Beginner · C PROGRAMMING

Reading standard input

Read values supplied by the learner

The standard input box supplies text to the program when you press Run code. Enter 12 8 to supply two integers. The & operator passes the address of each variable so that scanf can store a converted value there.

Validate the conversion

scanf returns the number of successful conversions. This program needs two. A failed conversion produces a message and ends the program. It then restricts both values to a small range before adding them.

Expected output

Sum: 20

Practise

Try 15 and -4, then try the text hello. Predict the result before running. Use the Example input button to return to the starting values.

Common mistake

Do not use a variable after a failed conversion. This introductory scanf example expects text representable as int; extremely long numbers are not a safe input. For robust untrusted input parsing, use fgets with strtol and explicit range checks.

Lesson discussion

#include <stdio.h>

int main(void) {
    int first, second;
    if (scanf("%d %d", &first, &second) != 2) {
        puts("Enter two integers.");
        return 1;
    }
    if (first < -1000 || first > 1000 || second < -1000 || second > 1000) {
        puts("Use values from -1000 to 1000.");
        return 1;
    }
    printf("Sum: %d\n", first + second);
    return 0;
}

PRACTICE LAB

Read. Edit. Run.

C source · .c

Experiment with the program below. Your code and input are sent to the selected compiler only when you run it.

Compiler

C source

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Standard input

Input / output example

Example input

12 8

Expected output for the original program

Sum: 20

Output will appear here.

Reading standard input

Read values supplied by the learner

Validate the conversion

Expected output

Practise

Common mistake

Lesson discussion

Read. Edit. Run.

Input / output example

Debugging Lab

Interactive Memory Visualizer

Stack frames

Memory and pointers

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