C - How to scan an int only entered after symbol

Question

I am having difficulty scanning from user input an integer (and storing it) only if printed directly after a !:

   char cmd[MAX_LINE/2 + 1];  
   if (strcmp(cmd, "history") == 0)
       history(hist, current);
   else if (strcmp(cmd, "!!") == 0)
       execMostRecHist(hist, current-1);
   else if (strcmp(cmd, "!%d") == 0)
       num = %d;
   else
       {//do stuff}

I understand this is completely wrong syntax for strcmp(), but just as an example of how I am gathering user input.

What is this suppose to mean `num = %d;` ? – ameyCU Sep 17 '15 at 15:25 — ameyCU, Sep 17 '15 at 15:25
Just set to num whatever int the user inputed after the ! – Sean Sep 17 '15 at 15:26 — Sean, Sep 17 '15 at 15:26

score 1 · Answer 1 · answered Sep 17 '15 at 15:27

Don't you like writing a checker by yourself?

#include <ctype.h>
#include <stdio.h>

int check(const char *code) {
    if (code == NULL || code[0] != '!') return 0;
    while(*(++code) != '\0') {
        if (!isdigit(*code)) return 0;
    }
    return 1;
}


/* ... */

if (check(cmd))
    sscanf(cmd + 1, "%d", &num);

score 1 · Accepted Answer · answered Sep 17 '15 at 15:28

strcmp doesn't know about format specifiers, it just compares two strings. sscanf does what you want: It tests whether a string has a certain format and converts parts of the string to other types.

For example:

int n = 0;

if (sscanf(cmd, " !%d", &num) == 1) {
    // Do stuff; num has already been assigned
}

The format specifier %d tells sscanf to look for a valid decimal integer. The exclamation mark has no special meaning and matches only if there is an exclamation mark. The space at the front means that the command may have leading white space. Nothe that there may be white space after the exclam and before the number and that the number may well be negative.

The format specifier is special to the scanf family and related to, but different from the ´%dformat ofprintf`. Is usually has no meaning in other strings and certainly not when it is found unquoted in the code.

score 0 · Answer 3 · answered Sep 17 '15 at 15:32

Use sscanf() and check its results.

char cmd[MAX_LINE/2 + 1];  
num = 0;  // Insure `num` has a known value
if (strcmp(cmd, "history") == 0)
   history(hist, current);
else if (strcmp(cmd, "!!") == 0)
   execMostRecHist(hist, current-1);
else if (sscanf(cmd, "!%d", &num) == 1)
   ;
else
   {//do stuff}

C - How to scan an int only entered after symbol

3 Answers3