Why is my string getting modified inside this code? [closed] - c++

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I am taking an integer as input in a string that is string s. I want to write integer in s in the form of "-"'s and "|". I am sure that my logic is right. The problem is string s is automatically getting modified inside the code. When I initially print the string s, it returns complete 12345 (My input is "2 12345") but when I tries to print it afterwards it is getting truncated or something. How do I resolve this?
#include <iostream>
#include <string>
using namespace std;
int main(){
int n;
std::string s;
cin >> n;
cin >> s;
cout << s.at(3) <<endl;
while(n!=0){
for (int l=0;l<3+2*n;l++){
// for (int i=0;i<s.length();i++){
if (l==0){
for (int j=0;j<s.length();j++){
if (s.at(j)=='1'||s.at(j)=='4'){
cout << " ";
for (int k=0;k<n;k++){
cout << " ";
}
cout << " ";
}
else if (s.at(j)=='0'||s.at(j)=='2'||s.at(j)=='3'||s.at(j)=='5'||s.at(j)=='6'||s.at(j)=='7'||s.at(j)=='8'||s.at(j)=='9'){
cout << " ";
for (int k=0;k<n;k++){
cout << "-";
}
cout << " ";
}
cout << " ";
}
}
else if (l==n+1){
for (int j=0;j<s.length();j++){
if (s.at(j)=='1'||s.at(j)=='7'||s.at(j)=='0'){
cout << " ";
for (int k=0;k<n;k++){
cout << " ";
}
cout << " ";
}
else if (s.at(j)=='2'||s.at(j)=='3'||s.at(j)=='4'||s.at(j)=='5'||s.at(j)=='6'||s.at(j)=='8'||s.at(j)=='9'){
cout << " ";
for (int k=0;k<n;k++){
cout << "-";
}
cout << " ";
}
cout << " ";
}
}
else if (l==2*n+2){
for (int j=0;j<s.length();j++){
if (s.at(j)=='1'||s.at(j)=='4'||s.at(j)=='7'){
cout << " ";
for (int k=0;k<n;k++){
cout << " ";
}
cout << " ";
}
else if (s.at(j)=='0'||s.at(j)=='2'||s.at(j)=='3'||s.at(j)=='6'||s.at(j)=='8'||s.at(j)=='9'||s.at(j)=='5'){
cout << " ";
for (int k=0;k<n;k++){
cout << "-";
}
cout << " ";
}
cout << " ";
}
}
else if ((l>0) && (l<n+1)){
for (int j=0;j<s.length();j++){
if (s.at(j)=='1'||s.at(j)=='2'||s.at(j)=='3'||s.at(j)=='7'){
cout << " ";
for (int k=0;k<n;k++){
cout << " ";
}
cout << "|";
}
else if(s.at(j)='4'||s.at(j)=='8'||s.at(j)=='9'||s.at(j)=='0') {
cout << "|";
cout << "s "<< s<< endl;
cout << "check 2";
for (int k=0;k<n;k++){
cout << " ";
}
cout << "|";
}
else if(s.at(j)=='5'||s.at(j)=='6'){
cout << "|";
cout << "check";
for (int k=0;k<n;k++){
cout <<" ";
}
cout << " ";
}
cout << " ";
}
}
else if ((l>n+1) && (l<2*n+2)){
for (int j=0;j<s.length();j++){
if (s.at(j)=='1'||s.at(j)=='3'||s.at(j)=='5'||s.at(j)=='7'||s.at(j)=='9'||s.at(j)=='4'){
cout << " ";
for (int k=0;k<n;k++){
cout << " ";
}
cout << "|";
}
else if(s.at(j)='6'||s.at(j)=='8'||s.at(j)=='0') {
cout << "|";
for (int k=0;k<n;k++){
cout << " ";
}
cout << "|";
}
else if(s.at(j)=='2'){
cout << "|";
for (int k=0;k<n;k++){
cout <<" ";
}
cout << " ";
}
cout << " ";
}
}
cout << s << endl;
cout << endl;
}
cin >> n;
cin >> s;
}
}

Answer 1

else if(s.at(j)='4'||
You forgot an = sign.

Answer 2

A quick look reveals that s.at(j)='6' and s.at(j)='4' modify your string. You can avoid this by using Yoda Conditions.
There are compilers that may warn you about an assignment, here. (See comments).
Also see Mats Petersson's answer, which I consider to be the best adivce: Use const objects, references or pointers to const memory if you do not want to alter contents in a certain situation.
If you'd use a function that takes a std::string const &s for it's argument, doing the job, you'd end up having
cleaner code structure and
compiler errors upon alteration of your argument s.

Answer 3

If you have a situation where you want to avoid unintentionally modifying a variable (for example a string), you can either break out the relevant code into a function and pass a const string& x to the function, or you can create a local const reference. In your code, one could do:
std::string ms;
cin >> n;
cin >> ms;
const &std::string s = ms;
Now you will get an error any time you try to make changes to s, because it's a const.