Tracing a loop

The table below shows an example of code tracing with a loop. The fragment of program find the sum of n terms of a geometric progression (Lab 5). I chose the value n=4, which is big enough to show several repetitions and small enough not to bore too much.
b+bq+bq²+...bq^n-1

       double b,q; /* initial term and quotient of geom.progression */
       double s;   /* sum-to-be (accumulator) */
       int n;      /* number of terms to be summed */
       double qi;    /* accumulator for powers q^i */ 
       int i;        /* counter of iterations in loop */        
   
10     b=3; /* initializing b */
11     q=2;  /*  ... and q */
12     n=4; /* ... and number of terms */

13     s=b; /* initializing sum */
14     qi=1; /* initializing power: qi=q^0=1 */
     
15     for (i=1; i<n; i++)
16     {
17        qi=qi*q;   /* updating power: qi was q^(i-1), becomes q^i */
18        s= s+ b*qi; /* updating sum */
19        printf("i=%d, q^i=%lf, sum=%lf\n", i,qi,s);
20     }   

21     printf("After loop: i=%d, q^i=%lf, sum=%lf\n", i,qi,s);

Please note the exact sequence of loop elements: the initialization section (i=1) is executed only once, condition check precedes the body. After the condition fails, program skips the loop body and proceeds to the line immediately after the closing brace.

    line #  of| b | q | n | s | i | qi | Comments 
    -------------------------------------------------
       10-12  | 3 | 2 | 4 | ? | ? | ?  |  
    -------------------------------------------------
       13-14  | 3 | 2 | 4 | 3 | ? | 1  |  values immediately before loop
    -------------------------------------------------
       15     | " | " | " | " | 1 | "  | initialization of loop: i=1
    -------------------------------------------------
       15     | Checking loop condition i<4? => True => to loop body  
    ----------------------------------------------------
       17     | 3 | 2 | 4 | 3 | 1 |  2 | qi=1*2=2
    ----------------------------------------------------
       18     | 3 | 2 | 4 | 9 | 1 |  2 | s=3+3*2=9
    -----------------------------------------------------
       19     | Printing: i=1, q^i=2, sum=9
    ------------------------------------------------------
       20     |                        | back to loop header (line 15)
    --------------------------------------------------------
       15     | 3 | 2 | 4 | 9 | 2 |  2 | loop update (i++)
    --------------------------------------------------------
       15     | Checking loop condition i<4? => True 
    -------------------------------------------------------
       17     | 3 | 2 | 4 | 9  | 2 |  4 |  qi=2*2=4
    -------------------------------------------------
       18     | 3 | 2 | 4 | 21 | 2 |  4 |  s=9+3*4=21
    -------------------------------------------------
       19     | Printing: i=2, q^i=4, sum=21
    -------------------------------------------------
       20     |                        | back to loop header (line 15)
    --------------------------------------------------------
       15     | 3 | 2 | 4 | 21 | 3 |  4 | loop update (i++)
    -------------------------------------------------------
       15     | Checking loop condition i<4? => True 
    --------------------------------------------------------
       17     | 3 | 2 | 4 | 21 | 3 |  8 |  qi=4*2=8
    -------------------------------------------------
       18     | 3 | 2 | 4 | 45 | 3 |  8 |  21=3+3*8=45
    -------------------------------------------------
       19     | Printing: i=3, q^i=8, sum=45
    -------------------------------------------------
       20     |                        | back to loop header (line 15)
    --------------------------------------------------------
       15     | 3 | 2 | 4 | 45 | 4 |  8 | loop update (i++)
    -------------------------------------------------
       15     | Checking loop condition i<4? => False => exit loop 
    --------------------------------------------------
       21     | Printing: i=4, q^i=8, sum=45

Note that the last printed values of i and q^i are inconsistent. This is because i has been incremented one more time, before the computer detected that the loop condition finally failed.