Program crashes

Question

I'm trying to write a code to check a password for an uppercase letter. When I type a password with an uppercase letter, the program runs as expected (prints out '1' and the entered password). However, if I type a password without an uppercase letter, the program crashes.

My code:

#include < stdio.h >
#include < stdlib.h >
#include < ctype.h >
#include < string.h >
#include <math.h>

int main() {

    char password[100];
    int i = 0;
    int upper = 0;

    printf("Enter a password with an uppercase letter: ");
    scanf("%s", password);

    for (i = 0; i <= 100; i++) {
        if (isupper(password[i])) {
            upper = 1;
            break;
        }
    }

    printf("%d\n", upper);
    printf("%s\n", password);

    system("pause");
    return (0);
}

The errors:

Debug Assertion Failed!

Program: ...15\\Projects\\ConsoleApplication3\\Debug\\ConsoleApplication3.exe File: minkernel\\crts\\ucrt\\src\\appcrt\\convert\\isctype.cpp Line: 36

Expression: c >= -1 && c <= 255

For information on how your program can cause an assertion failure, see the Visual C++ documentation on asserts.

(Press Retry to debug the application) ConsoleApplication3.exe has triggered a breakpoint.

Debug Assertion Failed!

Program: ...15\\Projects\\ConsoleApplication3\\Debug\\ConsoleApplication3.exe File: minkernel\\crts\\ucrt\\src\\appcrt\\convert\\isctype.cpp Line: 42

Expression: c >= -1 && c <= 255

For information on how your program can cause an assertion failure, see the Visual C++ documentation on asserts.

(Press Retry to debug the application) ConsoleApplication3.exe has triggered a breakpoint.

The program '[13188] ConsoleApplication3.exe' has exited with code 0 (0x0).

Answer 1

Change this:

for (i = 0; i <= 100; i++) {

to

for (i = 0; i < 100 && password[i] != '\0'; i++) {
              ^^^      ^^^^^^^^^^^^^^^^^^
        note: no =     check for end of string

You have

char password[100];

so the legal index is only 0 to 99. Further you need to check if you have reached end of string (aka password[I] != '\\0')

A better way of doing what you want to achieve (inspired by @Jean-Francois Fabre):

     scanf("%99s", password);
             ^^
             To prevent buffer overflow

     for (i = 0; i < strlen(password); i++) {
         ....

Answer 2

change :

for (i = 0; i <= 100; i++) 

to :

for (i = 0; i < strlen(password); i++)

Note : You are going out of bound if there is no uppercase letter.