In C scanf() if we use comma separated format specifier the first value gets assigned but the next one is assigned with some random values.Why?

Question

int main(void){
    char ch;
    int a;
    scanf("%c,%d",&ch,&a);
    printf("%c,%d",ch,a);
    return 0;
}

i am trying to take multiple inputs through scanf in C but here as you can see i have used ',' in format specifier which is causing the int variable to take garbage values.Why is that?If i don't use comma it works fine why?

Input-First input - C. now as soon as i hit enter to take the second input it prints C,1243421232 the second output that i am getting is some garbage value.

I can't see your code snippet, but are you checking `scanf`'s return value? (If not: do that.) — Steve Summit, Oct 05 '22 at 18:48
What did you type as input? You have to type a comma after the first character to match the format. — Barmar, Oct 05 '22 at 18:49
Please provide enough code so others can better understand or reproduce the problem. — Community, Oct 05 '22 at 18:51
Your `scanf` format string has to match your intended input. If the format says there are commas, the input has to have commas also. If you intend the input to have whitespace-separated values, then the individual format specifiers (`%c` and `%d`) should be separated in the format string by whitespace. (Or in some cases you don't need to separate them at all.) — Steve Summit, Oct 05 '22 at 18:53
You're probably assuming that `scanf` is a reasonable function, that you can just use, and will do what you expect. It is not, and it will not. You need to either (a) read and learn about `scanf` (and beware that there is a lot to learn), or (b) limit yourself to simplified, stylized uses, not anything complicated. And for our purposes here, trying to read two things at once is "complicated". — Steve Summit, Oct 05 '22 at 18:56
And it turns out that using `%c` at all is borderline complicated. — Steve Summit, Oct 05 '22 at 18:57
See [this list of secret rules](https://stackoverflow.com/questions/72178518#72178652) for using `scanf` simply but successfully. — Steve Summit, Oct 05 '22 at 18:57
There is a simpler set of rules for using `scanf`, so simple it can be expressed with a single word: **never**. — n. m. could be an AI, Oct 05 '22 at 19:35

score 2 · Accepted Answer · answered Oct 05 '22 at 19:08

In C scanf() if we use comma separated format specifier the first value gets assigned but the next one is assigned with some random values.Why?

I don't think I'm going too far out on a limb when I speculate that no, the second variable (a) does not get assigned a random value. Rather, it never gets any value assigned at all. Being a local variable without an initializer, it also has no defined default value. When you print it, you elicit undefined behavior, which seems to be manifesting as displaying a value that the program source code does not give you any means to predict.

scanf() is a rather tricky beast, but one very good rule about its use is always check its return value. That indicates the number of fields that were scanned and assigned, which in your case I'm sure you would find to be one, not two. It is possible also for the number to be zero, and if there is an error then it will be -1.

And why does your scanf() call match and assign only one field? Because the format string you present to it is a template for the expected input, and scanf stops processing it if it reaches an input character that it cannot match to the template. Characters that are not part of a field directive are taken literally, and the comma in your format falls into that category. Scanning stops after the first field because the input character matched to that field is not immediately followed by a comma to match to the comma in the format.

score 0 · Answer 2 · answered Oct 05 '22 at 19:21

Your format string is expecting to read a character that will be parsed by %c, followed by , followed by a number that will be parsed by %d. You can have whitespace before the number, but there needs to be a literal comma as the second character of the input.

You're typing C followed by Enter. The Enter key doesn't match the , in the format string, so scanf() stops processing input. It assigns to ch, but never assigns anything to a. Since it was never initialized, it contains an unpredictable value.

You have to type something like

C,123

with no Enter between them.

In C scanf() if we use comma separated format specifier the first value gets assigned but the next one is assigned with some random values.Why?

2 Answers2