Practice C right here — no installs, no signups.
C is the language operating systems, embedded firmware and every other language's runtime are written in. Learning it teaches you what a pointer actually is and where memory comes from. This online C compiler compiles and links your program, then prints the program output and any compiler diagnostics.
Use it to practise pointers and arrays, structs, dynamic allocation with malloc/free, string handling with the standard library, and file-free input via scanf. Compiler warnings are shown too — reading them is half of learning C.
Copy any snippet into the editor above and press Run. Each one is short on purpose — retype it from memory afterwards.
#include <stdio.h>
int main(void) {
int nums[4] = {10, 20, 30, 40};
int *p = nums;
for (int i = 0; i < 4; i++)
printf("%d at %p\n", *(p + i), (void *)(p + i));
return 0;
}An array name decays to a pointer to its first element, so *(p + i) is exactly nums[i].
#include <stdio.h>
#include <string.h>
struct Student { char name[20]; int marks; };
int main(void) {
struct Student s;
strcpy(s.name, "Aarav");
s.marks = 78;
printf("%s scored %d\n", s.name, s.marks);
return 0;
}Strings in C are char arrays — you copy them with strcpy, not with =.
warning: implicit declaration of function 'printf'
Why: The header that declares the function was not included.
Fix: Add #include <stdio.h> at the top.
Segmentation fault
Why: You dereferenced an invalid or freed pointer, or wrote past the end of an array.
Fix: Check array bounds and confirm every pointer is assigned before use.
error: expected ';' before '}' token
Why: A missing semicolon on the previous statement.
Fix: C needs a semicolon after every statement — look at the line above the reported one.
| Concept | Syntax |
|---|---|
| Entry point | int main(void) { return 0; } |
| Print an int | printf("%d\n", x); |
| Read an int | scanf("%d", &x); |
| Pointer | int *p = &x; |
| Allocate | int *a = malloc(n * sizeof(int)); |
| Struct | struct P { int x; }; |
A modern C compiler is used, so C99/C11 features such as declaring loop variables inside for work.
Either you never called printf, or the program crashed before reaching it — check the diagnostics panel for a segmentation fault.
Running snippets builds speed; the 17-chapter course builds understanding. Work through the C chapters, take the quiz, then claim your certificate.
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C teaches you what the machine actually does. These steps cover printf and scanf, types, loops, functions, arrays, strings and your first pointers.
Every C program starts at main and returns an int status. printf needs a format specifier for each value: %d for int, %f for float, %s for a string, %c for a char.
#include <stdio.h>
int main(void) {
int marks = 87;
printf("Marks: %d\n", marks);
return 0;
}scanf needs the address of the variable, hence the &. Type your input into the Input (stdin) box before pressing Run.
#include <stdio.h>
int main(void) {
int n;
scanf("%d", &n);
printf("You typed %d\n", n);
return 0;
}Declare a function above main (or prototype it) so the compiler knows its signature before the call.
#include <stdio.h>
int square(int n) { return n * n; }
int main(void) {
for (int i = 1; i <= 5; i++) printf("%d ", square(i));
printf("\n");
return 0;
}A C string is just a char array ending with a '\0' byte. That is why sizeof and strlen give different numbers.
#include <stdio.h>
#include <string.h>
int main(void) {
char name[] = "coding";
printf("%lu %lu\n", sizeof(name), strlen(name));
return 0;
}A pointer stores an address. * declares one and also reads through it; & takes the address of a variable. Pointers are how C passes data to a function without copying it.
#include <stdio.h>
void twice(int *n) { *n = *n * 2; }
int main(void) {
int v = 21;
twice(&v);
printf("%d\n", v);
return 0;
}Try each one in the editor above before opening the solution — the struggle is where the learning happens.
Hint: One loop for rows, a nested loop for the stars in each row.
#include <stdio.h>
int main(void) {
for (int i = 1; i <= 5; i++) {
for (int j = 0; j < i; j++) printf("*");
printf("\n");
}
return 0;
}Hint: Start max at the first element, not at 0 — negatives matter.
#include <stdio.h>
int main(void) {
int a[6] = {3, -9, 42, 7, 42, 1};
int max = a[0];
for (int i = 1; i < 6; i++) if (a[i] > max) max = a[i];
printf("%d\n", max);
return 0;
}Hint: Swap the characters at the ends and move both indexes inward.
#include <stdio.h>
#include <string.h>
int main(void) {
char s[] = "compiler";
int i = 0, j = strlen(s) - 1;
while (i < j) {
char t = s[i]; s[i] = s[j]; s[j] = t;
i++; j--;
}
printf("%s\n", s);
return 0;
}C makes memory, pointers and the cost of an operation visible, which is why it is the standard first language in computer-science courses.
Operating systems, databases and embedded firmware are still written in C, so its concepts show up under almost every other language you learn later.
Ready for structured practice? The 17-chapter C course takes these ideas one at a time, with a quiz and a certificate at the end.