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| Code Block | ||
|---|---|---|
| ||
int i=0;
for (i=0; i<10; i++) {
printf("%d, ", rand()); /* Always generates the same sequence */
}
output:
1st run: 41, 18467, 6334, 26500, 19169, 15724, 11478, 29358, 26962, 24464,
2nd run: 41, 18467, 6334, 26500, 19169, 15724, 11478, 29358, 26962, 24464,
...
nth run: 41, 18467, 6334, 26500, 19169, 15724, 11478, 29358, 26962, 24464,
|
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Noncompliant Code Example
Use srand() before rand() to seed the random sequence generated by rand(). The code produces different random number sequences at different calls.
| Code Block | ||
|---|---|---|
| ||
srand(time(NULL)); /* Create seed based on current time */
int i=0;
for (i=0; i<10; i++) {
printf("%d, ", rand()); /* Generates different sequences at different runs */
}
output:
1st run: 25121, 15571, 29839, 2454, 6844, 10186, 27534, 6693, 12456, 5756,
2nd run: 25134, 25796, 2992, 403, 15334, 25893, 7216, 27752, 12966, 13931,
3rd run: 25503, 27950, 22795, 32582, 1233, 10862, 31243, 24650, 11000, 7328,
...
|
Although the rand() function is now properly seeded, this solution is still noncompliant because the numbers generated by rand() have a comparatively short cycle, and the numbers may be predictable (see MSC30-C. Do not use the rand() function for generating pseudorandom numbers).
Noncompliant Code Example (POSIX)
This noncompliant code example generates a sequence of 10 pseudorandom numbers using the random() function. When random() is not seeded, it behaves like rand(), thus produces the same sequence of random numbers at different calls.
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