C-用于循环中argv的scanf char

Question

actually struggling with my code in C. I've added some program arguments and I want to print those beggining with letter user chooses. Glad for any advice.

Thank you

**Build messages:**
warning: multi-character character constant
warning: left-hand operand of comma expression has no effect 

int main(int argc, char *argv[]) {
    int i;
    char firstLetter;

    printf("What city r u looking for?\n");
    scanf("%c", &firstLetter);

    for(i=1;i<argc;i++) {
        if (argv[i][0] == '%c', "firstLetter") { //fault here?
            printf("%s\n", argv[i]);
        }
    }
    return 0;
}

This one works great:

int main(int argc, char *argv[]) {
    int i;
    for(i=1;i<argc;i++) {
        if (argv[i][1] == 'r') {
            printf("%s\n", argv[i]);
        }
    }
    return 0;
}

Answer 1

The error in your code is an issue with placing both an invalid character cosntant and a constant string literal inside of your if statement:

if (argv[i][0] == '%c', "firstLetter") //fault here?
{
    printf("%s\n", argv[i]);
}

This if statement's condition has several issues. First, '%c' is an invalid statement in general because anything surrounded by single quotes is expected to be a one-character literal. Therefore, '%c' would be invalid because it is two characters.

Next, you are comparing '%c' , but then you are discarding the result. The comma operator in C is rather useful and surprisingly underused. Within conditional expressions, the comma operator causes everything to the left of it to be ignored, and indicates that only the item(s) to the right of the comma should be considered when evaluating the condition. As an example if you say, if ( i = i+2, x != 'y' ){ ... } , then this will actually cause i to be incremented by two, then the comma causes the value to be ignored before the x != 'y' comparison is evaluated. So, in your case, it might actually be valid to say if (argv[i][0] == 'c', "firstLetter") . (Although, the result would still not be what you want.) In this case, argv[i][0] == 'c' would be evaluated, and then the result of that evaluation would discarded. After that is discarded, "firstLetter" would be evaluated for its truthiness. "firstLetter" is a truthy value, the condition if (argv[i][0] == 'c', "firstLetter") would almost certainly evaluate to true. (Assuming your compiler lets you compile that.)

Focusing on your specific use case, I would venture to guess you wanted to say if (argv[i][0] == firstLetter) , which will compare the first letter of the argv[i] string against the character stored in the firstLetter variable.