Others have already explained how the conversion works, but if you want to see for yourself the number into which a string is coerced, add 0 to the string.

print(0+'a',    "\n");  # 0
print(0+'b',    "\n");  # 0
print(0+'1',    "\n");  # 1
print(0+'2',    "\n");  # 2
print(0+'1foo', "\n");  # 1
print(0+'foo1', "\n");  # 0
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