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How do I check a string in C programming to see if each character is a number or '.'?
How do I use the isdigit() function in my code?
You can use the isdigit() function like this: 

```c
#include <ctype.h>

int main() {
    char str[] = "123.45";
    int i;
    for (i = 0; str[i] != '\0'; i++) {
        if (!isdigit(str[i]) && str[i] != '.') {
            printf("String contains non-numeric characters.\n");
            break;
        }
    }
    if (str[i] == '\0') {
        printf("String only contains numeric characters and decimal point.\n");
    }
    return 0;
}
```
This code will loop through each character in the string and use the isdigit() and ispunct() functions to check if it is a digit or a decimal point. If the character is neither, it will print a message saying that the string contains non-numeric characters. Otherwise, it will print a message saying that the string only contains numeric characters and decimal point.