Ignoring whitespace in string stream doesn't work with skipws flag

Question

I am trying to solve a problem that evaluates simple expressions in a string like 10+20 should return 30

The problem is the string can contain white spaces so i uses stringstring with skipws flag it works for white spaces within the string like 10 + 20 etc but when white space is in the end its not passing towards the correctly.

This is how I am parsing the string

void build_stack(const string& str) {
    istringstream ss(str);

    char op;
    int op1, op2;
    ss>>skipws>>op1;
    operands.push(op1);
    while(!ss.eof()) {
        ss>>op;
        operators.push(op);
        ss>>op2;
        operands.push(op2);
    }
}

full code with various string inputs here in rextester.

when the string is 3+5 / 2 my stacks gets built as operands => 2 2 5 3 operators => / / +

when the string is 3+5 / 2 (i.e with trailing whitespace) my stacks gets built as operands => 2 5 3 operators => / +

i tried

while(ss.peek() == ' ') {
    char c; ss>>c;
}

but this is removing all characters after seeing a whitespace.

Always test all input operations, such as use of operator >>, and report failures. And https://stackoverflow.com/questions/5605125/why-is-iostreameof-inside-a-loop-condition-i-e-while-stream-eof-cons — , Sep 17 '19 at 17:22
Please provide a [mcve]. Don't do `while(!ss.eof())` https://stackoverflow.com/questions/5837639/eof-bad-practice — Thomas Sablik, Sep 17 '19 at 17:42

Thomas Sablik · Answer 1 · 2019-09-17T18:34:23.053

Example "3+5 / 2 ":

ss>>skipws>>op1;

3 is written into op1
operands.push(op1);

op1 is pushed into operands
while(!ss.eof()) {

is true
ss>>op;

+ is written into op
operators.push(op);

op is pushed into operators
ss>>op2;

5 is written into op2
operands.push(op2);

op2 is pushed into operands
while(!ss.eof()) {

is true
ss>>op;

/ is written into op
operators.push(op);

op is pushed into operators
ss>>op2;

2 is written into op2
operands.push(op2);

op2 is pushed into operands, ss contains a whitespace, not empty
while(!ss.eof()) {

is true
ss>>op;

can't be read, / stays in op, eof is set
operators.push(op);

op is pushed into operators
ss>>op2;

can't be read, 2 stays in op2
operands.push(op2);

op2 is pushed into operands
while(!ss.eof()) {

is false

in contrast to example "3+5 / 2":

ss>>skipws>>op1;

3 is written into op1
operands.push(op1);

op1 is pushed into operands
while(!ss.eof()) {

is true
ss>>op;

+ is written into op
operators.push(op);

op is pushed into operators
ss>>op2;

5 is written into op2
operands.push(op2);

op2 is pushed into operands
while(!ss.eof()) {

is true
ss>>op;

/ is written into op
operators.push(op);

op is pushed into operators
ss>>op2;

2 is written into op2
operands.push(op2);

op2 is pushed into operands, ss is empty, eof is set
while(!ss.eof()) {

is false

You can fix it with

void build_stack(const string& str) {
    istringstream ss(str);

    char op;
    int op1, op2;
    ss>>skipws>>op1;
    operands.push(op1);
    while(true) {
        if (!(ss>>op)) break;
        operators.push(op);
        if (!(ss>>op2)) break;
        operands.push(op2);
    }
}

Thanks @ThomasSablik for the step by step explanation. So the value is being reused from the stack confirmed by modifying the values in the stack and see the dummy values gets inserted new source [location](https://rextester.com/MQQJ44129). My question still is since the ignore whitespace flag is set shouldnt `ss.eof()` check right away if remaining stream is valid or the last read adjust whitespace characters ? Rightnow i switched to `while(ss>>op)` instead. to avoid two breaks. — user3279954, Sep 17 '19 at 19:50
First, `ss>>skipws` does nothing in your example. skipws is the default behavior: https://en.cppreference.com/w/cpp/io/manip/skipws. Whitespaces are skipped until input, not after input. "_Enables or disables skipping of __leading__ whitespace"_ Without both breaks input like `5+5+` will yield `5 5 5` and `+ +`. — Thomas Sablik, Sep 17 '19 at 20:01

Ignoring whitespace in string stream doesn't work with skipws flag

1 Answers1