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Rather than only 6 digits, use the precision of the type:

Rather than only 6 digits, use the precision of the type:

Use an auto-formatter and present to the display width (e.g. ~ 92)

Missing break;

case '(': { .. deserves a break; at the end of the case or a comment indicating fall-through was desired.

double expr() missing return value;

TBD code needed to quiet "Problem description: No return, in function returning non-void".

Use an auto-formatter and present to the display width (e.g. ~82)

memcmp(str, name, len); lets memcmp() compare the first len bytes of the data pointed to by str and name, yet some of those are pointers to a string less than len in size. What code needs is strncmp() to not compare past the end of the string.

Pedantic Bug

str[len]-name[len] in compar() should be an unsigned char compare to well sort symbols that have a character outside the [0...CHAR_MAX] range. Yet since, presently, symbols[] only employs ASCII characters, it does not make a difference.

memcmp(str, name, len); lets memcmp() compare the first len bytes of the data pointed to by str and name, yet some of those are pointers to a string less than len in size. What code needs is strncmp() (or perhaps even strcmp()) to not compare past the end of the string.

Bug

str[len]-name[len] in compar() should be an unsigned char compare to well sort symbols that have a character outside the [0...CHAR_MAX] range. Yet since, presently, symbols[] only employs ASCII characters, it does not make a difference.