Q43808: C: srand() Sample Program Does Not Print Out the Array Values

Article: Q43808
Product(s): See article
Version(s): 5.00 5.10 | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | DOCERR S_QuickC | mspl13_c
Last Modified: 22-MAY-1989

In the following manuals, the sample program given for the function
srand() implies that it will print out random values for each of the
array elements. Although the array is filled with random values, these
values are not printed because the "for" loop is implemented
incorrectly. This example is given in both the "Microsoft C for the
MS-DOS Operating System Run-Time Library Reference" manual for
Versions 5.00 and 5.10, and the "Microsoft QuickC Run-Time Library
Reference" manual for Versions 1.00 and 1.10.

The following source code fragment is excerpted from the sample
program found in the "Microsoft QuickC Run-Time Library Reference"
manual on Page 564.

    srand(17);
    for (x = 0; x < 20; ranvals[x++] = rand())
      printf("Iteration %d, ranvals[%d] =%d\n",x+1, x, ranvals[x]);

If you execute the program as written, it produces a list of all zeros
instead of a list of the random numbers being put into the array. To
understand why this is so, the definition of a for loop must be
considered. The for loop defined by Kernighan and Ritchie is as
follows:

    for (expr1, expr2, expr3)
        statement;

    is equivalent to:

    expr1;
    while (expr2)
       {
       statement;
       expr3;
       }

If the above sample program is substituted into this definition, then
the program reads as follows:

    srand(17);
    x = 0;
    while (x < 20)
       {
       printf("Iteration %d, ranvals[%d] =%d\n",x+1,x,ranvals[x]);
       ranvals[x++] = rand();
       }

This program obviously prints the array elements one step before they
are initialized, which is why the sample program prints out all zeros
instead of the desired random numbers.

To display the correct results, you must take the call to  the
function rand() out of expr3 of the for statement. This can be
accomplished as follows:

    srand(17);
    for (x = 0; x < 20; x++)
       {
       ranvals[x] = rand();
       printf("Iteration %d, ranvals[%d] =%d\n",x+1, x, ranvals[x]);
       }

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