Majd Sadi
Majd Sadi

Reputation: 49

printf long changes variable when it's digits is more than 9 chars

I have this simple code in C, whenever I enter an id number that longer than 9 digits, it prints a different number. What's the wrong?

void main() {
    long id1;
    double l1, w1;
    printf("enter studint 1 id,lenght and weight: \n");
    scanf("%ld %lf %lf", &id1, &l1, &w1);

    printf("STUDINTS INFORMATION:");
    printf("\n\n%-10.0ld\t %-4.3lf\t %-4.3lf\n", id1, l1, w1); 
}

Upvotes: 1

Views: 544

Answers (3)

chqrlie
chqrlie

Reputation: 144770

You use type long for id1 which may have range of just -2147483647 to 2147483647 depending on the target system architecture. This type cannot be used to store larger numbers on your system.

Incidentally, the format %-10.0ld for id1 seems incorrect as the output for the value 0 will be just 10 spaces. The precision field does not specify the number of decimals for integers, but the minimum number of digits.

Note also that main without arguments should have a prototype of int main(void), and you should test the return value of scanf() to avoid undefined behavior on invalid input.

To solve your problem, here are possible solutions:

  • You could use type double but I would not advise so.

  • You could try type long long int, but this type may not be fully supported on your system.

  • I suggest you use a character string for your purpose.

Here is a modified version:

#include <stdio.h>

int main(void) {
    char id1[20];
    double l1, w1;

    printf("enter student 1 id, height and weight:\n");
    if (scanf("%19s%lf%lf", id1, &l1, &w1) != 3) {
        printf("invalid input\n");
        return 1;
    }
    printf("STUDENT INFORMATION:\n");
    printf("\n%-10s\t %-4.3f\t %-4.3f\n", id1, l1, w1);
    return 0;
}

Upvotes: 1

Mangaldeep Pannu
Mangaldeep Pannu

Reputation: 3987

That's because range of long in C is from -2,147,483,648 to 2,147,483,647. So, when you try to save a number longer than 9 digits it gets out of range.

For more details on the range of variables in C: https://www.tutorialspoint.com/cprogramming/c_data_types.htm

Upvotes: 0

John Zwinck
John Zwinck

Reputation: 249153

Usually, IDs are better stored as strings. Not only is it more flexible if the ID someday contains letters, but it avoids problems like this. IDs are not arithmetic, i.e. you cannot typically get anything out of adding or subtracting them, so you may as well just store them as strings. Then you can easily support any length, regardless of the width of various integer types on your system.

Upvotes: 0

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