"Debug assertion failed! expression: _p != nullptr" error - c

Question

error imageI tried to make a program that does calculation with multiple arguments.
Everything works as I intended except for last one in the picture i attached.
Every time I try to put more than 2 numbers in the command line, the error occurs.
The example of input is this : program1.exe 12 + 13 - 2 + 1
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
int a, b, rst;
int i;
char opr;
if (argc > 100)
{
printf("Too many arguments!\n");
return 0;
}
else if (argc <= 1)
{
printf("There are no arguments!\n");
return 0;
}
a = atoi(argv[1]);
rst = a;
if (argc == 2)
{
a = atoi(argv[1]);
}
else if (argc % 2 != 0)
{
printf("Invalid fomula!\n");
exit(0);
}
else
{
for (i = 1; 2*i <= argc; i++)
{
b = atoi(argv[2 * i]);
opr = argv[2 * i - 1];
switch (opr)
{
case '+':rst = a + b;
break;
case '-':rst = a - b;
break;
}
rst = a;
}
}
printf("%d", rst);
return 0;
}

Answer 1

Running the code as is, with input set to: <program.exe> 2 + 5
a null pointer error occurs here:
for (i = 1; 2*i <= argc; i++)
{
b = atoi(argv[2 * i]);//Null pointer for i==2
opr = argv[2 * i - 1];
switch (opr)
because there is no argv[4]. This results in undefined behavior.
It is impossible to make corrections accurately without knowing what the program expects as input. Because you do not specify, my best guess is the program attempts to read a set of arguments from the command line, eg:
argv[0] argv[1] argv[2] argv[3]
<program>.exe 1 + 2
Then convert the values read into variables, and perform a math operation using two numerics and a math operator such as + or -.
Assuming this is the basic goal, the following example simplifies your existing code to perform these steps. Note corrections to the argument input count test, and lower down, the looping indexes:
#include <stdio.h>
#include <stdlib.h>
/// test using:
// prog.exe 2 + 3 3 - 4 6 + 7
//results are:
//result 0: 5
//result 1: -1
//result 2: 13
typedef struct {
int n1;
char opr;
int n2;
int result;
} CALC;
int main(int argc, char *argv[])
{
int i;
// test for correct number of input arguments
if((argc-1)%3 !=0)
{
printf("Wrong number of inputs.\n");
return 0;
}
// read in, and assign variables (use array of struct)
int max = (argc-1)/3; //guaranteed viable only after determining multiples are correct from above
//Get ALL input into array variables
CALC calc[max];
for (i=0;i<max;i++)
{
//convert inputs into usable variables
calc[i].n1 = atoi(argv[i*3+1]);
calc[i].opr = argv[i*3+2][0];
calc[i].n2 = atoi(argv[i*3+3]);
// perform calulations
switch(calc[i].opr) {
case '+':
printf("result %d: %d\n", i, calc[i].n1 + calc[i].n2);
break;
case '-':
printf("result %d: %d\n", i, calc[i].n1 - calc[i].n2);
break;
}
}
return 0;
}
Most error checking, and testing of function return values are left to you.