Input and output: printf and scanf

printf: formatted output

int qty = 3;
double price = 249.5;
char grade = 'A';
const char *city = "Pune";

printf("%d items at Rs %.2f each\n", qty, price);   // 3 items at Rs 249.50 each
printf("Grade %c, city %s\n", grade, city);         // Grade A, city Pune
printf("[%5d] [%-6s] [%08.3f]\n", 42, "ok", 3.14159); // [   42] [ok    ] [0003.142]
printf("%x %X %o\n", 255, 255, 8);                   // ff FF 10
printf("100%% done\n");                               // %% prints a percent sign

Escape sequences

Inside string and character literals, a backslash starts an escape: \n new line, \t tab, \\ a backslash, \" a double quote, \' a single quote, and \0 the null character (used to end strings, module 4).

scanf: reading input

int age;
double height;
printf("Age and height: ");
int got = scanf("%d %lf", &age, &height);   // & gives the ADDRESS where scanf stores the value
if (got != 2) {
    printf("Please enter a number and a decimal.\n");
    return 1;
}
printf("Age %d, height %.2f\n", age, height);

scanf needs the address of each variable (written &variable) so it can store the value there; you will understand why with pointers in module 5. It returns how many values it successfully read (or EOF at end of input). Checking that number is the only way to know the input was valid.

Common scanf traps

• Forgetting &: scanf("%d", age) passes a garbage address and usually crashes.
• The leftover newline: after scanf("%d", &n), the Enter key stays in the input, so a following scanf("%c", &ch) reads that newline. Write " %c" (with a space) to skip whitespace first.
• Unbounded strings: scanf("%s", name) can write past the end of the array (a buffer overflow). Always give a width one less than the array size: char name[20]; scanf("%19s", name);
• %s stops at the first space, so "Ravi Kumar" reads only "Ravi".

Reading a whole line: fgets

char line[100];
if (fgets(line, sizeof line, stdin) != NULL) {   // reads at most 99 chars + '\0', including the newline
    line[strcspn(line, "\n")] = '\0';            // remove the newline (needs <string.h>)
    printf("You typed: %s\n", line);
}