Using index of to search for multiple characters

Question

In my Arduino project, a string will be received. I understand using .indexof to find the placement of a certain character in the string. My problem is that I need to find the first occurrence of any letter A-Z and haven't found any resources regarding using or statements in an .indexof aka string.indexof("a" || "b" ....).

Is this something that is not possible using indexof functions and if so is there any function that I can use to solve this! Thanks!

• dandavis: index is only for one comparison at a time. there's a built-in isupper(char): cplusplus.com/reference/cctype/isupper. you can find many chars at once using cplusplus.com/reference/cstring/strcspn
• Rowan McAlpin: You can also use a for loop that checks the value of i (or whatever variable) and then depending on what i is it runs indexof with different letters.

Answer 1

Here are two functions that I hope help in your endeavours. You can supply either a String or a char array and you should get the first position of an alpha character. Returns -1 if not present.

See https://www.arduino.cc/reference/en/language/functions/characters/isalpha/

Update:

Thanks to great comments from Edgar & Mat I have updated the functions with best practise and cost improvements.

int alphaPos(const char* str)
{
 for (int i = 0; str[i] !=0 ; ++i) {
 if (isAlpha(str[i]))
 {
 return i;
 }
 }
 
 return -1;
}
int alphaPos(const String& str)
{
 return alphaPos(str.c_str());
}
/*
 Usage:
 String bobble = "&&&88434a";
 int x = alphaPos(bobble);
 if (x != -1) 
 {
 ...x has the position in the string
 }
 else
 {
 ...There is no alpha char present.
 }
*/

• Edgar Bonet: I suggest passing a const reference to the second overload.
• voidPointer: @EdgarBonet, I admit I have much to learn so ask honestly what the benefit is?
• Mat: @voidPointer: it indicates to the caller that the function will not modify the passed String. It is good practice. Also you can avoid the strlen call (which doubles the cost of the function) - use str[i] != 0 in the loop condition instead.
• voidPointer: @Mat. Thanks. That strlen tip is pretty cool and will help others too.
• Edgar Bonet: To elaborate on Mat's comment: if you wan to get an idea of why it is considered good practice, see Const Correctness on the isocpp.org wiki.