Paul Staron

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10 years, 9 months ago.

Delete last character in a string

Silly question, but is there a simple way to remove the last character in a string and resize length accordingly?

Code

 char ssid[26];      
                    
                    
                    
 ssid.resize (ssid.size () - 1);     // Error: Expression must have class type in "main.cpp"

1 Answer

David Smart
10 years, 9 months ago.

Hi Paul,

Being more a c-programmer than c++, and starting with this:

char ssid[26];   // allocate 26 bytes to ssid, whether we use it or not.


Here's my thinking for what you are asking:

char * ssid = (char *)malloc(26);  // allocate 26 bytes (for now)
strcpy(ssid, "123456");            // ssid now has "123456\0"
ssid = (char *)realloc(ssid, 25);  // realloc preserves what it can, but caution: it will not move the NULL terminator
ssid = (char *)realloc(ssid, 3);   // preserves to "123", but we have no idea what comes after 3, a 4, a NULL, or garbage.
int ssidlen = strlen(ssid);        // so I won't predict the answer to this, and it might not be consistent depending on the memory manager.


Your title says to delete last character, so let me explore another possibility:

char ssid[26] = "123456";      // only occupying 7 of the 26 (7 counts the NULL terminator "123456\0")
int ssidlen = strlen(ssid);    // ssidlen has 6, since it stops at the NULL.
ssid[strlen(ssid) - 1] = '\0'; // find the last character position (the '6'), backup 1 and stuff NULL in there
ssidlen = strlen(ssid);        // newlen now has 5

This latter version does not reduce the 26 byte allocation, but all uses of ssid now have a shorter string. For this scenario I assumed ssid contains text, and therefore the NULL termination was needed.

And unrelated to your question - but if "ssid" refers to Wi-Fi ssid, the common references say it could be a maximum of 32 characters in length (and some indications that some systems only permit 31), so 26 might be a little shy of a fully flexible solution.

Thank you David, making a bit more sense now.

Nice if I could use pop_back

http://www.cplusplus.com/reference/vector/vector/pop_back/

I'm using a touch screen soft keyboard and the idea is if I mistype character's I need to delete or 'backspace' like you would on a PC keyboard. I would need to end up with the correct byte amount to pass to the Wi-Fi module. And yes I will need to extend the allocation to a maximum of 32 (when I get this bit working, I'll need to reduce font size to get it in one line, but that's no problem).

I'll give this a try and see what happens.

Edit...

Works fine David, many thanks. (can't accept answer for some reason)

Paul

posted by Paul Staron 22 Jan 2015