Not sure why one would want the "first pair" each time.
Below we define a function that produces an iterator
yielding random prime pairs one after the other,
with primes picked at random in a range ensuring
their product will have the correct number of digits.
```
def prime_pair_for_each_product_ndigits(nmin, nmax):
for n in range(nmin, nmax + 1):
lo = isqrt(10**(n-1))
hi = isqrt(10**n)
a = random_prime(hi, lbound=lo)
b = random_prime(hi, lbound=lo)
yield a, b
```
This might give something like the following.
```
sage: p_q_list = list(prime_pair_for_each_product_ndigits(19, 35))
sage: p_q_list
[(2257885207, 1029222697),
(7906953709, 8831254513),
(28123041337, 18086888887),
(59156603659, 75212477107),
(244478980829, 297135812329),
(878180166047, 926189502323),
(1390098443147, 1010518243127),
(5150835567223, 3603512345189),
(16533079879703, 15727405198679),
(34437803922611, 57693414719911),
(102979771890571, 141205565889301),
(941693653240727, 468548529571597),
(1375078260756671, 1726461802816903),
(8524331332200121, 4310462307066121),
(19014345158785303, 31601933129557883),
(92121426759262499, 64338223205154251),
(262242817443146321, 251117727289363427)]
```
We check the number of digits of each product.
```
sage: f = lambda n: f'{n:36d} has {n.ndigits():2d} digits'
sage: print('\n'.join(f(n) for n in (p*q for p, q in p_q_list)))
2323866702264943279 has 19 digits
69828320626688338717 has 20 digits
508658323826826921919 has 21 digits
4449314698430409934513 has 22 digits
72643460565990932840741 has 23 digits
813361250941000432227181 has 24 digits
1404719836542484333000669 has 25 digits
18561099554526665790140147 has 26 digits
260022446450216138134512337 has 27 digits
1986834503750174243684807621 has 28 digits
14541316964959210349671680871 has 29 digits
441229176532847985852022831019 has 30 digits
2374020093080293654628048809913 has 31 digits
36743808900391354263355551200641 has 32 digits
600890064210265812194221708193549 has 33 digits
5926928916814700284047132294733249 has 34 digits
65853820314282336219416564107002067 has 35 digits
```
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