How to return a character in string and convert a char to int ?

**desmond5** · 02-13-2008

I have a string and I'd like to check whether a character at a certain position is a digit or a letter.

I would do it like this:

char *mynumber - comes from function parameter

int a = atoi(mynumber[0]);
int b = atoi(mynumber[1]);
int c = atoi(mynumber[2]);

if (isdigit(mynumber[0]) && (b - a = 1) ...)

But it keeps telling me "passing arg 1 of `atoi' makes pointer from integer without a cast", so I guess it doesn't like mynumber[i] which a thought would work for returning a character from a string.

Any suggestions ?

Best wishes, D5

**Elysia** · 02-13-2008

You're passing a char, but atoi expects char*.
You must pass a string, not a char.
To check if a certain character is a digit, you can use the function isdigit().

**desmond5** · 02-13-2008

I am using the isdigit(), but I want to use it only on a specifit character in string not the whole string. Same thing with atoi(). So what's the workaround ?

**Elysia** · 02-13-2008

The answer is right in front of you. Isdigit can be used to validate a single char within a string.
Atoi doesn't work that way. Atoi converts a string to an int. Nothing more. Nothing less. And it does not work on single characters.

**laserlight** · 02-13-2008

I want to use it only on a specifit character in string not the whole string

You are already doing that correctly with isdigit(mynumber[0]). The error lies with your use of atoi() on characters instead of strings.

Same thing with atoi().

Use mynumber[0] - '0' instead.

**desmond5** · 02-13-2008

Originally Posted by laserlight

You are already doing that correctly with isdigit(mynumber[0]). The error lies with your use of atoi() on characters instead of strings.

Use mynumber[0] - '0' instead.

What's the point of using int a = atoi("0") ? I dont't want to convert "0" to 0 but the first (or second or fourth or whatever) character in the string to int.

If I wanted to loop throught all characters in a string and convert them to int one at a time, how would i do that ?

**laserlight** · 02-13-2008

What's the point of using int a = atoi("0") ?

I did not suggest that you do that. I suggested that you write: int a = munumber[0] - '0'.

**Elysia** · 02-13-2008

Originally Posted by desmond5

What's the point of using int a = atoi("0") ? I dont't want to convert "0" to 0 but the first (or second or fourth or whatever) character in the string to int.

Laserlight already told you. Take the char you want to convert and subtract '0' from it.

If I wanted to loop throught all characters in a string and convert them to int one at a time, how would i do that ?

That's exactly (or to a degree, at least) what atoi does - converts an entire string to its integer representation.

**desmond5** · 02-13-2008

Oh, thanks

Another question, how can I check if all characters in string are the same (ie "aaaaaa") ?

**laserlight** · 02-13-2008

Another question, how can I check if all characters in string are the same (ie "aaaaaa") ?

Check if every subsequent character is equal to the first character.

**dwks** · 02-13-2008

You could do that by setting a pointer to the beginning of the string and walking through it that way. Or you could use an integer index and increment that.

Code:

if (isdigit(mynumber[0]) && (b - a = 1) ...)

I know that's pseudocode, but note that you ought to use == instead of = in if statements, nearly all of the time.

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