C strcmp fails to compare with space

I try to see if formula[i] is a space and if it is number should be zero. But apparently it fails to compare them :S I'm new to C, so does somebody know the problem?

Note: this is not my whole code but only the essential part to understand the problem.

char formula[50] = "1 4 + 74 /";
int number = 0;
for (int i = 0; i != strlen(formula); i++)
{ if(!isdigit(formula[i])) { if (strcmp(&formula[i], " ") == 0) { number = 0; } }
}

4 Answers

It fails because the string " 4 + 74 /" is not equal to the string " ".

I think what you actually want to compare here are characters, which is as simple as

if (formula[i] == ' ')
{ // ...
}

For completeness sake, there is a way to perform a string comparison on a prefix of two strings, with the function strncmp, where you can specify how many characters of a string to match.

if (strncmp(&formula[i], " ", 1))
{ // ...
}

which would be equivalent to the caracter comparison above.

That is because they're not equal.

When you do &formula[i], you get a pointer to the character at location i. The string, viewed from that location, continues until the terminating '\0'-character, i.e. that's not a 1-character string.

Just do a direct comparison:

if(formula[i] == ' ')

strcmp compares two strings(two const char*s ending with a \0) and not two characters. In your case,use

if(formula[i]==' ')

The &formula[i] gives you a pointer to where i is allocated.

To tackle the problem , you have to do the following:

if (formula[i] == ' ')