On the ");
 177                 if(day == 1) printf("first");
 178                 else if(day == 2) printf("second");
 179                 else if(day == 3) printf("third");
 180                 else if(day == 4) printf("fourth");
 181                 else if(day == 5) printf("fifth");
 182                 else if(day == 6) printf("sixth");
 183                 else    printf("%d", day);
 184                 printf(" day of Christmas, ");
 185                 printf("my true love gave to me\n");
 186 </pre>
 187 The final
 188 <pre>
 189                 else printf("%d", day);
 190 </pre>
 191 is an example of ``defensive programming.''
 192 If the variable <TT>day</TT> ever ends up having a value
 193 outside the range 1-6
 194 (perhaps because we later added more verses to the song,
 195 but forgot to update the list of words),
 196 the program will at least print something.
 197
 198 <p>Exercise 2.
 199 <I>Write a program to compute the average of the ten numbers
 200 1, 4, 9, ..., 81, 100.
 201 </I><p><pre>
 202 #include &lt;stdio.h&gt;
 203
 204 int main()
 205 {
 206 int i;
 207 float sum = 0;
 208 int n = 0;
 209
 210 for(i = 1; i &lt;= 10; i = i + 1)
 211         {
 212         sum = sum + i * i;
 213         n = n + 1;
 214         }
 215
 216 printf("the average is %f\n", sum / n);
 217
 218 return 0;
 219 }
 220 </pre>
 221 <p>Exercise 3.
 222 <I>Write a program to print the numbers between 1 and 10,
 223 along with an indication of whether each is even or odd.
 224 </I><p><pre>
 225 #include &lt;stdio.h&gt;
 226
 227 int main()
 228 {
 229         int i;
 230
 231         for(i = 1; i &lt;= 10; i = i + 1)
 232                 {
 233                 if(i % 2 == 0)
 234                         printf("%d is even\n", i);
 235                 else    printf("%d is odd\n", i);
 236                 }
 237
 238         return 0;
 239 }
 240 </pre>
 241 <p>Exercise 4.
 242 <I>Print out the 8 points of the compass,
 243 based on the <TT>x</TT> and <TT>y</TT> components of the direction of travel.
 244 </I><p><pre>
 245 if(x &gt; 0)
 246         {
 247         if(y &gt; 0)
 248                 printf("Northeast.\n");
 249         else if(y &lt; 0)
 250                 printf("Southeast.\n");
 251         else    printf("East.\n");
 252         }
 253 else if(x &lt; 0)
 254         {
 255         if(y &gt; 0)
 256                 printf("Northwest.\n");
 257         else if(y &lt; 0)
 258                 printf("Southwest.\n");
 259         else    printf("West.\n");
 260         }
 261 else    {
 262         if(y &gt; 0)
 263                 printf("North.\n");
 264         else if(y &lt; 0)
 265                 printf("South.\n");
 266         else    printf("Nowhere.\n");
 267         }
 268 </pre>
 269 (You will notice that this code also prints
 270 ``Nowhere'' in the ninth case,
 271 namely when <TT>x == 0</TT> and <TT>y == 0</TT>.)
 272 <p>Exercise 5.
 273 <I>Write a program to print the first 7 positive integers
 274 and their factorials.
 275 </I><p><pre>
 276 #include &lt;stdio.h&gt;
 277
 278 int main()
 279 {
 280         int i;
 281         int factorial = 1;
 282
 283         for(i = 1; i &lt;= 7; i = i + 1)
 284                 {
 285                 factorial = factorial * i;
 286                 printf("%d   %d\n", i, factorial);
 287                 }
 288
 289         return 0;
 290 }
 291 </pre>
 292 <p>The answer to the extra credit question is that 8!,
 293 which is how mathematicians write ``eight factorial,''
 294 is 40320,
 295 which is <em>not</em> guaranteed to fit into a variable of type <TT>int</TT>.
 296 To write a portable program to compute factorials higher than 7,
 297 we would have to use <TT>long int</TT>.
 298 <p>Exercise 6.
 299 <I>Write a program to print the first 10 Fibonacci numbers.
 300 </I><p><pre>
 301 #include &lt;stdio.h&gt;
 302
 303 int main()
 304 {
 305         int i;
 306         int fibonacci = 1;
 307         int prevfib = 0;
 308         int tmp;
 309
 310         for(i = 1; i &lt;= 10; i = i + 1)
 311                 {
 312                 printf("%d   %d\n", i, fibonacci);
 313                 tmp = fibonacci;
 314                 fibonacci = fibonacci + prevfib;
 315                 prevfib = tmp;
 316                 }
 317
 318         return 0;
 319 }
 320 </pre>
 321 (There are many ways of writing this program.
 322 As long as the code you wrote printed
 323 1, 1, 2, 3, 5, 8, 13, 21, 34, 55,
 324 it's probably correct.
 325 If the code you wrote is clearer than my solution above,
 326 so much the
 327 better!)
 328 <p>Exercise 7.
 329 <I>Make some improvements to the prime number printing program.
 330 </I><p>Here is a version incorporating both of the suggested improvements:
 331 <pre>
 332 #include &lt;stdio.h&gt;
 333 #include &lt;math.h&gt;
 334
 335 int main()
 336 {
 337 int i, j;
 338 double sqrti;
 339
 340 printf("%d\n", 2);
 341
 342 for(i = 3; i &lt;= 100; i = i + 2)
 343         {
 344         sqrti = sqrt(i);
 345         for(j = 2; j &lt; i; j = j + 1)
 346                 {
 347                 if(i % j == 0)
 348                         break;                  /* not prime */
 349                 if(j &gt; sqrti)
 350                         {                       /* prime */
 351                         printf("%d\n", i);
 352                         break;
 353                         }
 354                 }
 355         }
 356
 357 return 0;
 358 }
 359 </pre>
 360 <hr>
 361 <hr>
 362 <p>
 363 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
 364 // <a href="copyright.html">Copyright</a> 1995-9
 365 // <a href="mailto:scs@eskimo.com">mail feedback</a>
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