atoi implementation in C

Question

I can\'t understand the following atoi implementation code, specifically this line:

k = (k << 3) + (k << 1) + (*p) - \'0\';
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Answer 1

k = (k << 3) + (k << 1);

means

k = k * 2³ + k * 2¹ = k * 8 + k * 2 = k * 10

Does that help?

The *p - '0' term adds the value of the next digit; this works because C requires that the digit characters have consecutive values, so that '1' == '0' + 1, '2' == '0' + 2, etc.

As for your second question (atof), that should be its own question, and it's the subject for a thesis, not something simple to answer...