checking for letters in a string of ints - c++

Question

bool isInt(string input) { //checks if string input is an int
if (input[0] == '-') { //checks if negative number
for (int a = 1; a < input.length(); a++) {
if (isdigit(input[a])) {
return true;
}
else {
return false;
}
}
}
for (int b = 0; b < input.length(); b++) {
if (isdigit(input[b])) {
return true;
} else {
return false;
}
}
}
So my code here is to check the string input the user enters and sees if its an integer. It works except cases where if the first character is either - or a number, but if thats followed by characters such as -23ab or -4b, my program won't catch that as an error as it isn't a valid number. I know the error is due to my return true/false statements, but I don't know of any method of how to fix it.
Any ideas?

Answer 1

It doesn't work at all!!
It only cares about the first character in the string because you return no matter the state of the character. You need to store a result in a variable and continue searching until the first failure.

Answer 2

You are returning true as soon as you see 1 digit. You should return true only if you make it to the end of the string w/o an error. You can return false early but for this problem, you should only return true at the end.

Answer 3

As others here said, you're only checking the first character and returning whether the first character is a digit or not....what you'll want to do instead is check if the character is a digit....if so then continue you on to the next digit. The only reason you'll return is if you either went through all the characters or if you found a non digit character.
for each character
if character is a digit
do nothing
else
return that it is false.
return true //you will only get to this statement if all characters are digits
This is what you have...
for each character
if character is a digit
return true
else
return false
So its basically returning before it checks the rest of the characters

Answer 4

Your code is trying too hard. You're testing for both success and failure rather than just looking for a fail case.
The following code takes a position index of 0, if the string isn't empty, it checks for a leading '-' and increments the index position to 1 if it finds that.
Then it simply checks through the string for a non-numeric character and fails if it finds one.
If it reaches the end of the string, then the string is all numeric.
bool isInt(const std::string& in)
{
size_t pos = 0;
if (!in.empty() && in[0] == '-')
pos = 1;
if (pos >= in.size()) // catch "" and "-"
return false;
do {
if (!isdigit(in[pos]))
return false;
} while (++pos < in.length());
return true;
}
This resolves your current problem but there is possible problem still: Is the string "123456789123456789" an int? It's too big to store in an int.