10.txt

一個計算機系教授的上課講義 主要是教導學生使用C語言編寫程序

CS 1355
Introduction to Programming in C
originally scheduled for Thursday 2006.10.12 (Week 5)
made-up on Monday 2006.10.16 (Week 6)
Lecture notes (at http://r638-2.cs.nthu.edu.tw/C/notes/10.txt)

(continued from last time: 
 - by-reference vs by-value
 - rand(), srand(), time()...)

* More experiments with the random number generator

Example from book: roll the dice 1000 times, count each face
(page 157-158, but paraphrased)
#include <stdio.h>
#include <stdlib.h>
int main() {
	int f1=0, f2=0, f3=0, f4=0, f5=0, f6=0;
	int roll;
	int face;
	for (roll=1; roll <= 6000; roll++) {
		switch(face = 1 + rand() % 6) {
		case 1: f1++; break;
		case 2: f2++; break;
		case 3: f3++; break;
		case 4: f4++; break;
		case 5: f5++; break;
		case 6: f6++; break;
		default: printf("error: should not happen: face=%d\n", face);
		}
	}
	printf("%4s%13s\n%4d%13d\n%4d%13d\n%4d%13d\n%4d%13d\n%4d%13d\n%4d%13d\n",
		"Face", "Frequency", 1, f1, 2, f2, 3, f3, 4, f4, 5, f5, 6, f6);
}
-----------
Output (on cs25; your results may vary on other platforms)
Face    Frequency
   1          945
   2          983
   3         1023
   4         1015
   5         1000
   6         1034



Try the following code: command line interface
/*
 * prompt for either a seed from the user, call srand(); 
 * or generate the next sequence of n random numbers
 */
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main() {
	int cmd = '\0';
	unsigned seed = 0;
	int n = 10 /* by default */, i;
	int minRandom = 0, maxRandom = 100; /* by default */
	srand(seed);
	while (cmd != 'q') {
		if (cmd != ' ') { printf("* "); /* prompt */ }
		cmd = getchar();
		switch (cmd) {
		case '?': /* help */
			printf("\
s setRandomSeed\n\
g nRandomNumbers\n\
m minRandomNumber\n\
M maxRandomNumber\n\
t  (uses timer to randomize)\n\
");
			continue;
		case 's': /* set random seed */
			switch (scanf("%u", &seed)) {
			case EOF:
				cmd = 'q';
				continue;
			case 0:
				printf("%u\n", seed);
				continue;
			}
			/* otherwise, set the random seed */
			srand(seed);
			continue;
		case 'g': /* generate the next n numbers */
			if (scanf("%d", &n) == EOF) {
				cmd = 'q'; 
				continue;
			}
			/* now generate n random numbers */
			for (i = 0; i < n; i++) {
				printf("%d ", rand() % (maxRandom - minRandom + 1) + minRandom);
			}
			printf("\n");
			continue;
		case 'm': /* set the minimum random */
			switch(scanf("%d", &minRandom)) {
			case EOF: cmd = 'q'; continue;
			case 0:   printf("%d\n", minRandom);
			case 1:   continue;
			}
		case 'M': /* set the maximum random */
			switch(scanf("%d", &maxRandom)) {
			case EOF: cmd = 'q'; continue;
			case 0: printf("%d\n", maxRandom);
			case 1: /* successful, nothing else to do */
				continue;
			}
		case 't':
			srand(seed = time(NULL));
			continue;
		case ' ': case '\n': case '\t': case ';':
			cmd = ' ';
			continue;
		case 'q': /* quit */
		case EOF:
			cmd = 'q';
		}
	}
}
-----
If you run it with the same seed,
it will generate the same sequence of random numbers
(result shown on cs25.cs.nthu.edu.tw)

% ./a.out
* g;				<<< initially, seed is 0
0 56 90 99 64 60 87 46 2 27 
* g;
40 85 3 62 79 19 48 74 62 87 
* g;
94 10 37 39 79 75 52 83 4 5 
* s 100				<<< set seed to 100
* g;
37 61 38 13 52 63 7 89 36 11 
* g;
20 20 22 1 75 8 40 79 11 4 
* s 0				<<< set seed to 0 again
* g;
0 56 90 99 64 60 87 46 2 27  	<<< same sequence as initially!!
* g;
40 85 3 62 79 19 48 74 62 87 
---------------
to randomize using timer:
./a.out
* t					<<< randomize using timer
* g;
41 17 10 83 43 26 63 91 100 16 
* g 20
81 66 51 37 27 21 66 95 1 51 58 25 91 14 98 36 44 38 73 43 
* s;					<<< display what the seed was
1160832289
---------------
you can use this program to roll dice
% ./a.out 
* m 1 M 6			<<< this sets min = 1, max = 6
* * g;				<<< generate n random numbers (default=10)
1 1 5 2 1 2 2 2 3 5 
* g;
4 5 2 3 4 4 5 2 3 4 
* g;
1 2 2 6 3 6 5 5 5 4 
---------