c - 使用 gcc -Wall 的 C 给出了一个数组下标类型为 'char' 的错误

Answer 1

Try changing this:

if (isdigit(input[0]))

to:

if (isdigit((unsigned char)input[0]))

or

if (isdigit((int)input[0]))

for more details see similar question: array subscript has type 'char'

Answer 2

This means that the index for the array can be negative.

This can be a problem as a char could represent a signed value, and you may be requesting a negative index.