How to check if a string is a number?

Question

I want to check if a string is a number with this code. I must check that all the chars in the string are integer, but the while returns always isDigit = 1. I don\'t know wh

Answer 1

In this part of your code:

if(tmp[j] > 57 && tmp[j] < 48)
  isDigit = 0;
else
  isDigit = 1;

Your if condition will always be false, resulting in isDigit always being set to 1. You are probably wanting:

if(tmp[j] > '9' || tmp[j] < '0')
  isDigit = 0;
else
  isDigit = 1;

But. this can be simplified to:

isDigit = isdigit(tmp[j]);

However, the logic of your loop seems kind of misguided:

int isDigit = 0;
int j=0;
while(j

As tmp is not a constant, it is uncertain whether the compiler will optimize the length calculation out of each iteration.

As @andlrc suggests in a comment, you can instead just check for digits, since the terminating NUL will fail the check anyway.

while (isdigit(tmp[j])) ++j;